In mammalian striated muscle, six actin-organizing formin proteins have been shown to influence sarcomere organization, but their precise molecular roles in this process remain obscure. In C. elegans, only the formin FHOD-1 works in a muscle cell-autonomous manner to influence sarcomere structure in body-wall muscle (BWM), providing a simpler system to determine the mechanism by which these highly conserved proteins influence the muscle cytoskeleton. Worms bearing a
fhod-1 allele that is a putative null for effects on actin organization display modest defects in BWM strength, cell growth and sarcomere assembly, arguing against a primary role in assembling the actin-based sarcomeric thin filaments. However,
fhod-1 muscle cells assemble fewer sarcomeres, and the muscle-specific myosin heavy chain MYO-3 is subject to elevated proteasome-dependent proteolysis. Moreover, the dense bodies that serve as thin filament-anchoring sites in BWM are malformed. Comparison of the distribution of immature and mature dense bodies in wild-type and
fhod-1 BWM cells, and the spatial relationship of FHOD-1 to those structures, suggests FHOD-1 accumulates at specific foci in the BWM cell to assist the formation of new dense bodies there.