This article was retracted on November 22, 2004

Normal muscle fiber development (top) goes wrong without paramyosin (bottom).

Liu et al. (page 899) genetically disrupted the gene for paramyosin, a major component of invertebrate muscle thick filaments, in Drosophila melanogaster. In addition to the expected finding, that the disruption damages myofibril structure, the work identifies a surprising requirement for paramyosin in myoblast fusion.

Mutant flies in which a P-element has been excised from the paramyosin promoter region die during embryonic development. In addition to defects in myofibril assembly, the embryos exhibit sporadic failures in myoblast fusion, leading to the absence of some muscle fibers. Antibody localization experiments show paramyosin in discrete foci at sites near the junctions of fusing myoblasts, and nonmuscle myosin appears to colocalize to the same sites.

The authors propose that, in the fly, paramyosin functions as a cytoskeletal protein during early development. In this model, paramyosin and nonmuscle myosin form minifilaments that interact with the actin cytoskeleton and promote myoblast fusion. After myoblast fusion, nonmuscle myosin is then replaced by muscle myosin to produce thick filament precursors, ultimately leading to the construction of normal thick filaments. With or without its myosin partners, paramyosin could function as intermediate filament proteins do in vertebrates, attaching to membrane bound proteins to aid in fusion. Since Drosophila lacks cytoskeletal intermediate filaments, paramyosin may have been borrowed for this function during evolution. ▪