Protein misfolding and clearance in the pathogenesis of a new infantile onset ataxia caused by mutations in PRDX3.

Peroxiredoxin 3 (PRDX3) encodes a mitochondrial antioxidant protein, which is essential for the control of reactive oxygen species homeostasis. So far, PRDX3 mutations are involved in mild-to-moderate progressive juvenile onset cerebellar ataxia. We aimed to unravel the molecular bases underlying the disease in an infant suffering from cerebellar ataxia that started at 19 months old and presented severe cerebellar atrophy and peripheral neuropathy early in the course of disease. By whole exome sequencing, we identified a novel homozygous mutation, PRDX3 p.D163E, which impaired the mitochondrial ROS defense system. In mouse primary cortical neurons, the exogenous expression of PRDX3 p.D163E was reduced and triggered alterations in neurite morphology and in mitochondria. Mitochondrial computational parameters showed that p.D163E led to serious mitochondrial alterations. In transfected HeLa cells expressing the mutation, mitochondria accumulation was detected by correlative light electron microscopy. Mitochondrial morphology showed severe changes, including extremely damaged outer and inner membranes with a notable cristae disorganization. Moreover, spherical structures compatible with lipid droplets were identified, which can be associated with a generalized response to stress and can be involved in the removal of unfolded proteins. In the patient's fibroblasts, PRDX3 expression was nearly absent. The biochemical analysis suggested that the mutation p.D163E would result in an unstable structure tending to form aggregates that trigger unfolded protein responses via mitochondria and endoplasmic reticulum. Altogether, our findings broaden the clinical spectrum of the recently described PRDX3-associated neurodegeneration and provide new insight into the pathological mechanisms underlying this new form of cerebellar ataxia.

Correction: The structural role of SARS-CoV-2 genetic background in the emergence and success of spike mutations: The case of the spike A222V mutation.

[This corrects the article DOI: 10.1371/journal.ppat.1010631.].

The structural role of SARS-CoV-2 genetic background in the emergence and success of spike mutations: The case of the spike A222V mutation.

The S:A222V point mutation, within the G clade, was characteristic of the 20E (EU1) SARS-CoV-2 variant identified in Spain in early summer 2020. This mutation has since reappeared in the Delta subvariant AY.4.2, raising questions about its specific effect on viral infection. We report combined serological, functional, structural and computational studies characterizing the impact of this mutation. Our results reveal that S:A222V promotes an increased RBD opening and slightly increases ACE2 binding as compared to the parent S:D614G clade. Finally, S:A222V does not reduce sera neutralization capacity, suggesting it does not affect vaccine effectiveness.

Use of pure recombinant human enzymes to assess the disease-causing potential of missense mutations in urea cycle disorders, applied to N-acetylglutamate synthase deficiency.

N-acetylglutamate synthase (NAGS) makes acetylglutamate, the essential activator of the first, regulatory enzyme of the urea cycle, carbamoyl phosphate synthetase 1 (CPS1). NAGS deficiency (NAGSD) and CPS1 deficiency (CPS1D) present identical phenotypes. However, they must be distinguished, because NAGSD is cured by substitutive therapy with the N-acetyl-L-glutamate analogue N-carbamyl-L-glutamate, while curative therapy of CPS1D requires liver transplantation. Since their differentiation is done genetically, it is important to ascertain the disease-causing potential of CPS1 and NAGS genetic variants. With this goal, we previously carried out site-directed mutagenesis studies with pure recombinant human CPS1. We could not do the same with human NAGS (HuNAGS) because of enzyme instability, leading to our prior utilization of a bacterial NAGS as an imperfect surrogate of HuNAGS. We now use genuine HuNAGS, stabilized as a chimera of its conserved domain (cHuNAGS) with the maltose binding protein (MBP), and produced in Escherichia coli. MBP-cHuNAGS linker cleavage allowed assessment of the enzymatic properties and thermal stability of cHuNAGS, either wild-type or hosting each one of 23 nonsynonymous single-base changes found in NAGSD patients. For all but one change, disease causation was accounted by the enzymatic alterations identified, including, depending on the variant, loss of arginine activation, increased Km Glutamate, active site inactivation, decreased thermal stability, and protein misfolding. Our present approach outperforms experimental in vitro use of bacterial NAGS or in silico utilization of prediction servers (including AlphaMissense), illustrating with HuNAGS the value for UCDs of using recombinant enzymes for assessing disease-causation and molecular pathogenesis, and for therapeutic guidance.

Genetic Heterogeneity Underlying Phenotypes with Early-Onset Cerebellar Atrophy.

Cerebellar atrophy (CA) is a frequent neuroimaging finding in paediatric neurology, usually associated with cerebellar ataxia. The list of genes involved in hereditary forms of CA is continuously growing and reveals its genetic complexity. We investigated ten cases with early-onset cerebellar involvement with and without ataxia by exome sequencing or by a targeted panel with 363 genes involved in ataxia or spastic paraplegia. Novel variants were investigated by in silico or experimental approaches. Seven probands carry causative variants in well-known genes associated with CA or cerebellar hypoplasia: SETX, CACNA1G, CACNA1A, CLN6, CPLANE1, and TBCD. The remaining three cases deserve special attention; they harbour variants in MAST1, PI4KA and CLK2 genes. MAST1 is responsible for an ultrarare condition characterised by global developmental delay and cognitive decline; our index case added ataxia to the list of concomitant associated symptoms. PIK4A is mainly related to hypomyelinating leukodystrophy; our proband presented with pure spastic paraplegia and normal intellectual capacity. Finally, in a patient who suffers from mild ataxia with oculomotor apraxia, the de novo novel CLK2 c.1120T>C variant was found. The protein expression of the mutated protein was reduced, which may indicate instability that would affect its kinase activity.

A novel TRMT5 mutation causes a complex inherited neuropathy syndrome: The role of nerve pathology in defining a demyelinating neuropathy.

AIMS: We aim to present data obtained from three patients belonging to three unrelated families with an infantile onset demyelinating neuropathy associated to somatic and neurodevelopmental delay and to describe the underlying genetic changes. METHODS: We performed whole-exome sequencing on genomic DNA from the patients and their parents and reviewed the clinical, muscle and nerve data, the serial neurophysiological studies, brain and muscle MRIs, as well as the respiratory chain complex activity in the muscle of the three index patients. Computer modelling was used to characterise the new missense variant detected. RESULTS: All three patients had a short stature, delayed motor milestone acquisition, intellectual disability and cerebellar abnormalities associated with a severe demyelinating neuropathy, with distinct morphological features. Despite the proliferation of giant mitochondria, the mitochondrial respiratory chain complex activity in skeletal muscle was normal, except in one patient in whom there was a mild decrease in complex I enzyme activity. All three patients carried the same two compound heterozygous variants of the TRMT5 (tRNA Methyltransferase 5) gene, one known pathogenic frameshift mutation [c.312_315del (p.Ile105Serfs*4)] and a second rare missense change [c.665 T > C (p.Ile222Thr)]. TRMT5 is a nuclear-encoded protein involved in the post-transcriptional maturation of mitochondrial tRNA. Computer modelling of the human TRMT5 protein structure suggests that the rare p.Ile222Thr mutation could affect the stability of tRNA binding. CONCLUSIONS: Our study expands the phenotype of mitochondrial disorders caused by TRTM5 mutations and defines a new form of recessive demyelinating peripheral neuropathy.

Mutations, Genes, and Phenotypes Related to Movement Disorders and Ataxias.

Our clinical series comprises 124 patients with movement disorders (MDs) and/or ataxia with cerebellar atrophy (CA), many of them showing signs of neurodegeneration with brain iron accumulation (NBIA). Ten NBIA genes are accepted, although isolated cases compatible with abnormal brain iron deposits are known. The patients were evaluated using standardised clinical assessments of ataxia and MDs. First, NBIA genes were analysed by Sanger sequencing and 59 patients achieved a diagnosis, including the detection of the founder mutation PANK2 p.T528M in Romani people. Then, we used a custom panel MovDisord and/or exome sequencing; 29 cases were solved with a great genetic heterogeneity (34 different mutations in 23 genes). Three patients presented brain iron deposits with Fe-sensitive MRI sequences and mutations in FBXO7, GLB1, and KIF1A, suggesting an NBIA-like phenotype. Eleven patients showed very early-onset ataxia and CA with cortical hyperintensities caused by mutations in ITPR1, KIF1A, SPTBN2, PLA2G6, PMPCA, and PRDX3. The novel variants were investigated by structural modelling, luciferase analysis, transcript/minigenes studies, or immunofluorescence assays. Our findings expand the phenotypes and the genetics of MDs and ataxias with early-onset CA and cortical hyperintensities and highlight that the abnormal brain iron accumulation or early cerebellar gliosis may resembling an NBIA phenotype.

Δ1 -Pyrroline-5-carboxylate synthetase deficiency: An emergent multifaceted urea cycle-related disorder.

The bifunctional homooligomeric enzyme Δ1 -pyrroline-5-carboxylate synthetase (P5CS) and its encoding gene ALDH18A1 were associated with disease in 1998. Two siblings who presented paradoxical hyperammonemia (alleviated by protein), mental disability, short stature, cataracts, cutis laxa, and joint laxity, were found to carry biallelic ALDH18A1 mutations. They showed biochemical indications of decreased ornithine/proline synthesis, agreeing with the role of P5CS in the biosynthesis of these amino acids. Of 32 patients reported with this neurocutaneous syndrome, 21 familial ones hosted homozygous or compound heterozygous ALDH18A1 mutations, while 11 sporadic ones carried de novo heterozygous ALDH18A1 mutations. In 2015 to 2016, an upper motor neuron syndrome (spastic paraparesis/paraplegia SPG9) complicated with some traits of the neurocutaneous syndrome, although without report of cutis laxa, joint laxity, or herniae, was associated with monoallelic or biallelic ALDH18A1 mutations with, respectively, dominant and recessive inheritance. Of 50 SPG9 patients reported, 14 and 36 (34/2 familial/sporadic) carried, respectively, biallelic and monoallelic mutations. Thus, two neurocutaneous syndromes (recessive and dominant cutis laxa 3, abbreviated ARCL3A and ADCL3, respectively) and two SPG9 syndromes (recessive SPG9B and dominant SPG9A) are caused by essentially different spectra of ALDH18A1 mutations. On the bases of the clinical data (including our own prior patients' reports), the ALDH18A1 mutations spectra, and our knowledge on the P5CS protein, we conclude that the four syndromes share the same pathogenic mechanisms based on decreased P5CS function. Thus, these syndromes represent a continuum of increasing severity (SPG9A < SPG9B < ADCL3 ≤ ARCL3A) of the same disease, P5CS deficiency, in which the dominant mutations cause loss-of-function by dominant-negative mechanisms.

A newly distal hereditary motor neuropathy caused by a rare AIFM1 mutation.

In two siblings, who suffer from an early childhood-onset axonal polyneuropathy with exclusive involvement of motor fibers, the c.629T>C (p.F210S) mutation was identified in the X-linked AIFM1 gene, which encodes for the apoptosis-inducing factor (AIF). The mutation was predicted as deleterious, according to in silico analysis. A decreased expression of the AIF protein, altered cellular morphology, and a fragmented mitochondrial network were observed in the proband's fibroblasts. This new form of motor neuropathy expands the phenotypic spectrum of AIFM1 mutations and therefore, the AIFM1 gene should be considered in the diagnosis of hereditary motor neuropathies.

Understanding N-Acetyl-L-Glutamate Synthase Deficiency: Mutational Spectrum, Impact of Clinical Mutations on Enzyme Functionality, and Structural Considerations.

N-acetyl-L-glutamate synthase (NAGS) deficiency (NAGSD), the rarest urea cycle defect, is clinically indistinguishable from carbamoyl phosphate synthetase 1 deficiency, rendering the identification of NAGS gene mutations key for differentiation, which is crucial, as only NAGSD has substitutive therapy. Over the last 13 years, we have identified 43 patients from 33 families with NAGS mutations, of which 14 were novel. Overall, 36 NAGS mutations have been found so far in 56 patients from 42 families, of which 76% are homozygous for the mutant allele. 61% of mutations are missense changes. Lack or decrease of NAGS protein is predicted for ∼1/3 of mutations. Missense mutations frequency is inhomogeneous along NAGS: null for exon 1, but six in exon 6, which reflects the paramount substrate binding/catalytic role of the C-terminal domain (GNAT domain). Correspondingly, phenotypes associated with missense mutations mapping in the GNAT domain are more severe than phenotypes of amino acid kinase domain-mapping missense mutations. Enzyme activity and stability assays with 12 mutations introduced into pure recombinant Pseudomonas aeruginosa NAGS, together with in silico structural analysis, support the pathogenic role of most NAGSD-associated mutations found. The disease-causing mechanisms appear to be, from higher to lower frequency, decreased solubility/stability, aberrant kinetics/catalysis, and altered arginine modulation.

C-2 Thiophenyl Tryptophan Trimers Inhibit Cellular Entry of SARS-CoV-2 through Interaction with the Viral Spike (S) Protein.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes COVID-19, by infecting cells via the interaction of its spike protein (S) with the primary cell receptor angiotensin-converting enzyme (ACE2). To search for inhibitors of this key step in viral infection, we screened an in-house library of multivalent tryptophan derivatives. Using VSV-S pseudoparticles, we identified compound 2 as a potent entry inhibitor lacking cellular toxicity. Chemical optimization of 2 rendered compounds 63 and 65, which also potently inhibited genuine SARS-CoV-2 cell entry. Thermofluor and microscale thermophoresis studies revealed their binding to S and to its isolated receptor binding domain (RBD), interfering with the interaction with ACE2. High-resolution cryoelectron microscopy structure of S, free or bound to 2, shed light on cell entry inhibition mechanisms by these compounds. Overall, this work identifies and characterizes a new class of SARS-CoV-2 entry inhibitors with clear potential for preventing and/or fighting COVID-19.

Understanding pyrroline-5-carboxylate synthetase deficiency: clinical, molecular, functional, and expression studies, structure-based analysis, and novel therapy with arginine.

Δ(1)-Pyrroline-5-carboxylate synthetase (P5CS) catalyzes the first two steps of ornithine/proline biosynthesis. P5CS deficiency has been reported in three families, with patients presenting with cutis/joint laxity, cataracts, and neurodevelopmental delay. Only one family exhibited metabolic changes consistent with P5CS deficiency (low proline/ornithine/citrulline/arginine; fasting hyperammonemia). Here we report a new P5CS-deficient patient presenting the complete clinical/metabolic phenotype and carrying p.G93R and p.T299I substitutions in the γ-glutamyl kinase (γGK) component of P5CS. The effects of these substitutions are (1) tested in mutagenesis/functional studies with E.coli γGK, (2) rationalized by structural modelling, and (3) reflected in decreased P5CS protein in patient fibroblasts (shown by immunofluorescence). Using optical/electron microscopy on skin biopsy, we show collagen/elastin fiber alterations that may contribute to connective tissue laxity and are compatible with our angio-MRI finding of kinky brain vessels in the patient. MR spectroscopy revealed decreased brain creatine, which normalized after sustained arginine supplementation, with improvement of neurodevelopmental and metabolic parameters, suggesting a pathogenic role of brain creatine decrease and the value of arginine therapy. Morphological and functional studies of fibroblast mitochondria show that P5CS deficiency is not associated with the mitochondrial alterations observed in Δ(1)-pyrroline-5-carboxylate reductase deficiency (another proline biosynthesis defect presenting cutis laxa and neurological alterations).

Discovery of 3H-pyrrolo[2,3-c]quinolines with activity against Mycobacterium tuberculosis by allosteric inhibition of the glutamate-5-kinase enzyme.

The therapeutic potential of 3H-pyrrolo[2,3-c]quinolines-the main core of Marinoquinoline natural products-has been explored for the development of new anti-TB agents. The chemical modification of various positions in this scaffold has led to the discovery of two pyrroloquinolines (compounds 50 and 54) with good in vitro activity against virulent strains of Mycobacterium tuberculosis (H37Rv, MIC = 4.1 µM and 4.2 µM, respectively). Enzymatic assays showed that both derivatives are inhibitors of glutamate-5-kinase (G5K, encoded by proB gene), an essential enzyme for this pathogen involved in the first step of the proline biosynthesis pathway. G5K catalyzes the phosphoryl-transference of the γ-phosphate group of ATP to L-glutamate to provide L-glutamyl-5-phosphate and ADP, and also regulates the synthesis of L-proline. The results of various molecular dynamics simulation studies revealed that the inhibition of G5K would be caused by allosteric interaction of these compounds with the interface between enzyme domains, against different pockets and with distinct recognition patterns. The binding of compound 54 promotes long-distance conformational changes at the L-glutamate binding site that would prevent it from anchoring for catalysis, while compound 50 alters the ATP binding site architecture for recognition. Enzyme assays revealed that compound 50 caused a substancial increase in the Kmapp for ATP, while no significant effect was observed for derivative 54. This work also demonstrates the potential of the G5K enzyme as a biological target for the development of new anti-TB drugs.

Congenital hypomyelinating neuropathy due to a novel MPZ mutation.

Congenital hypomyelinating neuropathy (CHN) is a severe inherited neuropathy with neonatal or early infancy onset, reduced nerve conduction velocity, and pathological evidence of hypomyelination. We describe a case of CHN that presented with neonatal hypotonia and a progressive downhill clinical course, developing cranial nerve dysfunction, and respiratory failure. The nerve conduction velocities were severely slowed and sural nerve biopsy revealed non-myelinated and poorly myelinated axons, with no typical onion bulbs. The mutational screening showed that our proband harbored a novel missense mutation, p.S121F, in the MPZ gene. In silico analyses and molecular modeling predicted that the replacement of a serine by a phenylalanine is a non-tolerated change and may affect the folding and the stability of the protein. Subcellular location studies were performed and revealed that the mutant protein loses its correct location on the cell membrane surface and is mainly expressed in the cytosol, reducing its adhesive properties. This case illustrates the clinical heterogeneity that exists in neuropathies associated with MPZ mutations and highlights that in patients with mild hypotonia in the first months that develop a very severe demyelinating neuropathy, the MPZ gene must be taken into account.

Functional and structural characterization of PII-like protein CutA does not support involvement in heavy metal tolerance and hints at a small-molecule carrying/signaling role.

The PII-like protein CutA is annotated as being involved in Cu2+ tolerance, based on analysis of Escherichia coli mutants. However, the precise cellular function of CutA remains unclear. Our bioinformatic analysis reveals that CutA proteins are universally distributed across all domains of life. Based on sequence-based clustering, we chose representative cyanobacterial CutA proteins for physiological, biochemical, and structural characterization and examined their involvement in heavy metal tolerance, by generating CutA mutants in filamentous Nostoc sp. and in unicellular Synechococcus elongatus. However, we were unable to find any involvement of cyanobacterial CutA in metal tolerance under various conditions. This prompted us to re-examine experimentally the role of CutA in protecting E. coli from Cu2+ . Since we found no effect on copper tolerance, we conclude that CutA plays a different role that is not involved in metal protection. We resolved high-resolution CutA structures from Nostoc and S. elongatus. Similarly to their counterpart from E. coli and to canonical PII proteins, cyanobacterial CutA proteins are trimeric in solution and in crystal structure; however, no binding affinity for small signaling molecules or for Cu2+ could be detected. The clefts between the CutA subunits, corresponding to the binding pockets of PII proteins, are formed by conserved aromatic and charged residues, suggesting a conserved binding/signaling function for CutA. In fact, we find binding of organic Bis-Tris/MES molecules in CutA crystal structures, revealing a strong tendency of these pockets to accommodate cargo. This highlights the need to search for the potential physiological ligands and for their signaling functions upon binding to CutA. DATABASES: Structural data are available in Protein Data Bank (PDB) under the accession numbers 6GDU, 6GDV, 6GDW, 6GDX, 6T76, and 6T7E.

Nitrogen storage regulation by PII protein: lessons learned from taxonomic outliers.

The paper 'Interaction of N-acetyl-l-glutamate kinase with the PII signal transducer in the non-photosynthetic alga Polytomella parva: Co-evolution towards a hetero-oligomeric enzyme' by Selim et al. highlights how the study of a true taxonomic oddity, the heterotrophic unicellular alga P. parva, has been instrumental in uncovering the large potential for adaptive variation in the signaling complex of PII with the enzyme N-acetylglutamate kinase (NAGK). This complex modifies the regulatory properties of NAGK, allowing nitrogen stockpiling as arginine. In P. parva, a stable PII-NAGK complex is formed which lacks regulation by canonical PII effectors but which exhibits novel adaptive responses to nitrogen abundance mediated by glutamine, a neo-effector of PII proteins of photosynthetic eukaryotes.

P5CS expression study in a new family with ALDH18A1-associated hereditary spastic paraplegia SPG9.

In 2015-2016, we and others reported ALDH18A1 mutations causing dominant (SPG9A) or recessive (SPG9B) spastic paraplegia. In vitro production of the ALDH18A1 product, Δ1 -pyrroline-5-carboxylate synthetase (P5CS), appeared necessary for cracking SPG9 disease-causing mechanisms. We now describe a baculovirus-insect cell system that yields mgs of pure human P5CS and that has proven highly valuable with two novel P5CS mutations reported here in new SPG9B patients. We conclude that both mutations are disease-causing, that SPG9B associates with partial P5CS deficiency and that it is clinically more severe than SPG9A, as reflected in onset age, disability, cognitive status, growth, and dysmorphic traits.

A novel two-domain architecture within the amino acid kinase enzyme family revealed by the crystal structure of Escherichia coli glutamate 5-kinase.

Glutamate 5-kinase (G5K) makes the highly unstable product glutamyl 5-phosphate (G5P) in the initial, controlling step of proline/ornithine synthesis, being feedback-inhibited by proline or ornithine, and causing, when defective, clinical hyperammonaemia. We determined two crystal structures of G5K from Escherichia coli, at 2.9 A and 2.5 A resolution, complexed with glutamate and sulphate, or with G5P, sulphate and the proline analogue 5-oxoproline. E. coli G5K presents a novel tetrameric (dimer of dimers) architecture. Each subunit contains a 257 residue AAK domain, typical of acylphosphate-forming enzymes, with characteristic alpha(3)beta(8)alpha(4) sandwich topology. This domain is responsible for catalysis and proline inhibition, and has a crater on the beta sheet C-edge that hosts the active centre and bound 5-oxoproline. Each subunit contains a 93 residue C-terminal PUA domain, typical of RNA-modifying enzymes, which presents the characteristic beta(5)beta(4) sandwich fold and three alpha helices. The AAK and PUA domains of one subunit associate non-canonically in the dimer with the same domains of the other subunit, leaving a negatively charged hole between them that hosts two Mg ions in one crystal, in line with the G5K requirement for free Mg. The tetramer, formed by two dimers interacting exclusively through their AAK domains, is flat and elongated, and has in each face, pericentrically, two exposed active centres in alternate subunits. This would permit the close apposition of two active centres of bacterial glutamate-5-phosphate reductase (the next enzyme in the proline/ornithine-synthesising route), supporting the postulated channelling of G5P. The structures clarify substrate binding and catalysis, justify the high glutamate specificity, explain the effects of known point mutations, and support the binding of proline near glutamate. Proline binding may trigger the movement of a loop that encircles glutamate, and which participates in a hydrogen bond network connecting active centres, which is possibly involved in the cooperativity for glutamate.

The PII-NAGK-PipX-NtcA Regulatory Axis of Cyanobacteria: A Tale of Changing Partners, Allosteric Effectors and Non-covalent Interactions.

PII, a homotrimeric very ancient and highly widespread (bacteria, archaea, plants) key sensor-transducer protein, conveys signals of abundance or poorness of carbon, energy and usable nitrogen, converting these signals into changes in the activities of channels, enzymes, or of gene expression. PII sensing is mediated by the PII allosteric effectors ATP, ADP (and, in some organisms, AMP), 2-oxoglutarate (2OG; it reflects carbon abundance and nitrogen scarcity) and, in many plants, L-glutamine. Cyanobacteria have been crucial for clarification of the structural bases of PII function and regulation. They are the subject of this review because the information gathered on them provides an overall structure-based view of a PII regulatory network. Studies on these organisms yielded a first structure of a PII complex with an enzyme, (N-acetyl-Lglutamate kinase, NAGK), deciphering how PII can cause enzyme activation, and how it promotes nitrogen stockpiling as arginine in cyanobacteria and plants. They have also revealed the first clear-cut mechanism by which PII can control gene expression. A small adaptor protein, PipX, is sequestered by PII when nitrogen is abundant and is released when is scarce, swapping partner by binding to the 2OG-activated transcriptional regulator NtcA, co-activating it. The structures of PII-NAGK, PII-PipX, PipX alone, of NtcA in inactive and 2OG-activated forms and as NtcA-2OG-PipX complex, explain structurally PII regulatory functions and reveal the changing shapes and interactions of the T-loops of PII depending on the partner and on the allosteric effectors bound to PII. Cyanobacterial studies have also revealed that in the PII-PipX complex PipX binds an additional transcriptional factor, PlmA, thus possibly expanding PipX roles beyond NtcA-dependency. Further exploration of these roles has revealed a functional interaction of PipX with PipY, a pyridoxal-phosphate (PLP) protein involved in PLP homeostasis whose mutations in the human ortholog cause epilepsy. Knowledge of cellular levels of the different components of this PII-PipX regulatory network and of KD values for some of the complexes provides the basic background for gross modeling of the system at high and low nitrogen abundance. The cyanobacterial network can guide searches for analogous components in other organisms, particularly of PipX functional analogs.