Mi-Mi et al. describe two actin-nucleating proteins that assemble actin filaments into the sarcomeres of worm muscle cells.

Formin family proteins stimulate actin polymerization in vitro, but the functions of individual family members in vivo remain unclear. Mi-Mi et al. found that a formin called FHOD-1 was strongly expressed in the body wall muscle cells of C. elegans larvae. These cells contract to help worms swim and crawl, and, just like vertebrate skeletal muscle, they appear striated due to the regular arrangement of actin and myosin filaments into repeating units called sarcomeres.

FHOD-1 localized around each sarcomere's Z lines, the sites where actin filaments are anchored. Worms lacking FHOD-1 had relatively normal muscles, however, so Mi-Mi et al. looked for additional formins that might help organize sarcomeric actin. The formin CYK-1 also localized to Z lines, and, although CYK-1 mutant worms also had a mild muscle phenotype, worms lacking CYK-1 and FHOD-1 developed very small body wall muscles with only a few sarcomeres in each cell.

FHOD-1 expression was switched off in adult worms, leaving CYK-1 in sole charge of maintaining the sarcomeric actin network. Adult worms lacking CYK-1 developed increasingly disorganized contractile lattices. Senior author David Pruyne says it's unclear why nematodes need two formins to build muscle sarcomeres; one possibility is that FHOD-1 and CYK-1 generate two distinct sets of actin filaments at slightly different sites in the contractile network. Pruyne now wants to investigate how the formins are recruited to developing sarcomeres and to determine how their activity is regulated.

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J. Cell Biol.