Characterization of connectin-like proteins of obliquely striated muscle of a polychaete (Annelida)

Summary In obliquely striated muscle of polychaete, Neanthes sp., three kinds of connectin (titin)-like high molecular weight proteins, 4000 kDa, 1200 kDa and 700 kDa, were detected by SDS gel electrophoresis and immunoblots using antibodies to vertebrate skeletal muscle connectin and antiserum to the protein in question. The 700 kDa protein was isolated and characterized as a sheet-rich filament 170 nm long and 4 nm wide. Using polyclonal antibodies to the 700 kDa protein, the binding of the immunogold to the thick filament was only demonstrated in high ionic strength relaxing solution which solubilized some myosin. This observation suggested that the 700 kDa protein was localized below the layers of myosin in the thick filament and this localization is different from that of twitchin of C. elegans bodywall muscle that is on the surface of thick filament. The 4000 kDa protein was identified as a very thin filament linking the thick filament to the dense body. The very thin filaments were visualized in gelsolin-treated actin filament-free fibres. The 1200 kDa protein was located in the periphery of the dense body. A model of the elastic filament in polychaete bodywall muscle is presented.