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J. Cell Biol.,
Volume 145, Number 3, May 3, 1999 491-502
* Department of Pathology and Department of Cell Biology, Emory University, Atlanta, Georgia 30322; and The Caenorhabditis elegans unc-60 gene encodes two functionally distinct isoforms of ADF/cofilin
that are implicated in myofibril assembly. Here, we
show that one of the gene products, UNC-60B, is specifically required for proper assembly of actin into myofibrils. We found that all homozygous viable unc-60 mutations resided in the unc-60B coding region, indicating
that UNC-60B is responsible for the Unc-60 phenotype.
Wild-type UNC-60B had F-actin binding, partial actin
depolymerizing, and weak F-actin severing activities in
vitro. However, mutations in UNC-60B caused various
alterations in these activities. Three missense mutations
resulted in weaker F-actin binding and actin depolymerizing activities and complete loss of severing activity. The r398 mutation truncated three residues from
the COOH terminus and resulted in the loss of severing
activity and greater actin depolymerizing activity. The
s1307 mutation in a putative actin-binding helix caused
greater activity in actin-depolymerizing and severing.
Using a specific antibody for UNC-60B, we found varying protein levels of UNC-60B in mutant animals, and
that UNC-60B was expressed in embryonic muscles.
Regardless of these various molecular phenotypes, actin was not properly assembled into embryonic myofibrils in all unc-60 mutants to similar extents. We conclude that precise control of actin filament dynamics by UNC-60B is required for proper integration of actin
into myofibrils.
Institute of
Molecular Biology and Biochemistry, Department of Biological Sciences, Simon Fraser University, Burnaby, British
Columbia, Canada V5A 1S6
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