Two separate functions of NME3 critical for cell survival underlie a neurodegenerative disorder.

We report a patient who presented with congenital hypotonia, hypoventilation, and cerebellar histopathological alterations. Exome analysis revealed a homozygous mutation in the initiation codon of the NME3 gene, which encodes an NDP kinase. The initiation-codon mutation leads to deficiency in NME3 protein expression. NME3 is a mitochondrial outer-membrane protein capable of interacting with MFN1/2, and its depletion causes dysfunction in mitochondrial dynamics. Consistently, the patient's fibroblasts were characterized by a slow rate of mitochondrial dynamics, which was reversed by expression of wild-type or catalytic-dead NME3. Moreover, glucose starvation caused mitochondrial fragmentation and cell death in the patient's cells. The expression of wild-type and catalytic-dead but not oligomerization-attenuated NME3 restored mitochondrial elongation. However, only wild-type NME3 sustained ATP production and viability. Thus, the separate functions of NME3 in mitochondrial fusion and NDP kinase cooperate in metabolic adaptation for cell survival in response to glucose starvation. Given the critical role of mitochondrial dynamics and energy requirements in neuronal development, the homozygous mutation in NME3 is linked to a fatal mitochondrial neurodegenerative disorder.

Mitochondrial UQCRC1 mutations cause autosomal dominant parkinsonism with polyneuropathy.

Parkinson's disease is a neurodegenerative disorder with a multifactorial aetiology. Nevertheless, the genetic predisposition in many families with multi-incidence disease remains unknown. This study aimed to identify novel genes that cause familial Parkinson's disease. Whole exome sequencing was performed in three affected members of the index family with a late-onset autosomal-dominant parkinsonism and polyneuropathy. We identified a novel heterozygous substitution c.941A>C (p.Tyr314Ser) in the mitochondrial ubiquinol-cytochrome c reductase core protein 1 (UQCRC1) gene, which co-segregates with disease within the family. Additional analysis of 699 unrelated Parkinson's disease probands with autosomal-dominant Parkinson's disease and 1934 patients with sporadic Parkinson's disease revealed another two variants in UQCRC1 in the probands with familial Parkinson's disease, c.931A>C (p.Ile311Leu) and an allele with concomitant splicing mutation (c.70-1G>A) and a frameshift insertion (c.73_74insG, p.Ala25Glyfs*27). All substitutions were absent in 1077 controls and the Taiwan Biobank exome database from healthy participants (n = 1517 exomes). We then assayed the pathogenicity of the identified rare variants using CRISPR/Cas9-based knock-in human dopaminergic SH-SY5Y cell lines, Drosophila and mouse models. Mutant UQCRC1 expression leads to neurite degeneration and mitochondrial respiratory chain dysfunction in SH-SY5Y cells. UQCRC1 p.Tyr314Ser knock-in Drosophila and mouse models exhibit age-dependent locomotor defects, dopaminergic neuronal loss, peripheral neuropathy, impaired respiratory chain complex III activity and aberrant mitochondrial ultrastructures in nigral neurons. Furthermore, intraperitoneal injection of levodopa could significantly improve the motor dysfunction in UQCRC1 p.Tyr314Ser mutant knock-in mice. Taken together, our in vitro and in vivo studies support the functional pathogenicity of rare UQCRC1 variants in familial parkinsonism. Our findings expand an additional link of mitochondrial complex III dysfunction in Parkinson's disease.

A Celecoxib Derivative Eradicates Antibiotic-Resistant Staphylococcus aureus and Biofilms by Targeting YidC2 Translocase.

The treatment of Staphylococcus aureus infections is impeded by the prevalence of MRSA and the formation of persisters and biofilms. Previously, we identified two celecoxib derivatives, Cpd36 and Cpd46, to eradicate MRSA and other staphylococci. Through whole-genome resequencing, we obtained several lines of evidence that these compounds might act by targeting the membrane protein translocase YidC2. Our data showed that ectopic expression of YidC2 in S. aureus decreased the bacterial susceptibility to Cpd36 and Cpd46, and that the YidC2-mediated tolerance to environmental stresses was suppressed by both compounds. Moreover, the membrane translocation of ATP synthase subunit c, a substrate of YidC2, was blocked by Cpd46, leading to a reduction in bacterial ATP production. Furthermore, we found that the thermal stability of bacterial YidC2 was enhanced, and introducing point mutations into the substrate-interacting cavity of YidC2 had a dramatic effect on Cpd36 binding via surface plasmon resonance assays. Finally, we demonstrated that these YidC2 inhibitors could effectively eradicate MRSA persisters and biofilms. Our findings highlight the potential of impeding YidC2-mediated translocation of membrane proteins as a new strategy for the treatment of bacterial infections.

Protein activity regulation by conformational entropy.

How the interplay between protein structure and internal dynamics regulates protein function is poorly understood. Often, ligand binding, post-translational modifications and mutations modify protein activity in a manner that is not possible to rationalize solely on the basis of structural data. It is likely that changes in the internal motions of proteins have a major role in regulating protein activity, but the nature of their contributions remains elusive, especially in quantitative terms. Here we show that changes in conformational entropy can determine whether protein-ligand interactions will occur, even among protein complexes with identical binding interfaces. We have used NMR spectroscopy to determine the changes in structure and internal dynamics that are elicited by the binding of DNA to several variants of the catabolite activator protein (CAP) that differentially populate the inactive and active DNA-binding domain states. We found that the CAP variants have markedly different affinities for DNA, despite the CAP−DNA-binding interfaces being essentially identical in the various complexes. Combined with thermodynamic data, the results show that conformational entropy changes can inhibit the binding of CAP variants that are structurally poised for optimal DNA binding or can stimulate the binding activity of CAP variants that only transiently populate the DNA-binding-domain active state. Collectively, the data show how changes in fast internal dynamics (conformational entropy) and slow internal dynamics (energetically excited conformational states) can regulate binding activity in a way that cannot be predicted on the basis of the protein's ground-state structure.

Structural basis of antizyme-mediated regulation of polyamine homeostasis.

Polyamines are organic polycations essential for cell growth and differentiation; their aberrant accumulation is often associated with diseases, including many types of cancer. To maintain polyamine homeostasis, the catalytic activity and protein abundance of ornithine decarboxylase (ODC), the committed enzyme for polyamine biosynthesis, are reciprocally controlled by the regulatory proteins antizyme isoform 1 (Az1) and antizyme inhibitor (AzIN). Az1 suppresses polyamine production by inhibiting the assembly of the functional ODC homodimer and, most uniquely, by targeting ODC for ubiquitin-independent proteolytic destruction by the 26S proteasome. In contrast, AzIN positively regulates polyamine levels by competing with ODC for Az1 binding. The structural basis of the Az1-mediated regulation of polyamine homeostasis has remained elusive. Here we report crystal structures of human Az1 complexed with either ODC or AzIN. Structural analysis revealed that Az1 sterically blocks ODC homodimerization. Moreover, Az1 binding triggers ODC degradation by inducing the exposure of a cryptic proteasome-interacting surface of ODC, which illustrates how a substrate protein may be primed upon association with Az1 for ubiquitin-independent proteasome recognition. Dynamic and functional analyses further indicated that the Az1-induced binding and degradation of ODC by proteasome can be decoupled, with the intrinsically disordered C-terminal tail fragment of ODC being required only for degradation but not binding. Finally, the AzIN-Az1 structure suggests how AzIN may effectively compete with ODC for Az1 to restore polyamine production. Taken together, our findings offer structural insights into the Az-mediated regulation of polyamine homeostasis and proteasomal degradation.

Exploiting E. coli auxotrophs for leucine, valine, and threonine specific methyl labeling of large proteins for NMR applications.

A simple and cost effective method to independently and stereo-specifically incorporate [(1)H,(13)C]-methyls in Leu and Val in proteins is presented. Recombinant proteins for NMR studies are produced using a tailored set of auxotrophic E. coli strains. NMR active isotopes are routed to either Leu or Val methyl groups from the commercially available and scrambling-free precursors α-ketoisovalerate and acetolactate. The engineered strains produce deuterated proteins with stereospecific [(1)H,(13)C]-methyl labeling separately at Leu or Val amino acids. This is the first method that achieves Leu-specific stereospecific [(1)H,(13)C]-methyl labeling of proteins and scramble-free Val-specific labeling. Use of auxotrophs drastically decreases the amount of labeled precursor required for expression without impacting the yield. The concept is extended to Thr methyl labeling by means of a Thr-specific auxotroph that provides enhanced efficiency for use with the costly L-[4-(13)C,2,3-(2)H2,(15)N]-Thr reagent. The Thr-specific strain allows for the production of Thr-[(13)CH3](γ2) labeled protein with an optimal isotope incorporation using up to 50 % less labeled Thr than the traditional E. coli strain without the need for (2)H-glycine to prevent scrambling.

Allosteric inhibition through suppression of transient conformational states.

The ability to inhibit binding or enzymatic activity is key to preventing aberrant behaviors of proteins. Allosteric inhibition is desirable as it offers several advantages over competitive inhibition, but the mechanisms of action remain poorly understood in most cases. Here we show that allosteric inhibition can be effected by destabilizing a low-populated conformational state that serves as an on-pathway intermediate for ligand binding, without altering the protein's ground-state structure. As standard structural approaches are typically concerned with changes in the ground-state structure of proteins, the presence of such a mechanism can go easily undetected. Our data strongly argue for the routine use of NMR tools suited to detect and characterize transiently formed conformational states in allosteric systems. Structure information on such important intermediates can ultimately result in more efficient design of allosteric inhibitors.

Dynamic activation of an allosteric regulatory protein.

Allosteric regulation is used as a very efficient mechanism to control protein activity in most biological processes, including signal transduction, metabolism, catalysis and gene regulation. Allosteric proteins can exist in several conformational states with distinct binding or enzymatic activity. Effectors are considered to function in a purely structural manner by selectively stabilizing a specific conformational state, thereby regulating protein activity. Here we show that allosteric proteins can be regulated predominantly by changes in their structural dynamics. We have used NMR spectroscopy and isothermal titration calorimetry to characterize cyclic AMP (cAMP) binding to the catabolite activator protein (CAP), a transcriptional activator that has been a prototype for understanding effector-mediated allosteric control of protein activity. cAMP switches CAP from the 'off' state (inactive), which binds DNA weakly and non-specifically, to the 'on' state (active), which binds DNA strongly and specifically. In contrast, cAMP binding to a single CAP mutant, CAP-S62F, fails to elicit the active conformation; yet, cAMP binding to CAP-S62F strongly activates the protein for DNA binding. NMR and thermodynamic analyses show that despite the fact that CAP-S62F-cAMP(2) adopts the inactive conformation, its strong binding to DNA is driven by a large conformational entropy originating in enhanced protein motions induced by DNA binding. The results provide strong evidence that changes in protein motions may activate allosteric proteins that are otherwise structurally inactive.

Nerve pathology of microangiopathy and thromboinflammation in hereditary transthyretin amyloidosis.

OBJECTIVE: Despite amyloid deposition as a hallmark of hereditary transthyretin amyloidosis (ATTRv) with polyneuropathy, this pathology could not completely account for nerve degeneration. ATTRv patients frequently have vasomotor symptoms, but microangiopathy hypothesis in ATTRv was not systemically clarified. METHODS: This study examined the vascular pathology of sural nerves in ATTRv patients with transthyretin (TTR) mutation of p.Ala117Ser (TTR-A97S), focusing on morphometry and patterns of molecular expression in relation to nerve degeneration. We further applied human microvascular endothelial cell (HMEC-1) culture to examine the direct effect of TTR-A97S protein on endothelial cells. RESULTS: In ATTRv nerves, there was characteristic microangiopathy compared to controls: increased vessel wall thickness and decreased luminal area; both were correlated with the reduction of myelinated fiber density. Among the components of vascular wall, the area of collagen IV in ATTRv nerves was larger than that of controls. This finding was validated in a cell model of HMEC-1 culture in which the expression of collagen IV was upregulated after exposure to TTR-A97S. Apoptosis contributed to the endothelial cell degeneration of microvasculatures in ATTRv endoneurium. ATTRv showed prothrombotic status with intravascular fibrin deposition, which was correlated with (1) increased tissue factor and coagulation factor XIIIA and (2) reduced tissue plasminogen activator. This cascade led to intravascular thrombin deposition, which was colocalized with upregulated p-selectin and thrombomodulin, accompanied by complement deposition and macrophages infiltration, indicating thromboinflammation in ATTRv. INTERPRETATION: Microangiopathy with thromboinflammation is characteristic of advanced-stage ATTRv nerves, which provides an add-on mechanism and therapeutic target for nerve degeneration.

Altered gut microbiota in Taiwanese A97S predominant transthyretin amyloidosis with polyneuropathy.

Increasing evidence suggests that gut microbiota alterations are related to development and phenotypes of many neuropsychiatric diseases. Here, we evaluated the fecal microbiota and its clinical correlates in patients with hereditary transthyretin amyloidosis (ATTRv) and polyneuropathy. Fecal microbiota from 38 ATTRv patients and 39 age-matched controls was analyzed by sequencing 16S V3-V4 ribosomal RNA, and its relationships with clinical characteristics of polyneuropathy and cardiomyopathy were explored. The familial amyloidotic polyneuropathy stage was stage I, II, and III in 13, 18, and 7 patients. 99mTc-PYP SPECT showed a visual score of 2 in 15 and 3 in 21 patients. The gut microbiota of ATTRv patients showed higher alpha diversity (ASV richness and Shannon effective numbers) and dissimilar beta diversity compared to controls. Relative abundance of microbiota was dominated by Firmicutes and decreased in Bacteroidetes in ATTRv patients than in controls. Patients with more myocardial amyloid deposition were associated with increased alpha diversity, and the abundance of Clostridia was significantly correlated with pathophysiology of polyneuropathy in ATTRv patients. These findings demonstrated alterations in the gut microbiota, especially Firmicutes, in ATTRv. The association between altered microbiota and phenotypes of cardiomyopathy and polyneuropathy might suggest potential contributions of gut microbiota to ATTRv pathogenesis.

Structural basis for recruitment of peptidoglycan endopeptidase MepS by lipoprotein NlpI.

Peptidoglycan (PG) sacculi surround the cytoplasmic membrane, maintaining cell integrity by withstanding internal turgor pressure. During cell growth, PG endopeptidases cleave the crosslinks of the fully closed sacculi, allowing for the incorporation of new glycan strands and expansion of the peptidoglycan mesh. Outer-membrane-anchored NlpI associates with hydrolases and synthases near PG synthesis complexes, facilitating spatially close PG hydrolysis. Here, we present the structure of adaptor NlpI in complex with the endopeptidase MepS, revealing atomic details of how NlpI recruits multiple MepS molecules and subsequently influences PG expansion. NlpI binding elicits a disorder-to-order transition in the intrinsically disordered N-terminal of MepS, concomitantly promoting the dimerization of monomeric MepS. This results in the alignment of two asymmetric MepS dimers respectively located on the two opposite sides of the dimerization interface of NlpI, thus enhancing MepS activity in PG hydrolysis. Notably, the protein level of MepS is primarily modulated by the tail-specific protease Prc, which is known to interact with NlpI. The structure of the Prc-NlpI-MepS complex demonstrates that NlpI brings together MepS and Prc, leading to the efficient MepS degradation by Prc. Collectively, our results provide structural insights into the NlpI-enabled avidity effect of cellular endopeptidases and NlpI-directed MepS degradation by Prc.

Diflunisal versus tafamidis on neuropathy and cardiomyopathy in hereditary transthyretin amyloidosis.

OBJECTIVES: Hereditary transthyretin (TTR) amyloidosis (ATTRv) is frequently complicated by polyneuropathy (ATTRv-PN) and cardiomyopathy (ATTRv-CM). The long-term efficacy of diflunisal on both polyneuropathy and cardiomyopathy in ATTRv patients, especially those with non-V30M genotypes, has not been fully investigated and compared with that of tafamidis. METHODS: We compared the structural and biochemical characteristics of A97S-TTR complexed with tafamidis with those of diflunisal, and prospectively followed up and compared the progression of polyneuropathy and cardiomyopathy between ATTRv-PN patients taking diflunisal and those taking tafamidis. RESULTS: Both diflunisal and tafamidis effectively bind to the two thyroxine-binding sites at the A97S-TTR dimer-dimer interface and equally and almost sufficiently reduce amyloid fibril formation. Thirty-five ATTRv-PN patients receiving diflunisal and 22 patients receiving tafamidis were enrolled. Compared with no treatment, diflunisal treatment significantly delayed the transition of FAP Stage 1 to 2 and Stage 2 to 3 and decreased the deterioration in parameters of the ulnar nerve conduction study (NCS). The progression of FAP stage or NCS parameters did not differ between patients treated with diflunisal and those treated with tafamidis. Both diflunisal and tafamidis treatments significantly decreased radiotracer uptake on 99mTc-PYP SPECT and stabilized cardiac wall thickness and blood pro-B-type natriuretic peptide levels. No significant adverse events occurred during diflunisal or tafamidis treatment. INTERPRETATIONS: The binding patterns of both tafamidis and diflunisal to A97S-TTR closely resembled those observed in the wild type. Diflunisal can effectively delay the progression of polyneuropathy and cardiomyopathy with similar efficacy to tafamidis and may become a cost-effective alternative treatment for late-onset ATTRv-PN.

Structural basis for regulation of the Crk signaling protein by a proline switch.

Proline switches, controlled by cis-trans isomerization, have emerged as a particularly effective regulatory mechanism in a wide range of biological processes. Here we report the structures of both the cis and trans conformers of a proline switch in the Crk signaling protein. Proline isomerization toggles Crk between two conformations: an autoinhibitory conformation, stabilized by the intramolecular association of two tandem SH3 domains in the cis form, and an uninhibited, activated conformation promoted by the trans form. In addition to acting as a structural switch, the heterogeneous proline recruits cyclophilin A, which accelerates the interconversion rate between the isomers, thereby regulating the kinetics of Crk activation. The data provide atomic insight into the mechanisms that underpin the functionality of this binary switch and elucidate its remarkable efficiency. The results also reveal new SH3 binding surfaces, highlighting the binding versatility and expanding the noncanonical ligand repertoire of this important signaling domain.

A molecular basis for tetramer destabilization and aggregation of transthyretin Ala97Ser.

Transthyretin (TTR)-related amyloidosis (ATTR) is a syndrome of diseases characterized by the extracellular deposition of fibrillar materials containing TTR variants. Ala97Ser (A97S) is the major mutation reported in Taiwanese ATTR patients. Here, we combine atomic resolution structural information together with the biochemical data to demonstrate that substitution of polar Ser for a small hydrophobic side chain of Ala at residue 97 of TTR largely influences the local packing density of the FG-loop, thus leading to the conformational instability of native tetramer, the increased monomeric species, and thus the enhanced amyloidogenicity of apo-A97S. Based on calorimetric studies, the tetramer destabilization of A97S can be substantially altered by interacting with native stabilizers via similarly energetic patterns compared to that of wild-type (WT) TTR; however, stabilizer binding partially rearranges the networks of hydrogen bonding in TTR variants while FG-loops of tetrameric A97S still remain relatively flexible. Moreover, TTR in complexed with holo-retinol binding protein 4 is slightly influenced by the structural and dynamic changes of FG-loop caused by A97S substitution with an approximately five-fold difference in binding affinity. Collectively, our findings suggest that the amyloidogenic A97S mutation destabilizes TTR by increasing the flexibility of the FG-loop in the monomer, thus modulating the rate of amyloid fibrillization.

Clinical Characteristics, Genetic Features, and Long-Term Outcome of Wilson's Disease in a Taiwanese Population: An 11-Year Follow-Up Study.

OBJECTIVE: aaWilson's disease (WD) is a rare genetic disorder of copper metabolism, and longitudinal follow-up studies are limited. We performed a retrospective analysis to determine the clinical characteristics and long-term outcomes in a large WD cohort. METHODS: aaMedical records of WD patients diagnosed from 2006-2021 at National Taiwan University Hospital were retrospectively evaluated for clinical presentations, neuroimages, genetic information, and follow-up outcomes. RESULTS: aaThe present study enrolled 123 WD patients (mean follow-up: 11.12 ± 7.41 years), including 74 patients (60.2%) with hepatic features and 49 patients (39.8%) with predominantly neuropsychiatric symptoms. Compared to the hepatic group, the neuropsychiatric group exhibited more Kayser-Fleischer rings (77.6% vs. 41.9%, p < 0.01), lower serum ceruloplasmin levels (4.9 ± 3.9 vs. 6.3 ± 3.9 mg/dL, p < 0.01), smaller total brain and subcortical gray matter volumes (p < 0.0001), and worse functional outcomes during follow-up (p = 0.0003). Among patients with available DNA samples (n = 59), the most common mutations were p.R778L (allelic frequency of 22.03%) followed by p.P992L (11.86%) and p.T935M (9.32%). Patients with at least one allele of p.R778L had a younger onset age (p = 0.04), lower ceruloplasmin levels (p < 0.01), lower serum copper levels (p = 0.03), higher percentage of the hepatic form (p = 0.03), and a better functional outcome during follow-up (p = 0.0012) compared to patients with other genetic variations. CONCLUSION: aaThe distinct clinical characteristics and long-term outcomes of patients in our cohort support the ethnic differences regarding the mutational spectrum and clinical presentations in WD.

Subunit vaccines with a saponin-based adjuvant boost humoral and cellular immunity to MERS coronavirus.

Middle East respiratory syndrome coronavirus (MERS-CoV) outbreaks have constituted a public health issue with drastic mortality higher than 34%, necessitating the development of an effective vaccine. During MERS-CoV infection, the trimeric spike protein on the viral envelope is primarily responsible for attachment to host cellular receptor, dipeptidyl peptidase 4 (DPP4). With the goal of generating a protein-based prophylactic, we designed a subunit vaccine comprising the recombinant S1 protein with a trimerization motif (S1-Fd) and examined its immunogenicity and protective immune responses in combination with various adjuvants. We found that sera from immunized wild-type and human DPP4 transgenic mice contained S1-specific antibodies that can neutralize MERS-CoV infection in susceptible cells. Vaccination with S1-Fd protein in combination with a saponin-based QS-21 adjuvant provided long-term humoral as well as cellular immunity in mice. Our findings highlight the significance of the trimeric S1 protein in the development of MERS-CoV vaccines and offer a suitable adjuvant, QS-21, to induce robust and prolonged memory T cell response.

Structural basis for cAMP-mediated allosteric control of the catabolite activator protein.

The cAMP-mediated allosteric transition in the catabolite activator protein (CAP; also known as the cAMP receptor protein, CRP) is a textbook example of modulation of DNA-binding activity by small-molecule binding. Here we report the structure of CAP in the absence of cAMP, which, together with structures of CAP in the presence of cAMP, defines atomic details of the cAMP-mediated allosteric transition. The structural changes, and their relationship to cAMP binding and DNA binding, are remarkably clear and simple. Binding of cAMP results in a coil-to-helix transition that extends the coiled-coil dimerization interface of CAP by 3 turns of helix and concomitantly causes rotation, by approximately 60 degrees , and translation, by approximately 7 A, of the DNA-binding domains (DBDs) of CAP, positioning the recognition helices in the DBDs in the correct orientation to interact with DNA. The allosteric transition is stabilized further by expulsion of an aromatic residue from the cAMP-binding pocket upon cAMP binding. The results define the structural mechanisms that underlie allosteric control of this prototypic transcriptional regulatory factor and provide an illustrative example of how effector-mediated structural changes can control the activity of regulatory proteins.

Molecular insights into substrate recognition and discrimination by the N-terminal domain of Lon AAA+ protease.

The Lon AAA+ protease (LonA) is a ubiquitous ATP-dependent proteolytic machine, which selectively degrades damaged proteins or native proteins carrying exposed motifs (degrons). Here we characterize the structural basis for substrate recognition and discrimination by the N-terminal domain (NTD) of LonA. The results reveal that the six NTDs are attached to the hexameric LonA chamber by flexible linkers such that the formers tumble independently of the latter. Further spectral analyses show that the NTD selectively interacts with unfolded proteins, protein aggregates, and degron-tagged proteins by two hydrophobic patches of its N-lobe, but not intrinsically disordered substrate, α-casein. Moreover, the NTD selectively binds to protein substrates when they are thermally induced to adopt unfolded conformations. Collectively, our findings demonstrate that NTDs enable LonA to perform protein quality control to selectively degrade proteins in damaged states and suggest that substrate discrimination and selective degradation by LonA are mediated by multiple NTD interactions.

There are many different types of protein which each have different roles in biology. Most proteins are surrounded by water and are folded so that their water-attracting regions are on the outside and more fat-like regions, which repel water, are on the inside. When a protein becomes damaged or is assembled incorrectly, some of the fat-like regions end up on the outside of the protein and become exposed to water. This can prevent the protein from performing its role and harm the cell instead. LonA proteases are responsible for dismantling and recycling these harmful proteins, as well as proteins that have been labelled for destruction. They do this by unfolding the unwanted protein and transporting it into an enclosed chamber made of six LonA molecules. Once inside the chamber, the target protein is broken down into smaller fragments that can be used to build other structures. LonA proteases contain a region called the N-terminal domain, or NTD for short, which is thought to be responsible for identifying which proteins need degrading. Yet it remained unclear how the NTD recognizes and binds to these target proteins. To answer this question, Tzeng et al. studied the detailed structure of a LonA protease that had been purified from bacteria cells. This revealed that the NTD of LonA contains two water-repelling regions which bind to fat-like segments on the surface of proteins that have become unfolded or tagged for destruction. Further experiments showed that the NTD is bound to the main body of LonA via a 'flexible linker'. This led Tzeng et al. to propose that the NTD sways around loosely at the end of LonA searching for proteins with exposed water-repelling regions. Once an NTD identifies and attaches to a target, the NTDs of the other LonA molecules then bind to the protein and help insert it into the chamber. Proteases are a vital component of all biological systems. Controlling protein destruction and recycling is a key factor in how cells divide and respond to a changing environment. This study provides new insights into how LonA operates in bacteria, which may apply to proteases more widely. This contributes to our knowledge of fundamental biology and may also be relevant in a range of diseases where protein recycling is defective or inefficient.

Solution structure of the Ubp-M BUZ domain, a highly specific protein module that recognizes the C-terminal tail of free ubiquitin.

The BUZ/Znf-UBP domain is a distinct ubiquitin-binding module found in the cytoplasmic deacetylase HDAC6, the E3 ubiquitin ligase BRAP2/IMP, and a subfamily of deubiquitinating enzymes. Here, we report the solution structure of the BUZ domain of Ubp-M, a ubiquitin-specific protease, and its interaction with ubiquitin. Unlike the BUZ domain from isopeptidase T (isoT) that contains a single zinc finger, the Ubp-M BUZ domain features three zinc-binding sites consisting of 12 residues. These zinc ligands form a pair of cross-braced ring fingers encapsulated within a third zinc finger in the primary structure. In contrast to isoT, which can form an N-terminal loop swapped dimer in the crystal state, the formation of additional zinc fingers in the Ubp-M BUZ domain restricts its N-terminal loop to intra-domain interactions. The ubiquitin-binding site of the Ubp-M BUZ domain is mapped to the highly conserved, concave surface formed by the alpha 3 helix and the central beta-sheet. We further show that this site binds to the C-terminal tail of free ubiquitin, and corresponding peptides display essentially the same binding affinities as full-length ubiquitin does for the Ubp-M BUZ domain. However, modification of the G76(Ub) carboxylate group either by a peptide or isopeptide bond abolishes BUZ-domain interaction. The unique ubiquitin-recognition mode of the BUZ domain family suggests that they may function as "sensors" of free ubiquitin in cells to achieve regulatory roles in many aspects of ubiquitin-dependent processes.

Structural basis of adaptor-mediated protein degradation by the tail-specific PDZ-protease Prc.

Peptidoglycan (PG) is a highly cross-linked, protective mesh-like sacculus that surrounds the bacterial cytoplasmic membrane. Expansion of PG is tightly coupled to growth of a bacterial cell and requires hydrolases to cleave the cross-links for insertion of nascent PG material. In Escherichia coli, a proteolytic system comprising the periplasmic PDZ-protease Prc and the lipoprotein adaptor NlpI contributes to PG enlargement by regulating cellular levels of MepS, a cross-link-specific hydrolase. Here, we demonstrate how NlpI binds Prc to facilitate the degradation of its substrate MepS by structural and mutational analyses. An NlpI homodimer binds two molecules of Prc and forms three-sided MepS-docking cradles using its tetratricopeptide repeats. Prc forms a monomeric bowl-shaped structure with a lid-like PDZ domain connected by a substrate-sensing hinge that recognizes the bound C terminus of the substrate. In summary, our study reveals mechanistic details of protein degradation by the PDZ-protease Prc bound to its cognate adaptor protein.