The Myosin-Binding UCS Domain but not the Hsp90-Binding TPR Domain of the UNC-45 Chaperone is Essential for Myosin Accumulation and Assembly in Caenorhabditis elegans

The UNC-45 family of molecular chaperones is expressed in metazoan organisms from C. elegans to humans. The UNC-45 protein is essential in C. elegans for early body-wall muscle cell development and A band assembly. We show that the myosin-binding UCS domain of UNC-45 alone is sufficient to rescue embryonic lethal unc-45 null mutants arrested in embryonic muscle development and temperature-sensitive loss-of-function unc-45 mutants defective in worm A band assembly. Removal of the Hsp90-binding TPR domain of UNC-45 does not affect rescue. Similar results were obtained with overexpression of the same fragments in wild-type nematodes when assayed by diminution of myosin accumulation and assembly. Titration experiments show that on a per molecule basis, UCS has greater activity in C. elegans muscle in vivo than full-length UNC-45 protein, suggesting it may be inhibited by either the TPR domain or its interaction with the general chaperone Hsp90. In vitro experiments with purified recombinant C. elegans Hsp90 and UNC-45 proteins show that they compete for binding to C. elegans myosin. Our in vivo genetic and in vitro biochemical experiments are consistent with a novel inhibitory role for Hsp90 with respect to UNC-45 action.