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The Caenorhabditis elegans unc-78 Gene Encodes a Homologue of Actin-interacting Protein 1 Required for Organized Assembly of Muscle Actin Filaments

Correspondence to: Shoichiro Ono, Department of Pathology, Emory University, 1639 Pierce Dr., Woodruff Memorial Building, Room 7109C, Atlanta, GA 30322. Tel:(404) 727-3916 Fax:(404) 727-8540 E-mail:sono{at}emory.edu.

Assembly and maintenance of myofibrils require dynamic regulation of the actin cytoskeleton. In Caenorhabditis elegans, UNC-60B, a muscle-specific actin depolymerizing factor (ADF)/cofilin isoform, is required for proper actin filament assembly in body wall muscle (Ono, S., D.L. Baillie, and G.M. Benian. 1999. J. Cell Biol. 145:491–502). Here, I show that UNC-78 is a homologue of actin-interacting protein 1 (AIP1) and functions as a novel regulator of actin organization in myofibrils. In unc-78 mutants, the striated organization of actin filaments is disrupted, and large actin aggregates are formed in the body wall muscle cells, resulting in defects in their motility. Point mutations in unc-78 alleles change conserved residues within different WD repeats of the UNC-78 protein and cause less severe phenotypes than a deletion allele, suggesting that these mutations partially impair the function of UNC-78. UNC-60B is normally localized in the diffuse cytoplasm and to the myofibrils in wild type but mislocalized to the actin aggregates in unc-78 mutants. Similar Unc-78 phenotypes are observed in both embryonic and adult muscles. Thus, AIP1 is an important regulator of actin filament organization and localization of ADF/cofilin during development of myofibrils.

Key Words: myofibrils, AIP1, ADF/cofilin, WD repeats, actin filament dynamics

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