β-Filagenin, a Newly Identified Protein Coassembling with Myosin and Paramyosin in Caenorhabditis elegans

Muscle thick filaments are stable assemblies of myosin and associated proteins whose dimensions are precisely regulated. The mechanisms underlying the stability and regulation of the assembly are not understood. As an approach to these problems, we have studied the core proteins that, together with paramyosin, form the core structure of the thick filament backbone in the nematode Caenorhabditis elegans. We obtained partial peptide sequences from one of the core proteins, beta-filagenin, and then identified a gene that encodes a novel protein of 201-amino acid residues from databases using these sequences. beta-Filagenin has a calculated isoelectric point at 10.61 and a high percentage of aromatic amino acids. Secondary structure algorithms predict that it consists of four beta-strands but no alpha-helices. Western blotting using an affinity-purified antibody showed that beta-filagenin was associated with the cores. beta-Filagenin was localized by immunofluorescence microscopy to the A bands of body-wall muscles, but not the pharynx. beta-filagenin assembled with the myosin homologue paramyosin into the tubular cores of wild-type nematodes at a periodicity matching the 72-nm repeats of paramyosin, as revealed by immunoelectron microscopy. In CB1214 mutants where paramyosin is absent, beta-filagenin assembled with myosin to form abnormal tubular filaments with a periodicity identical to wild type. These results verify that beta-filagenin is a core protein that coassembles with either myosin or paramyosin in C. elegans to form tubular filaments.