unc-45 is essential for normal thick filament development in body wall muscles. The protein product (UNC-45) contains TPR repeats and has similarity to the yeast She4p protein which is involved in asymmetric segregation of specific mRNAs, but its biochemical function is unknown. We have previously shown that UNC-45 differentially co-localizes in body wall muscle thick filaments with myosin heavy chain B (MHC B) but not MHC A. In contrast to other body wall muscle thick filament components,
unc-45 is also maternally contributed and the protein and mRNA are present in all cells of the early embryo. However, zygotic
unc-45 expression is only detected in the developing muscle cells. Yeast two-hybrid screens show that UNC-45 interacts with at least two non-muscle myosins, including NMY-2, a type II myosin necessary for asymmetric cell division. UNC-45 and NMY-2 also co-localize in vivo, at the cortex of the cell in early embryos, and localization of UNC-45 is dependent on the presence of NMY-2. Rather than being a muscle-specific protein, this would argue that the UNC-45 protein may have a more general role as a myosin chaperone, stabilizer, or assemblase. We have recently found UNC-45 sequence homologues in Drosophila and humans, and a lethal mutation in the Drosophila gene, leading us to propose that this family of proteins may be an evolutionarily conserved class of myosin interacting proteins.