Worm Breeder's Gazette 16(2): 35
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Dept. of Biological Sciences, University of Alberta, Edmonton, Canada.
The unc-45 gene was one of the first ever reported in worms (Epstein and Thomson, 1974) yet we still do not have a satisfactory explanation of its role in development. Missense alleles at the locus show a temperature-sensitive disorganization of muscle thick filament structure, while nonsense alleles are lethal, with varying maternal effects. unc-45 reporter constructs show zygotic expression which is exclusive to muscle cells, and antibodies against the protein show a co-localization with MHC B, but not MHC A, in thick filaments. The phenotype of unc-45 (ts) alleles can be suppressed if all MHC B is removed, and MHC A overexpressed, as unc-45(ts);unc-54(0);myo-3(sup-3) animals are almost normal. This would suggest that the function of UNC-45 is restricted to some sort of MHC B-specific interaction.
However, UNC-45 must be playing another role in embryonic development, since the phenotype of strong unc-45 alleles (or unc-45 RNAi animals) is 'pat' (paralyzed and arrested at two-fold stage), while null alleles of unc-54 (encoding for MHC B) are viable. Further, myo-3(sup-3) cannot suppress the lethality of strong unc-45 alleles.
We consistently detect strong UNC-45 staining in eggs and early embryos, which is relatively uniform throughout the embryo, and diminishes over time. At the comma stage, UNC-45 staining strengthens in the muscle cells, and we believe this represents the zygotic expression. In early embryos, there is a definite concentration of UNC-45 staining at the cleavage furrow. Another protein that shows the same behavior is the non-muscle myosin NMY-2 (Guo and Kemphues, 1996), and we have detected nmy-2 cDNA using UNC-45 as a bait in the yeast two-hybrid system. The staining pattern of the two proteins overlaps, and the localized staining of UNC-45 at the cleavage furrow of two-cell embryos is disrupted in embryos from mothers treated with nmy-2 RNAi. We have not been able to demonstrate the reciprocal disruption.
UNC-45 shows similarity to several fungal proteins in a C-terminal domain. The S. cerevisiae homologue is SHE4p, which is thought to interact with a non-muscle myosin in the asymmetric distribution of cytoplasmic material between mother and daughter cells during mating type switching. We have no evidence that UNC-45 plays a functional role in embryos outside of muscle development, but we are open to suggestions about how to test this.