I04-Macromolecular Crystallography
I24-Microfocus Macromolecular Crystallography
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Abstract: The assembly of a septin filament requires that homologous monomers must distinguish between one another in establishing appropriate interfaces with their neighbors. To understand this phenomenon at the molecular level, we present the first four crystal structures of heterodimeric septin complexes. We describe in detail the two distinct types of G-interface present within the octameric particles which must polymerize to form filaments. These are formed between SEPT2 and SEPT6 and between SEPT7 and SEPT3, and their description permits an understanding of the structural basis for the selectivity necessary for correct filament assembly. By replacing SEPT6 by SEPT8 or SEPT11, it is possible to rationalize Kinoshita's postulate which predicts the exchangeability of septins from within a subgroup. Switches I and II, which in classical small GTPases provide a mechanism for nucleotide-dependent conformational change, have been repurposed in septins to play a fundamental role in molecular recognition. Specifically, it is switch I which holds the key to discriminating between the two different G-interfaces. Moreover, residues which are characteristic for a given subgroup play subtle, but pivotal, roles in guaranteeing that the correct interfaces are formed.
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Sep 2020
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I04-Macromolecular Crystallography
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Diamond Proposal Number(s):
[15991]
Open Access
Abstract: Copper-containing nitrite reductases (CuNiRs) are found in all three kingdoms of life and play a major role in the denitrification branch of the global nitrogen cycle where nitrate is used in place of dioxygen as an electron acceptor in respiratory energy metabolism. Several C- and N-terminal redox domain tethered CuNiRs have been identified and structurally characterized during the last decade. Our understanding of the role of tethered domains in these new classes of three-domain CuNiRs, where an extra cytochrome or cupredoxin domain is tethered to the catalytic two-domain CuNiRs, has remained limited. This is further compounded by a complete lack of substrate-bound structures for these tethered CuNiRs. There is still no substrate-bound structure for any of the as-isolated wild-type tethered enzymes. Here, structures of nitrite and product-bound states from a nitrite-soaked crystal of the N-terminal cupredoxin-tethered enzyme from the Hyphomicrobium denitrificans strain 1NES1 (Hd1NES1NiR) are provided. These, together with the as-isolated structure of the same species, provide clear evidence for the role of the N-terminal peptide bearing the conserved His27 in water-mediated anchoring of the substrate at the catalytic T2Cu site. Our data indicate a more complex role of tethering.
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May 2020
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I02-Macromolecular Crystallography
I24-Microfocus Macromolecular Crystallography
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Diamond Proposal Number(s):
[14493, 5810]
Open Access
Abstract: Human septins 3, 9 and 12 are the only members of a specific subgroup of septins that display several unusual features, including the absence of a C-terminal coiled coil. This particular subgroup (the SEPT3 septins) are present in rod-like octameric protofilaments but are lacking in similar hexameric assemblies, which only contain representatives of the three remaining subgroups. Both hexamers and octamers can self-assemble into mixed filaments by end-to-end association, implying that the SEPT3 septins may facilitate polymerization but not necessarily function. These filaments frequently associate into higher order complexes which associate with biological membranes, triggering a wide range of cellular events. In the present work, a complete compendium of crystal structures for the GTP-binding domains of all of the SEPT3 subgroup members when bound to either GDP or to a GTP analogue is provided. The structures reveal a unique degree of plasticity at one of the filamentous interfaces (dubbed NC). Specifically, structures of the GDP and GTPγS complexes of SEPT9 reveal a squeezing mechanism at the NC interface which would expel a polybasic region from its binding site and render it free to interact with negatively charged membranes. On the other hand, a polyacidic region associated with helix α5′, the orientation of which is particular to this subgroup, provides a safe haven for the polybasic region when retracted within the interface. Together, these results suggest a mechanism which couples GTP binding and hydrolysis to membrane association and implies a unique role for the SEPT3 subgroup in this process. These observations can be accounted for by constellations of specific amino-acid residues that are found only in this subgroup and by the absence of the C-terminal coiled coil. Such conclusions can only be reached owing to the completeness of the structural studies presented here.
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May 2020
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I24-Microfocus Macromolecular Crystallography
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Diamond Proposal Number(s):
[14496]
Abstract: Septins are GTP-binding proteins that will often spontaneously assemble into filaments. In some species, particularly budding yeast, it is well known that these are capable of associating with membranes in order to fulfill their cellular role as a component of the cytoskeleton. Different from other human septins, SEPT7 appears to be unique in that it is an essential component of all hetero-oligomeric complexes described to date. As a step towards understanding the molecular basis of filament assembly, here we present two high-resolution structures of the SEPT7 GTPase domain complexed with GDP. One of these reveals a previously unreported coordination for the magnesium ion involving four water molecules and only a tenuous connection to the protein. The higher resolution structures provide unambiguous insight into the interactions at the G-interface where a structural motif based on an antiparallel β-bridge allows for the rationalization of why some septins show nucleotide-dependent β-strand slippage and others do not.
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Apr 2019
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I03-Macromolecular Crystallography
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Open Access
Abstract: Superoxide dismutase-1 (SOD1) maturation comprises a string of posttranslational modifications which transform the nascent peptide into a stable and active enzyme. The successive folding, metal ion binding, and disulphide acquisition steps in this pathway can be catalysed through a direct interaction with the copper chaperone for SOD1 (CCS). This process confers enzymatic activity and reduces access to noncanonical, aggregation-prone states. Here, we present the functional mechanisms of human copper chaperone for SOD1 (hCCS)–catalysed SOD1 activation based on crystal structures of reaction precursors, intermediates, and products. Molecular recognition of immature SOD1 by hCCS is driven by several interface interactions, which provide an extended surface upon which SOD1 folds. Induced-fit complexation is reliant on the structural plasticity of the immature SOD1 disulphide sub-loop, a characteristic which contributes to misfolding and aggregation in neurodegenerative disease. Complexation specifically stabilises the SOD1 disulphide sub-loop, priming it and the active site for copper transfer, while delaying disulphide formation and complex dissociation. Critically, a single destabilising amino acid substitution within the hCCS interface reduces hCCS homodimer affinity, creating a pool of hCCS available to interact with immature SOD1. hCCS substrate specificity, segregation between solvent and biological membranes, and interaction transience are direct results of this substitution. In this way, hCCS-catalysed SOD1 maturation is finessed to minimise copper wastage and reduce production of potentially toxic SOD1 species.
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Feb 2019
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I04-1-Macromolecular Crystallography (fixed wavelength)
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Abstract: One central goal in molecular evolution is to pinpoint the mechanisms and evolutionary forces that cause an enzyme to change its substrate specificity; however, these processes remain largely unexplored. Using the glycolytic ADP-dependent kinases of archaea, including the orders Thermococcales, Methanosarcinales, and Methanococcales, as a model and employing an approach involving paleoenzymology, evolutionary statistics, and protein structural analysis, we could track changes in substrate specificity during ADP-dependent kinase evolution along with the structural determinants of these changes. To do so, we studied five key resurrected ancestral enzymes as well as their extant counterparts. We found that a major shift in function from a bifunctional ancestor that could phosphorylate either glucose or fructose 6-phosphate (fructose-6P) as a substrate to a fructose-6P-specific enzyme was started by a single amino acid substitution resulting in negative selection with a ground-state mode against glucose and a subsequent 1,600-fold change in specificity of the ancestral protein. This change rendered the residual phosphorylation of glucose a promiscuous and physiologically irrelevant activity, highlighting how promiscuity may be an evolutionary vestige of ancestral enzyme activities, which have been eliminated over time. We also could reconstruct the evolutionary history of substrate utilization by using an evolutionary model of discrete binary characters, indicating that substrate uses can be discretely lost or acquired during enzyme evolution. These findings exemplify how negative selection and subtle enzyme changes can lead to major evolutionary shifts in function, which can subsequently generate important adaptive advantages, for example, in improving glycolytic efficiency in Thermococcales.
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Jul 2017
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I04-1-Macromolecular Crystallography (fixed wavelength)
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Andressa P. A.
Pinto
,
Humberto M.
Pereira
,
Ana E.
Zeraik
,
Heloisa
Ciol
,
Frederico M.
Ferreira
,
Jose
Brandao-neto
,
Ricardo
Demarco
,
Marcos V. A. S.
Navarro
,
Cristina
Risi
,
Vitold E.
Galkin
,
Richard C.
Garratt
,
Ana P. U.
Araujo
Diamond Proposal Number(s):
[5073]
Abstract: Septins are filament−forming GTP−binding proteins involved in many essential cellular events related to cytoskeletal dynamics and maintenance. Septins can self−assemble into hetero−complexes, which polymerize into highly organized, cell membrane−interacting filaments. The number of septin genes varies among organisms, and although their structure and function have been thoroughly studied in opisthokonts (including animals and fungi), no structural studies have been reported for other organisms. This makes the single septin from Chlamydomonas (CrSEPT) a particularly attractive model for investigating whether functional homopolymeric septin filaments also exist. CrSEPT was detected at the base of the flagella in Chlamydomonas, suggesting that CrSEPT is involved in the formation of a membrane diffusion barrier. Using transmission electron microscopy, we observed that recombinant CrSEPT forms long filaments with dimensions comparable to those of the canonical structure described for opisthokonts. The GTP−binding domain of CrSEPT purified as a nucleotide−free monomer that hydrolyzes GTP and readily binds its analog GTPγS. We also found that on nucleotide binding, CrSEPT forms dimers that are stabilized by an interface involving the ligand (G−interface). Across this interface, one monomer supplied a catalytic arginine to the opposing subunit, greatly accelerating the rate of GTP hydrolysis. This is the first report of an arginine finger observed in a septin and suggests that CrSEPT may act as its own GTP−activating protein (GAP). The finger is conserved in all algal septin sequences, suggesting a possible correlation between the ability to form homo−polymeric filaments and the accelerated rate of hydrolysis which it provides.
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May 2017
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Abstract: Reports of Schistosoma mansoni strains resistant to praziquantel, the only therapeutic strategy available for the treatment of schistosomiasis, have motivated the scientific community towards the search for new possible therapies. Biochemical characterization of the parasite's metabolism is an essential component for the rational development of new therapeutic alternatives. One of the so far uncharacterized enzymes is uridine phosphorylase (UP) (EC 2.4.2.3), for which the parasite genome presents two isoforms (SmUPa and SmUPb) that share 92% sequence identity. In this paper, we present crystal structures for SmUPa and SmUPb in their free states as well as bound to different ligands. This we have complemented by enzyme kinetic characterization and phylogenetic analyses. Both enzymes present an overall fold and active site structure similar to other known UPs. The kinetic analyses showed conclusively that SmUPa is a regular uridine phosphorylase but by contrast SmUPb presented no detectable activity. This is particularly noteworthy given the high level of sequence identity between the two isoforms and is probably the result of the significant differences observed for SmUPb in the vicinity of the active site itself, suggesting that it is not a UP at all. On the other hand, it was not possible to identify an alternative function for SmUPb, although our phylogenetic analyses and expression data suggest that SmUPb is still functional and plays a role in parasite metabolism. The unusual UPb isoform may open up new opportunities for understanding unique features of S. mansoni metabolism.
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Jun 2016
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I04-1-Macromolecular Crystallography (fixed wavelength)
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Abstract: The sequences of all seven polypeptide chains from the giant haemoglobin of the free-living earthworm Glossoscolex paulistus (HbGp) are reported together with the three-dimensional structure of the 3.6 MDa complex which they form. The refinement of the full particle, which has been solved at 3.2 Å resolution, the highest resolution reported to date for a hexagonal bilayer haemoglobin composed of 12 protomers, is reported. This has allowed a more detailed description of the contacts between subunits which are essential for particle stability. Interpretation of features in the electron-density maps suggests the presence of metal-binding sites (probably Zn2+ and Ca2+) and glycosylation sites, some of which have not been reported previously. The former appear to be important for the integrity of the particle. The crystal structure of the isolated d chain (d-HbGp) at 2.1 Å resolution shows different interchain contacts between d monomers compared with those observed in the full particle. Instead of forming trimers, as seen in the complex, the isolated d chains associate to form dimers across a crystallographic twofold axis. These observations eliminate the possibility that trimers form spontaneously in solution as intermediates during the formation of the dodecameric globin cap and contribute to understanding of the possible ways in which the particle self-assembles.
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Jun 2015
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I04-1-Macromolecular Crystallography (fixed wavelength)
I24-Microfocus Macromolecular Crystallography
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Abstract: Background: Septins are filament-forming proteins involved in membrane-remodeling events.
Results: Two crystal structures of a septin with the highest resolution to date reveal the phenomenon of -strand slippage.
Conclusion: A novel mechanistic framework for the influence of the nature of the bound nucleotide and the presence of Mg2
in septins is proposed.
Significance: Identification of strand slippage might contribute to elucidating the mechanism of septin association with
membranes.
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Jan 2014
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