MYOSIN ROD PROTEIN - A NOVEL THICK FILAMENT COMPONENT OF DROSOPHILA MUSCLE

Myosin rod protein (MRP), a 155 kDa protein encoded by a gene internal to the Drosophila muscle myosin heavy chain (Mhc) gene, contains the MHC rod domain, but has 77 unique N-terminal residues that exactly rep lace the MHC motor and light chain binding domains. Originally describ ed as an abundant testis protein, we now demonstrate the MRP also is a major component of myofilaments in Drosophila. Specifically, the Mrp promoter directs the expression of a LacZ reporter transgene in somati c, cardiac and visceral muscles. MRP-specific antibodies detect the pr otein in detergent-insoluble fractions of muscle extracts and co-local ize the protein with MHC to the sarcomeric A-band in immunostained mus cles. Immunoblot analysis shows that in a set of adult direct flight m uscles (DFM), the ratio of MRP to MHC is 1:3. Chemical cross-link and co-immunoprecipitation experiments using 0.5 M KCl-extracted thick fil ament proteins indicate that native MRP is a homodimer. Electron micro scopy of DFM49, which has a high MRP content, shows in cross section, disordered myofilament packing and a variable thin to thick filament r atio and, in longitudinal section, severely bent thin filaments that a re not well associated with thick filaments. In rigor, thick filaments from DFM49 consist of segments with cross bridges that are interspers ed with smooth domains lacking cross bridges. These data indicate that MRP is a novel contractile protein that co-integrates With myosin int o the thick filament, thereby changing structure and function or: the sarcomere. (C) 1997 Academic Press Limited