Sequences in the myosin A rod interact with UNC‐89/obscurin and the zinc‐finger protein UNC‐98 during thick filament assembly and M‐line formation in C. elegans striated muscle

Abstract

The M‐line of striated muscle is a complex structure that anchors myosin‐containing thick filaments and also participates in signaling and proteostasis. While the physical associations among many M‐line components have been defined, the mechanism of thick filament attachment is not completely understood. In Caenorhabditis elegans, myosin A is essential for viability and forms the site of M‐line attachment at the center of the filament, whereas myosin B forms the filament arms. Using a mutant myosin A that forms ectopic filaments, we examined interactions between myosin A and M‐line proteins in intact muscle cells. Ectopic myosin A recruits the giant kinase UNC‐89/obscurin, a presumed scaffolding protein, in an interaction that requires the zinc‐finger protein UNC‐98, but not UNC‐82/NUAK, UNC‐97/PINCH, or UNC‐96. In myosin A mutants, UNC‐89/obscurin patterning is highly defective in embryos and adults. A chimeric myosin containing 169 residues of the myosin A C‐terminal rod, coincident with the UNC‐98/ZnF binding site, is sufficient for colocalization of UNC‐89/obscurin and UNC‐98/ZnF in M‐line structures whereas a myosin chimera lacking these residues colocalizes with UNC‐89/obscurin in M‐lines that lack UNC‐98. Thus, at least two myosin A rod regions contribute independently to M‐line organization. We hypothesize that these M‐line‐organizing functions correspond to the essential “filament initiation function” performed by this isoform.