Genetic analysis of the Sep2 and Sep5 septins. K.G. Hales , M. Peifer , J.R. Pringle. Dept Biol, Univ North Carolina, Chapel Hill, NC.

   Septins are conserved filament-forming proteins that contain GTPase and coiled-coil domains. A role for septins in cytokinesis has been demonstrated in several organisms; for example, in yeast, septins act at the bud neck to help assemble components of both the actomyosin ring and the septum-construction machinery. The immunolocalization patterns of septins in various nondividing cell types suggests that septins may have a general role in defining regions of the cell cortex. In yeast, the seven septins do not all act together; different subsets have roles in cytokinesis, sporulation, and perhaps mating projection extension. Septins in multicellular organisms may also differ in their functions. Of the five known Drosophila septins, only Pnut has been analyzed genetically. Here we report mutations in two additional Drosophila septin genes, sep2 and sep5. We mobilized a P element and identified a new insertion in the 5' UTR of sep2 through Southern blot analysis of 100 strains. This sep2 allele is homozygous viable and fertile but shows incompletely penetrant pharate adult lethality over a deletion. Excision of the P element led to the identification of small deletions for sep2, which cause recessive pharate adult lethality with a few escapers. Male homozygotes make motile sperm, but females show oogenesis and egg-laying defects. Since Sep2 and Sep5 are more closely related to each other (~70% identity) than to the other Drosophila septins (35-45%), we suspected possible redundancy in at least some stages or tissue types. We generated a P insertion (with a small upstream deletion) at the eighth codon of sep5. sep5 single mutants also appear to cause variably penetrant recessive lethality at late stages of development. We are analyzing the cellular and molecular phenotypes of sep2 and sep5 single and double mutants, as well as of germline clones, focusing on how the functions of Sep2 and Sep5 relate to those of the other septins and functionally related proteins.