mec-7 is a , -tubulin gene required for the production of lS- protofilament microtubules, a unique feature of the touch receptor neurons. 54 alleles of
mec-7 have been isolated and characterized as strong or weak depending upon whether the touch insensitive phenotype is complete or partial. About 60% of the alleles are expressed dominantly or semi-dominantly. To characterize the relationship between structure and function of this alpha-tubulin, we have sequenced the
mec-7 mutations. We hypothesize that dominant effects are caused by mutant proteins that dimerize with alpha-tubulin, then disrupt polymerization. Recessive alleles may be caused by defects in protein stability or alpha-tubulin binding. We will test the stability and o-tubulin binding capacity of mutant proteins by immunoprecipitation of
mec-7 and associated proteins. We have sequenced mutations in 45 mec- 7 alleles . All of these mutations are localized in the coding region, confirming that
mec-7 encodes this alpha-tubulin. Six of thirteen null alleles are caused by stop mutations, one changes the initial AUG codon, and two are the result of TcS insertion into the coding sequence. Three other null alleles have mutations in a putative GTP-binding site, and one null allele may disrupt alpha-tubulin binding. All four of these missense mutations accumulate more
mec-7 mRNA than wild type, as measured by in situ hybridization, although they contain normal levels of
mec-7 gene product by immunocytochemistry. Normally, cellular tubulin levels are autoregulated via a mechanism in which unpolymerized tubulin reduces the stability of its own message. Thus, it appears that the non- functional
mec-7 gene products fail to autoregulate. One additional stop mutation encodes a partially functional product, suggesting that this truncated product still contains an alpha-tubulin binding site. One strong semi-dominant allele disrupts a putative GTPase domain. Therefore, GTPase activity may be required for microtubule polymerization. Other strong semidominant and dominant alleles have mutations that are clustered into three regions; these may represent functional domains which interact with other microtubule components during polymerization.