POLYMERIZATION OF PURIFIED YEAST SEPTINS - EVIDENCE THAT ORGANIZED FILAMENT ARRAYS MAY NOT BE REQUIRED FOR SEPTIN FUNCTION

The septins are a family of proteins required for cytokinesis in a num ber of eukaryotic cell types. In budding yeast, these proteins are tho ught to be the structural components of a filament system present at t he mother-bud neck, called the neck filaments. In this study, we repor t the isolation of a protein complex containing the yeast septins Cdc3 p, Cdc10p, Cdc11p, and Cdc12p that is capable of forming long filament s in vitro. To investigate the relationship between these filaments an d the neck filaments, we purified septin complexes from cells deleted for CDC10 or CDC11. These complexes were not capable of the polymeriza tion exhibited by wild-type preparations, and analysis of the neck reg ion by electron microscopy revealed that the cdc10 Delta and cdc11 Del ta cells did not contain detectable neck filaments. These results stre ngthen the hypothesis that the septins are the major structural compon ents of the neck filaments. Surprisingly, we found that septin depende nt processes like cytokinesis and the localization of Bud4p to the nec k still occurred in cdc10 Delta cells. This suggests that the septins may be able to function in the absence of normal polymerization and th e formation of a higher order filament structure.