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MBC in Press, published online ahead of print January 11, 2006
Mol. Biol. Cell 10.1091/mbc.E05-09-0886

A more recent version of this article appeared on March 1, 2006
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Submitted on September 23, 2005
Revised on December 30, 2005
Accepted on January 4, 2006

Conditional Dominant Mutations in the C. elegans Gene act-2 Identify Cytoplasmic and Muscle Roles for a Redundant Actin Isoform

John H. Willis,* Edwin Munro,{dagger} Rebecca Lyczak,*{ddagger} and Bruce Bowerman*

*Institute of Molecular Biology, University of Oregon, Eugene, OR 97403; {dagger}Center for Cell Dynamics and Friday Harbor Laboratories, University of Washington, Friday Harbor, WA 98250

Monitoring Editor: Sandra Schmid

Animal genomes each encode multiple highly conserved actin isoforms that polymerize to form the microfilament cytoskeleton. Previous studies of vertebrates and invertebrates have shown that many actin isoforms are restricted to either nonmuscle (cytoplasmic) functions, or to myofibril force generation in muscle cells. We have identified two temperature-sensitive and semidominant embryonic-lethal C. elegans mutants, each with a single mis-sense mutation in act-2, one of five C. elegans genes that encode actin isoforms. These mutations alter conserved and adjacent amino acids predicted to form part of the ATP binding pocket of actin. At the restrictive temperature, both mutations resulted in aberrant distributions of cortical microfilaments associated with abnormal and striking membrane ingressions and protrusions. In contrast to the defects caused by these dominant mis-sense mutations, an act-2 deletion did not result in early embryonic cell division defects, suggesting that additional and redundant actin isoforms are involved. Accordingly, we found that two additional actin isoforms, act-1 and act-3, were required redundantly with act-2 for cytoplasmic function in early embryonic cells. The act-1 and -3 genes also have been implicated previously in muscle function. We found that an ACT-2::GFP reporter was expressed cytoplasmically in embryonic cells and also was incorporated into contractile filaments in adult muscle cells. Furthermore, one of the dominant act-2 mutations resulted in uncoordinated adult movement. We conclude that redundant C. elegans actin isoforms function in both muscle and nonmuscle contractile processes.

{ddagger}Present Address: Biology Department, Ursinus College, Collegeville, PA 19426.

Address correspondence to: Bruce Bowerman (bbowerman{at}molbio.uoregon.edu)






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