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J. Cell Biol.,
Volume 143, Number 5, November 30, 1998 1215-1225
* Department of Biochemistry and The Caenorhabditis elegans unc-45 locus has
been proposed to encode a protein machine for myosin
assembly. The UNC-45 protein is predicted to contain
an NH2-terminal domain with three tetratricopeptide repeat motifs, a unique central region, and a COOH-terminal domain homologous to CRO1 and She4p.
CRO1 and She4p are fungal proteins required for the
segregation of other molecules in budding, endocytosis, and septation. Three mutations that lead to temperature-sensitive (ts) alleles have been localized to conserved residues within the CRO1/She4p-like domain,
and two lethal alleles were found to result from stop
codon mutations in the central region that would prevent translation of the COOH-terminal domain. Electron microscopy shows that thick filament accumulation in vivo is decreased by ~50% in the CB286 ts
mutant grown at the restrictive temperature. The thick
filaments that assemble have abnormal structure. Immunofluorescence and immunoelectron microscopy
show that myosins A and B are scrambled, in contrast
to their assembly into distinct regions at the permissive temperature and in wild type. This abnormal structure
correlates with the high degree of instability of the filaments in vitro as reflected by their extremely low yields
and shortened lengths upon isolation. These results implicate the UNC-45 CRO1/She4p-like region in the assembly of myosin isoforms in C. elegans and suggest a
possible common mechanism for the function of this
UCS (UNC-45/CRO1/She4p) protein family.
Department of Neurology, Baylor College of Medicine, Houston, Texas 77030
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