Deleterious genetic changes in AGTPBP1 result in teratozoospermia with sperm head and flagella defects.

Approximately 10%-15% of couples worldwide are infertile, and male factors account for approximately half of these cases. Teratozoospermia is a major cause of male infertility. Although various mutations have been identified in teratozoospermia, these can vary among ethnic groups. In this study, we performed whole-exome sequencing to identify genetic changes potentially causative of teratozoospermia. Out of seven genes identified, one, ATP/GTP Binding Protein 1 (AGTPBP1), was characterized, and three missense changes were identified in two patients (Affected A: p.Glu423Asp and p.Pro631Leu; Affected B: p.Arg811His). In those two cases, severe sperm head and tail defects were observed. Moreover, AGTPBP1 localization showed a fragmented pattern compared to control participants, with specific localization in the neck and annulus regions. Using murine models, we found that AGTPBP1 is localized in the manchette structure, which is essential for sperm structure formation. Additionally, in Agtpbp1-null mice, we observed sperm head and tail defects similar to those in sperm from AGTPBP1-mutated cases, along with abnormal polyglutamylation tubulin and decreasing △-2 tubulin levels. In this study, we established a link between genetic changes in AGTPBP1 and human teratozoospermia for the first time and identified the role of AGTPBP1 in deglutamination, which is crucial for sperm formation.

CCP1, a Regulator of Tubulin Post-Translational Modifications, Potentially Plays an Essential Role in Cerebellar Development.

The cytosolic carboxypeptidase (CCP) 1 protein, encoded by CCP1, is expressed in cerebellar Purkinje cells (PCs). The dysfunction of CCP1 protein (caused by CCP1 point mutation) and the deletion of CCP1 protein (caused by CCP1 gene knockout) all lead to the degeneration of cerebellar PCs, which leads to cerebellar ataxia. Thus, two CCP1 mutants (i.e., Ataxia and Male Sterility [AMS] mice and Nna1 knockout [KO] mice) are used as disease models. We investigated the cerebellar CCP1 distribution in wild-type (WT), AMS and Nna1 KO mice on postnatal days (P) 7-28 to investigate the differential effects of CCP protein deficiency and disorder on cerebellar development. Immunohistochemical and immunofluorescence studies revealed significant differences in the cerebellar CCP1 expression in WT and mutant mice of P7 and P15, but no significant difference between AMS and Nna1 KO mice. Electron microscopy showed slight abnormality in the nuclear membrane structure of PCs in the AMS and Nna1 KO mice at P15 and significant abnormality with depolymerization and fragmentation of microtubule structure at P21. Using two CCP1 mutant mice strains, we revealed the morphological changes of PCs at postnatal stages and indicated that CCP1 played an important role in cerebellar development, most likely via polyglutamylation.

Nna1, Essential for Purkinje Cell Survival, Is also Associated with Emotion and Memory.

Nna1/CCP1 is generally known as a causative gene for a spontaneous autosomal recessive mouse mutation, Purkinje cell degeneration (pcd). There is enough evidence that the cytosolic function of the zinc carboxypeptidase (CP) domain at the C-terminus of the Nna1 protein is associated with cell death. On the other hand, this molecule's two nuclear localization signals (NLSs) suggest some other functions exist. We generated exon 3-deficient mice (Nna1N KO), which encode a portion of the N-terminal NLS. Despite the frameshift occurring in these mice, there was an expression of the Nna1 protein lacking the N-terminal side. Surprisingly, the pcd phenotype did not occur in the Nna1N KO mouse. Behavioral analysis revealed that they were less anxious when assessed by the elevated plus maze and the light/dark box tests compared to the control. Furthermore, they showed impairments in context-dependent and sound stimulus-dependent learning. Biochemical analysis of Nna1N KO mice revealed a reduced level of the AMPA-type glutamine receptor GluA2 in the hippocampal synaptosomal fraction. In addition, the motor protein kinesin-1, which transports GluA2 to dendrites, was also decreased. These results indicate that Nna1 is also involved in emotion and memory learning, presumably through the trafficking and expression of synaptic signaling molecules, besides a known role in cell survival.

Cell density-dependent antibiotic tolerance to inhibition of the elongation machinery requires fully functional PBP1B.

The peptidoglycan (PG) cell wall provides shape and structure to most bacteria. There are two systems to build PG in rod shaped organisms: the elongasome and divisome, which are made up of many proteins including the essential MreB and PBP2, or FtsZ and PBP3, respectively. The elongasome is responsible for PG insertion during cell elongation, while the divisome is responsible for septal PG insertion during division. We found that the main elongasome proteins, MreB and PBP2, can be inhibited without affecting growth rate in a quorum sensing-independent density-dependent manner. Before cells reach a particular cell density, inhibition of the elongasome results in different physiological responses, including intracellular vesicle formation and an increase in cell size. This inhibition of MreB or PBP2 can be compensated for by the presence of the class A penicillin binding protein, PBP1B. Furthermore, we found this density-dependent growth resistance to be specific for elongasome inhibition and was consistent across multiple Gram-negative rods, providing new areas of research into antibiotic treatment.

The AGTPBP1 gene in neurobiology.

The function of the Agtpbp1 gene has mainly been delineated by studying Agtpbp1pcd (pcd) mutant mice, characterized by losses in cerebellar Purkinje and granule cells along with degeneration of retinal photoreceptors, mitral cells of the olfactory bulb, thalamic neurons, and alpha-motoneurons. As a result of cerebellar degeneration, cerebellar GABA and glutamate concentrations in Agtpbp1pcd mutants decreased while monoamine concentrations increased. The salient behavioral phenotypes include cerebellar ataxia, a loss in motor coordination, and cognitive deficits. Similar neuropathogical and behavioral profiles have been described in childhood-onset human subjects with biallelic variants of AGTPBP1, including cerebellar ataxia and hypotonia.

Identification and validation of CALCRL-associated prognostic genes in acute myeloid leukemia.

In the past few decades, several advances have been made in the field of acute myeloid leukemia (AML), especially in the development of novel drugs. However, the overall survival rate remains particularly disappointing due to a high rate of chemotherapy resistance and relapse. The calcitonin receptor-like receptor (CALCRL) is a novel promising therapeutic target of AML and has been indicated to be strongly correlated with chemotherapy resistance and relapse driven by leukemic stem cells. Nevertheless, the CALCRL downstream genes associated with the drug resistance and relapse of AML remain to be elucidated. Within this study, we used multiple gene expression datasets from the Gene Expression Omnibus (GEO) database and cBioPortal to explore the candidate CALCRL-associated genes that could potentially mediate the chemoresistance and relapse of AML. Then, we investigated the prognostic value, coexpression relationship with CALCRL, and expression characteristics of these genes using independent data from The Cancer Genome Atlas (TCGA). Eventually, three genes were screened out as CALCRL-associated prognostic genes. The expression of AGTPBP1 and LYST was negatively correlated with CALCRL, high expression of which was associated with favorable prognosis in AML. In contrast, the expression of ETS2 was positively correlated with CALCRL, high expression of which was associated with poor prognosis in AML. The results indicated that the three prognostic genes are potential CALCRL downstream genes that mediate drug resistance and relapse in AML. This study helps to further explore the role and molecular pathways of CALCRL in mediating drug resistance and relapse of AML.

Mutations in PBP2 from ceftriaxone-resistant Neisseria gonorrhoeae alter the dynamics of the ß3-ß4 loop to favor a low-affinity drug-binding state.

Resistance to the extended-spectrum cephalosporin ceftriaxone in the pathogenic bacteria Neisseria gonorrhoeae is conferred by mutations in penicillin-binding protein 2 (PBP2), the lethal target of the antibiotic, but how these mutations exert their effect at the molecular level is unclear. Using solution NMR, X-ray crystallography, and isothermal titration calorimetry, we report that WT PBP2 exchanges dynamically between a low-affinity state with an extended ß3-ß4 loop conformation and a high-affinity state with an inward ß3-ß4 loop conformation. Histidine-514, which is located at the boundary of the ß4 strand, plays an important role during the exchange between these two conformational states. We also find that mutations present in PBP2 from H041, a ceftriaxone-resistant strain of N. gonorrhoeae, increase resistance to ceftriaxone by destabilizing the inward ß3-ß4 loop conformation or stabilizing the extended ß3-ß4 loop conformation to favor the low-affinity drug-binding state. These observations reveal a unique mechanism for ceftriaxone resistance, whereby mutations in PBP2 lower the proportion of target molecules in the high-affinity drug-binding state and thus reduce inhibition at lower drug concentrations.

Molecular characterization of decreased susceptibility to ceftriaxone and genotyping of Neisseria gonorrheae isolates in New Delhi, India.

Data on genetic characteristics of Neisseria gonorrhoeae isolates exhibiting decreased susceptibility to extended-spectrum cephalosporins in India is deficient. In this study, we have sequenced penA, porB, mtrR and ponA and blaTEM genes in 70 clinical isolates of NG with varying ceftriaxone MICs. Amongst these, 22 (31.4%) were PPNG. Additionally, N. gonorrheae Multiantigen Sequence Typing was performed. Fisher exact and χ2 were used to evaluate significance of mutations with MICs. A total of six non-mosaic penA (Penicillin binding protein 2 [PBP2]) amino acid patterns were seen (II, IV, IX, XII, XIX, XXII) of which, pattern IX was significantly associated with decreased susceptibility to ceftriaxone. Other significant associations were noted in porB & mtrR genes. There were no mutations in blaTEM gene. ST6069 was significantly associated with decreased susceptibility to ceftriaxone. To conclude, development of decreased susceptibility to ceftriaxone in gonococci involves cumulation of different mutations in the four chromosomal genes investigated.

A novel pathogenic variant in the 3' end of the AGTPBP1 gene gives rise to neurodegeneration without cerebellar atrophy: an expansion of the disease phenotype?

Childhood-onset neurodegeneration with cerebellar atrophy (CONDCA) is a recently described form of the large group of infantile hereditary lower motor neuron diseases (Teoh et al. 2017), resulting from biallelic damaging variants in the AGTPBP1 gene, first described by Shashi et al. in EMBO J 37(23):e100540, 2018. AGTPBP-related neurodegeneration is a severe neurodevelopmental disorder that progresses with global developmental delay and intellectual disability, often accompanied with peripheral nerve damage and lower motor degeneration and a fatal course in the early years of life. The encoded protein is ATP/GTP-Binding Protein1, also known as cytosolic carboxypeptidase 1 (CCP1) or nervous system nuclear protein induced by axotomy (NNA1). Here we report a consanguineous family with four offspring, two of whom are affected. The index patient is a 21-month-old male with global developmental delay and hypotonia. The proband's 17-year-old sister, diagnosed with cerebral palsy, had severe hypotonia accompanied by motor and cognitive retardation. WES analysis revealed a novel homozygous c.3293G > A variant in the AGTPBP1 gene with high pathogenicity scores. Targeted Sanger sequencing confirmed the variant in both affected children and in heterozygous form in the parents. The affected siblings present with hypotonia and motor and cognitive retardation, in line with the studies previously reported. However, in our patients, no signs of cerebellar atrophy in cranial MRI were present, so the acronym CONDCA is not applicable; lower motor neuron findings were also absent. The matching and distinguishing aspects of our patients will add to the present literature and expand our understanding of this rare genetic neurodegenerative disease of early childhood.

Real-time monitoring of peptidoglycan synthesis by membrane-reconstituted penicillin-binding proteins.

Peptidoglycan is an essential component of the bacterial cell envelope that surrounds the cytoplasmic membrane to protect the cell from osmotic lysis. Important antibiotics such as ß-lactams and glycopeptides target peptidoglycan biosynthesis. Class A penicillin-binding proteins (PBPs) are bifunctional membrane-bound peptidoglycan synthases that polymerize glycan chains and connect adjacent stem peptides by transpeptidation. How these enzymes work in their physiological membrane environment is poorly understood. Here, we developed a novel Förster resonance energy transfer-based assay to follow in real time both reactions of class A PBPs reconstituted in liposomes or supported lipid bilayers and applied this assay with PBP1B homologues from Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter baumannii in the presence or absence of their cognate lipoprotein activator. Our assay will allow unravelling the mechanisms of peptidoglycan synthesis in a lipid-bilayer environment and can be further developed to be used for high-throughput screening for new antimicrobials.

Loss of cell wall integrity genes cpxA and mrcB causes flocculation in Escherichia coli.

Flocculation has been recognized for hundreds of years as an important phenomenon in brewing and wastewater treatment. However, the underlying molecular mechanisms remain elusive. The lack of a distinct phenotype to differentiate between slow-growing mutants and floc-forming mutants prevents the isolation of floc-related gene by conventional mutant screening. To overcome this, we performed a two-step Escherichia coli mutant screen. The initial screen of E. coli for mutants conferring floc production during high salt treatment yielded a mutant containing point mutations in 61 genes. The following screen of the corresponding single-gene mutants identified two genes, mrcB, encoding a peptidoglycan-synthesizing enzyme and cpxA, encoding a histidine kinase of a two-component signal transduction system that contributed to salt tolerance and flocculation prevention. Both single mutants formed flocs during high salt shock, these flocs contained cytosolic proteins. ΔcpxA exhibited decreased growth with increasing floc production and addition of magnesium to ΔcpxA suppressed floc production effectively. In contrast, the growth of ΔmrcB was inconsistent under high salt conditions. In both strains, flocculation was accompanied by the release of membrane vesicles containing inner and outer membrane proteins. Of 25 histidine kinase mutants tested, ΔcpxA produced the highest amount of proteins in floc. Expression of cpxP was up-regulated by high salt in ΔcpxA, suggesting that high salinity and activation of CpxR might promote floc formation. The finding that ΔmrcB or ΔcpxA conferred floc production indicates that cell envelope stress triggered by unfavorable environmental conditions cause the initiation of flocculation in E. coli.

Identification of 2-PMPA as a novel inhibitor of cytosolic carboxypeptidases.

Cytosolic carboxypeptidases (CCPs) comprise a unique subfamily of M14 carboxypeptidases and are erasers of the reversible protein posttranslational modification- polyglutamylation. Potent inhibitors for CCPs may serve as leading compounds targeting imbalanced polyglutamylation. However, no efficient CCP inhibitor has yet been reported. Here, we showed that 2-phosphonomethylpentanedioic acid (2-PMPA), a potent inhibitor of the distant M28 family member glutamate carboxypeptidase II (GCPII), rather than the typical M14 inhibitor 2-benzylsuccinic acid, could efficiently inhibit CCP activities. 2-PMPA inhibited the recombinant Nna1 (a.k.a. CCP1) for hydrolyzing a synthetic peptide in a mixed manner, with Ki and Ki' being 0.11 µM and 0.24 µM respectively. It inhibited Nna1 for deglutamylating tubulin, the best-known polyglutamylated protein, with an IC50 of 0.21 mM. Homology modeling predicted that the R-form of 2-PMPA is more favorable to bind Nna1, unlike that GCPII prefers to S-form. This work for the first time identified a potent inhibitor for CCP family.

The bacterial cell division protein fragment EFtsN binds to and activates the major peptidoglycan synthase PBP1b.

Peptidoglycan (PG) is an essential constituent of the bacterial cell wall. During cell division, the machinery responsible for PG synthesis localizes mid-cell, at the septum, under the control of a multiprotein complex called the divisome. In Escherichia coli, septal PG synthesis and cell constriction rely on the accumulation of FtsN at the division site. Interestingly, a short sequence of FtsN (Leu75-Gln93, known as EFtsN) was shown to be essential and sufficient for its functioning in vivo, but what exactly this sequence is doing remained unknown. Here, we show that EFtsN binds specifically to the major PG synthase PBP1b and is sufficient to stimulate its biosynthetic glycosyltransferase (GTase) activity. We also report the crystal structure of PBP1b in complex with EFtsN, which demonstrates that EFtsN binds at the junction between the GTase and UB2H domains of PBP1b. Interestingly, mutations to two residues (R141A/R397A) within the EFtsN-binding pocket reduced the activation of PBP1b by FtsN but not by the lipoprotein LpoB. This mutant was unable to rescue the ΔponB-ponAts strain, which lacks PBP1b and has a thermosensitive PBP1a, at nonpermissive temperature and induced a mild cell-chaining phenotype and cell lysis. Altogether, the results show that EFtsN interacts with PBP1b and that this interaction plays a role in the activation of its GTase activity by FtsN, which may contribute to the overall septal PG synthesis and regulation during cell division.

Mutations in penicillin-binding protein 2 from cephalosporin-resistant Neisseria gonorrhoeae hinder ceftriaxone acylation by restricting protein dynamics.

The global incidence of the sexually transmitted disease gonorrhea is expected to rise due to the spread of Neisseria gonorrhoeae strains with decreased susceptibility to extended-spectrum cephalosporins (ESCs). ESC resistance is conferred by mosaic variants of penicillin-binding protein 2 (PBP2) that have diminished capacity to form acylated adducts with cephalosporins. To elucidate the molecular mechanisms of ESC resistance, we conducted a biochemical and high-resolution structural analysis of PBP2 variants derived from the decreased-susceptibility N. gonorrhoeae strain 35/02 and ESC-resistant strain H041. Our data reveal that mutations both lower affinity of PBP2 for ceftriaxone and restrict conformational changes that normally accompany acylation. Specifically, we observe that a G545S substitution hinders rotation of the ß3 strand necessary to form the oxyanion hole for acylation and also traps ceftriaxone in a noncanonical configuration. In addition, F504L and N512Y substitutions appear to prevent bending of the ß3-ß4 loop that is required to contact the R1 group of ceftriaxone in the active site. Other mutations also appear to act by reducing flexibility in the protein. Overall, our findings reveal that restriction of protein dynamics in PBP2 underpins the ESC resistance of N. gonorrhoeae.

Involvement of penicillin-binding proteins in the metabolism of a bacterial peptidoglycan containing a non-canonical D-amino acid.

Bacteria produce various D-amino acids, including non-canonical D-amino acids, to adapt to environmental changes and overcome a variety of threats. These D-amino acids are largely utilized as components of peptidoglycan, and they promote peptidoglycan remodeling and biofilm disassembly. The biosynthesis, maturation, and recycling of peptidoglycan are catalyzed by penicillin-binding proteins (PBPs). However, although non-canonical D-amino acids are known to be incorporated into peptidoglycan, the maturation and recycling of peptidoglycan containing such residues remain uncharacterized. Therefore, we investigated whether PBP4 and PBP5, low molecular mass (LMM) PBPs from Escherichia coli and Bacillus subtilis, are involved in these events of peptidoglycan metabolism. Enzyme assays using p-nitroaniline (pNA)-derivatized D-amino acids and peptidoglycan-mimicking peptides revealed that PBP4 and PBP5 from both species have peptidase activity toward substrates containing D-Asn, D-His, or D-Trp. These D-amino acids slowed the growth of dacA- or dacB-deficient E. coli (∆dacA or ∆dacB) relative to the wild-type strain. Additionally, these D-amino acids affected biofilm formation by the ∆dacB strain. Collectively, PBP4 and PBP5 are involved in the cleavage of peptidoglycan containing non-canonical D-amino acids, and these properties affect growth and biofilm formation.

Genotyping Neisseria gonorrhoeae gyrA and penA antimicrobial genes from remnant Neisseria gonorrhoeae positive Cepheid Xpert® clinical specimens - A feasibility study.

Neisseria gonorrhoeae (NG) has developed resistance to most antibiotics, making it increasingly difficult to treat. Previous studies have predicted antimicrobial NG susceptibility based on the antimicrobial gene target DNA gyrase subunit A (gyrA) codon serine 91 and the penicillin-binding protein 2 (penA) using Roche Cobas® and Hologic APTIMA™ clinical specimens. We studied whether similar methods could be used on remnant NG-positive Cepheid Xpert® specimens.

The D-alanyl-d-alanine carboxypeptidase enzyme is essential for virulence in the Schu S4 strain of Francisella tularensis and a dacD mutant is able to provide protection against a pneumonic challenge.

Low molecular mass penicillin binding proteins (LMM PBP) are bacterial enzymes involved in the final steps of peptidoglycan biosynthesis. In Escherichia coli, most LMM PBP exhibit dd-carboxypeptidase activity, are not essential for growth in routine laboratory media, and contributions to virulent phenotypes remain largely unknown. The Francisella tularensis Schu S4 genome harbors the dacD gene (FTT_1029), which encodes a LMM PBP with homology to PBP6b of E. coli. Disruption of this locus in the fully virulent Schu S4 strain resulted in a mutant that could not grow in Chamberlain's Defined Medium and exhibited severe morphological defects. Further characterization studies demonstrated that the growth defects of the dacD mutant were pH-dependent, and could be partially restored by growth at neutral pH or fully restored by genetic complementation. Infection of murine macrophage-like cells showed that the Schu S4 dacD mutant is capable of intracellular replication. However, this mutant was attenuated in BALB/c mice following intranasal challenge (LD50 = 603 CFU) as compared to mice challenged with the parent (LD50 = 1 CFU) or complemented strain (LD50 = 1 CFU). Additionally, mice that survived infection with the dacD mutant showed significant protection against subsequent challenge with the parent strain. Collectively, these results indicate that the DacD protein of F. tularensis is essential for growth in low pH environments and virulence in vivo. These results also suggest that a PBP mutant could serve as the basis of a novel, live attenuated vaccine strain.

Recognition of the ß-lactam carboxylate triggers acylation of Neisseria gonorrhoeae penicillin-binding protein 2.

Resistance of Neisseria gonorrhoeae to extended-spectrum cephalosporins (ESCs) has become a major threat to human health. The primary mechanism by which N. gonorrhoeae becomes resistant to ESCs is by acquiring a mosaic penA allele, encoding penicillin-binding protein 2 (PBP2) variants containing up to 62 mutations compared with WT, of which a subset contribute to resistance. To interpret molecular mechanisms underpinning cephalosporin resistance, it is necessary to know how PBP2 is acylated by ESCs. Here, we report the crystal structures of the transpeptidase domain of WT PBP2 in complex with cefixime and ceftriaxone, along with structures of PBP2 in the apo form and with a phosphate ion bound in the active site at resolutions of 1-7-1.9 Å. These structures reveal that acylation of PBP2 by ESCs is accompanied by rotation of the Thr-498 side chain in the KTG motif to contact the cephalosporin carboxylate, twisting of the ß3 strand to form the oxyanion hole, and rolling of the ß3-ß4 loop toward the active site. Recognition of the cephalosporin carboxylate appears to be the key trigger for formation of an acylation-competent state of PBP2. The structures also begin to explain the impact of mutations implicated in ESC resistance. In particular, a G545S mutation may hinder twisting of ß3 because its side chain hydroxyl forms a hydrogen bond with Thr-498. Overall, our data suggest that acylation is initiated by conformational changes elicited or trapped by binding of ESCs and that these movements are restricted by mutations associated with resistance against ESCs.

Upregulation of PBP1B and LpoB in cysB Mutants Confers Mecillinam (Amdinocillin) Resistance in Escherichia coli.

Mecillinam (amdinocillin) is a ß-lactam antibiotic that inhibits the essential penicillin-binding protein 2 (PBP2). In clinical isolates of Escherichia coli from urinary tract infections, inactivation of the cysB gene (which encodes the main regulator of cysteine biosynthesis, CysB) is the major cause of resistance. How a nonfunctional CysB protein confers resistance is unknown, however, and in this study we wanted to examine the mechanism of resistance. Results show that cysB mutations cause a gene regulatory response that changes the expression of ∼450 genes. Among the proteins that show increased levels are the PBP1B, LpoB, and FtsZ proteins, which are known to be involved in peptidoglycan biosynthesis. Artificial overexpression of either PBP1B or LpoB in a wild-type E. coli strain conferred mecillinam resistance; conversely, inactivation of either the mrcB gene (which encodes PBP1B) or the lpoB gene (which encodes the PBP1B activator LpoB) made cysB mutants susceptible. These results show that expression of the proteins PBP1B and LpoB is both necessary and sufficient to confer mecillinam resistance. The addition of reducing agents to a cysB mutant converted it to full susceptibility, with associated downregulation of PBP1B, LpoB, and FtsZ. We propose a model in which cysB mutants confer mecillinam resistance by inducing a response that causes upregulation of the PBP1B and LpoB proteins. The higher levels of these two proteins can then rescue cells with mecillinam-inhibited PBP2. Our results also show how resistance can be modulated by external conditions such as reducing agents.

Novel and Improved Crystal Structures of H. influenzae, E. coli and P. aeruginosa Penicillin-Binding Protein 3 (PBP3) and N. gonorrhoeae PBP2: Toward a Better Understanding of ß-Lactam Target-Mediated Resistance.

Even with the emergence of antibiotic resistance, penicillin and the wider family of ß-lactams have remained the single most important family of antibiotics. The periplasmic/extra-cytoplasmic targets of penicillin are a family of enzymes with a highly conserved catalytic activity involved in the final stage of bacterial cell wall (peptidoglycan) biosynthesis. Named after their ability to bind penicillin, rather than their catalytic activity, these key targets are called penicillin-binding proteins (PBPs). Resistance is predominantly mediated by reducing the target drug concentration via ß-lactamases; however, naturally transformable bacteria have also acquired target-mediated resistance by inter-species recombination. Here we focus on structural based interpretations of amino acid alterations associated with the emergence of resistance within clinical isolates and include new PBP3 structures along with new, and improved, PBP-ß-lactam co-structures.