The mechanisms that regulate cytokinesis remain poorly understood. Over the past few years, a class of gene called septins has been identified and shown to play a critical role in cytokinsis in several organisms including S. cerevisiae and D. melanogaster. We have identified, cloned, and characterized the C. elegans
unc-61 locus that encodes for a C. elegans septin,
sep-2.
sep-2 gives a 2.1 kb transcript that encodes a polypeptide of 568 amino acids with a predicted molecular weight of 65 Kd. When translated, SEP-2 has significant homology to other known septins including C. elegans SEP-1. Like most septins, SEP-2 has the GTPase domain at the N-terminus and a predicted coiled-coiled domain at the C-terminus Phenotypically,
unc-61 (
e228 and
n3169) have vulva protrusion, egg-laying defects, gonad extrusion, and male abnormality. These phenotypes are classical characteristics of a mutant that has post-embryonic failures in cytokinesis. Genetically,
unc-61 maps to 6.49 map unit to the right arm of chromosome V and is covered by a cosmid C18G4 that contains the full length
sep-2. Cosmid C18G4 and its subclones rescued
unc-61 mutants. Furthermore, RNAi using
sep-2 cDNA phenocopies all aspects of
unc-61 mutants. Sequencing analysis reveals that both alleles of
unc-61(
e228 and
n3169) have a premature stop codon resulting in truncated protein. We generated antibodies against SEP-2 peptides and performed indirect immunostaining to determine the localization of this gene product during cytokinesis in early embryos. SEP-2 localized to the anterior cortex in the early embryos and became localized in all cleavage furrows at early telophase. It formed a ring-like structure in close association with an actomyosin contractile ring. Eventually, SEP-2 localized to the persisting intercellular bridge remaining there until the next cell division. Further characterization showed that SEP-2 was not detected in
unc-61 mutants. In addition, SEP-2 also failed to localize in
unc-59(
e261 and
e1005), mutant alleles of
sep-1. These and the reciprocal observations suggest that SEP-1 is required for SEP-2 localization and vice versa.