UNC-60B, an ADF cofilin family protein, is required for proper assembly ofactin into myofibrils in Caenorhabditis elgans body wall muscle

The Caenorhabditis elegans unc-60 gene encodes two functionally distinct is oforms of ADF/cofilin that are implicated in myofibril assembly. Here, we s how that one of the gene products, UNC-60B, is specifically required for pr oper 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 activitie s in vitro. However, mutations in UNC-60B caused various alterations in the se 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 resu lted in the loss of severing activity and greater actin depolymerizing acti vity. The s1307 mutation in a putative actin-binding helix caused greater a ctivity 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 vario us molecular phenotypes, actin was not properly assembled into embryonic my ofibrils in all unc-60 mutants to similar extents. We conclude that precise control of actin filament dynamics by UNC-60B is required for proper integ ration of actin into myofibrils.