Role of GTP Binding in Yeast Septins

Abstract

The septins are nucleotide-binding, filament forming protein present in all eukaryotes with the exception of plants. Septins as a complex localize to the cortex at the mother-bud neck, and facilitate the localization of proteins involved in several cell-cycle processes. Septins might also have a role in regulating the functions of these septin-dependent proteins. Septin nucleotide binding and/or hydrolysis may regulate the interaction of septins with septins or non-septin proteins. By UV-crosslinking analysis we have clearly indicated that septins bind GTP and mutations predicted to disrupt nucleotide binding does disrupt nucleotide binding. Co-immunoprecipitation and in vitro transcription and translation assays showed that GTP is required for septin-septin interactions. Compared to complexes from cells expressing wild-type Cdc11p, complexes from mutant cells contain sub-stoichiometric amounts of cdc11p indicating that nucleotide binding is affected even at 23˚C. However, this defect does not show a detectable phenotype. Septin P-loop mutations show a range of minimal restrictive temperatures for viability, which correlates with the predicted defects in nucleotide binding and the extent of defects in septin localization. Thus, septin nucleotide binding promotes septin-septin interactions, septin filament formation, and septin localization.