An Atypical Tropomyosin in Drosophila with Intermediate Filament-like Properties

• Flies express Tm1-I/C, an atypical Tropomyosin that forms 13- to16-nm filaments in vitro
• Canonical isoforms Tm1-A and TM1-L redundantly recruit myosin to stress fibers in vivo
• Tm1-A and Tm1-L localize with actin in vivo, but Tm1-I/C filaments localize with microtubules
• Tm1-I/C is essential in migratory border cells, germ cells, and epithelial follicle cells

SummaryA 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.

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