The roles of tubulins in Caenorhabditis elegans meiotic and mitotic spindle formation

C. elegans female meiotic spindle formation requires two meiosis-specific genes, mei-1 and mei-2. The proteins they encode, MEI-1 and MEI-2, are similar to the p60 (severing) and p80 (localization) subunits, respectively, of the sea urchin microtubule-severing complex katanin. In addition to their sequence similarities to katanin, MEI-1 and MEI-2 disassemble interphase microtubules when coexpressed in Hela cells. In wild-type embryos, MEI-1 and MEI-2 localize exclusively to the female meiotic spindle. MEI-1 and MEI-2 likely regulate meiotic spindle microtubules to maintain the unique morphology of the small anteriorly-located, barrel-shaped meiotic spindle, but MEI-1/MEI-2 severing activity must be inactivated prior to mitosis. The post-meiotic inactivation of MEI-1/MEI-2 likely requires ubiquitination and proteosome dependent degradation of MEI-1.; This thesis characterized three extragenic suppressors of a mei-1(gf) mutant that results in ectopic microtubule-severing activity during mitosis. These suppressors show semi-dominant suppression of the mei-1(gf) defects, but are phenotypically wild-type by themselves. One suppressor is a missense allele of the beta tubulin gene, tbb-2; the other two are missense alleles of the alpha tubulin gene, tba-2. All three suppressors genetically behave as if they generally inhibit microtubule severing: they suppress the mitotic phenotype resulting from ectopic severing activity while enhancing meiotic defects seen when MEI-1/MEI-2 severing activity is compromised during meiosis. Although functional redundancies were seen with the other alpha and beta-tubulin isotypes that are also expressed during embryonic development, use of a sensitized genetic background revealed tubulin isotype preferences of the MEI-1/MEI-2 severing complex: when MEI-1 and MEI-2 are limiting, removing TBB-2 by either RNAi or a deletion allele resulted in meiotic defects while removing TBB-1 did not; similarly, tba-2(RNAi) enhanced meiotic defects caused by limiting MEI-1/MEI-2 activity but tba-1(RNAi) did not. Thus, MEI-1/MEI-2 prefers the TBA-2 and the TBB-2 containing microtubules. In addition, I identified sb51 a dominant temperature-sensitive tba-2 allele that disrupts both meiotic and mitotic spindle formation independent of MEI-1/MEI-2 katanin. TBA-2(sb51) likely has a poisonous effect on microtubule structure and disrupts spindle organization when incorporated.