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Investigation of Aspects Important in the Structural Assembly of Human Septin Filaments

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Sabrina Matos de Oliveira da Silva
Total Authors: 1
Document type: Doctoral Thesis
Press: São Carlos.
Institution: Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT)
Defense date:
Examining board members:
Richard Charles Garratt; Ana Paula Ulian de Araujo; Fernanda Canduri; Flávio Henrique da Silva
Advisor: Richard Charles Garratt

Septins belong to a highly conserved family of guanine nucleotide binding proteins, capable of forming filaments. Despite their importance for several critical cellular events including cell division, little information is available about their molecular functions and the mechanism which leads to the formation of hetero-oligomers. The aim of this thesis was the cloning, co-expression and crystallization of septins and their complexes (SEPT9-SEPT11-SEPT7; SEPT2-SEPT6-SEPT7-SEPT9; SEPT4-SEPT6-SEPT7 and SEPT4-SEPT8-SEPT7), followed by comparative structural analysis. The cDNAs coding for the proteins SEPT9, SEPT6, SEPT2, SEPT11, SEPT8, SEPT4, SEPT9GCα0 (residues 262-568) and SEPT9GC (residues 279-568) were successfully cloned into transferable or expression vectors. The best characterized complex was that composed of SEPT2-SEPT6-SEPT7-SEPT9GCα0, which was confirmed by LC-MS/MS analysis after triptic digestion, unveiling the in vitro production of a tetrameric septin complex. Additionally, we characterized the hetero-dimer formed by SEPT7NGc-SEPT9GC. However, the most fully investigated proteins were the constructs SEPT9GCα0 and SEPT9GC which were studied individually for biophysical and structural characterizations. SEPT9GCα0 was highly unstable contrasting with SEPT9GC that was efficiently produced and characterized. The presence of the oligomeric state of SEPT9GC (monomer) was confirmed by molecular exclusion chromatography. The protein was produced in the apo form and was capable of hydrolyzing GTP. The affinity of SEPT9GC for GDP and GTPγS nucleotides was evaluated by Isothermal Titration Calorimetry (ITC) revealing that SEPT9GC has a higher affinity for GTPγS, with a Kd of 25.9 µM, which is dependent on the presence of Mg2+. Crystallization studies of this protein were performed, obtaining high quality crystals of protein complexes with GDP and GTPγS. Interestingly, the structure of the complex of SEPT9GC with the nucleotide GTPγS was obtained by soaking a previously grown crystal of the GDP complex. This is the first report of the application of this method for septins. Finally, we have obtained three sets of crystallographic data, two for SEPT9GC-GDP, at resolutions of 2.8 Å and 2.1 Å, and one set of data for SEPT9GC-GTPγS at 2.8 Å. Among the main characteristics observed in the SEPT9GC structure is the NC interface responsible for filament polymerization, which is shown to vary according to the type of bound nucleotide, with foreshortening of the filament in the presence of GTPγS. This indicates communication between consecutive G and NC interfaces along the filament. The conformational change at the interface involves the rearrangement of residues which are highly conserved in all septins as well as several which are SEPT9 group specific and may have implications for filament assembly and membrane association. (AU)

FAPESP's process: 12/00268-0 - Investigation of Aspects Important in the Structural Assembly of Human Septin Filaments
Grantee:Sabrina Matos de Oliveira da Silva
Support Opportunities: Scholarships in Brazil - Doctorate