An Atypical Tropomyosin in Drosophila with Intermediate Filament-like Properties.

A longstanding mystery has been the absence of cytoplasmic intermediate filaments (IFs) from Drosophila despite their importance in other organisms. In the course of characterizing the in vivo expression and functions of Drosophila Tropomyosin (Tm) isoforms, we discovered an essential but unusual product of the Tm1 locus, Tm1-I/C, which resembles an IF protein in some respects. Like IFs, Tm1-I/C spontaneously forms filaments in vitro that are intermediate in diameter between F-actin and microtubules. Like IFs but unlike canonical Tms, Tm1-I/C contains N- and C-terminal low-complexity domains flanking a central coiled coil. In vivo, Tm1-I/C forms cytoplasmic filaments that do not associate with F-actin or canonical Tms. Tm1-I/C is essential for collective border cell migration, in epithelial cells for proper cytoarchitecture, and in the germline for the formation of germ plasm. These results suggest that flies have evolved a distinctive type of cytoskeletal filament from Tm.