title: Higher-order septin assembly is driven by GTP-promoted conformational changes: evidence from unbiased mutational analysis in Saccharomyces cerevisiae.
abstract: Septin proteins bind GTP and heterooligomerize into filaments with conserved functions across a wide range of eukaryotes. Most septins hydrolyze GTP, altering the oligomerization interfaces; yet mutations designed to abolish nucleotide binding or hydrolysis by yeast septins perturb function only at high temperatures. Here, we apply an unbiased mutational approach to this problem. Mutations causing defects at high temperature mapped exclusively to the oligomerization interface encompassing the GTP-binding pocket, or to the pocket itself. Strikingly, cold-sensitive defects arise when certain of these same mutations are coexpressed with a wild-type allele, suggestive of a novel mode of dominance involving incompatibility between mutant and wild-type molecules at the septin-septin interfaces that mediate filament polymerization. A different cold-sensitive mutant harbors a substitution in an unstudied but highly conserved region of the septin Cdc12. A homologous domain in the small GTPase Ran allosterically regulates GTP-binding domain conformations, pointing to a possible new functional domain in some septins. Finally, we identify a mutation in septin Cdc3 that restores the high-temperature assembly competence of a mutant allele of septin Cdc10, likely by adopting a conformation more compatible with nucleotide-free Cdc10. Taken together, our findings demonstrate that GTP binding and hydrolysis promote, but are not required for, one-time events--presumably oligomerization-associated conformational changes--during assembly of the building blocks of septin filaments. Restrictive temperatures impose conformational constraints on mutant septin proteins, preventing new assembly and in certain cases destabilizing existing assemblies. These insights from yeast relate directly to disease-causing mutations in human septins.
authors.affiliation: Department of Cell and Developmental Biology, University of Colorado Anschutz Medical Campus, Aurora, Colorado 80045.
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Weems
authors.forename: Courtney R
authors.initials: CR
authors.lastname: Johnson
authors.forename: Juan Lucas
authors.initials: JL
authors.lastname: Argueso
authors.forename: Michael A
authors.initials: MA
authors.lastname: McMurray
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D018797:Cell Cycle Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D055028:Comparative Genomic Hybridization; D017124:Conserved Sequence; D003598:Cytoskeletal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D016681:Genome, Fungal; D006160:Guanosine Triphosphate / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D051304:Profilins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011487:Protein Conformation; D055503:Protein Multimerization; D012441:Saccharomyces cerevisiae / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D058112:Septins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D013696:Temperature
chemical_list: C099504:CDC12 protein, S cerevisiae; C086662:CDC3 protein, S cerevisiae; D018797:Cell Cycle Proteins; D003598:Cytoskeletal Proteins; D008565:Membrane Proteins; D051304:Profilins; D029701:Saccharomyces cerevisiae Proteins; D006160:Guanosine Triphosphate; C512892:CDC10 protein, S cerevisiae; D020558:GTP Phosphohydrolases; D058112:Septins
keywords: allosteric regulation; higher-order cytoskeletal assembly; ligand binding; temperature-sensitive mutants; yeast forward genetics