Paramyosin gene (unc-15) of Caenorhabditis elegans. Molecular cloning, nucleotide sequence and models for thick filament structure

Paramyosin is a major structural component of thick filaments isolated from many invertebrate muscles. The Caenorhabditis elegans paramyosin gene (unc-15) was identified bv screening with specific antibodies an "exon-expression" library containing lacZ/nematode gene fusions. Short probes recovered from the library were used to identify bacteriophage λ and cosmid clones that encompass the entire paramyosin (unc-15) gene. From these clones, numerous subclones containing epitopes reacting with anti-paramyosin sera were obtained, providing strong evidence that the initial cloned fragment was, in fact, derived from the structural gene for paramyosin. The complete nucleotide sequence of a 12 × 10³ base-pair region spanning the gene was obtained. The gene is composed of ten short exons encoding a protein of 868 amino acid residues. Paramyosin is highly similar to residues 267 to 1089 of myosin heavy chain rods. For most of its length, paramyosin appears to form an α-helical coiled-coil and shows the expected heptad repeat of hydrophobic amino acid residues and the 28-residue repeat of charged amino acids characteristic of myosin heavy chain rods. However, paramyosin differs from myosin in having non-helical extensions at both the N and C termini and an additional "skip" residue that interrupts the 28-residue repeat. The distribution of charges along the length of the paramyosin rod is also significantly different from that of myosin heavy chain rods. Potential charge-mediated interactions between paramyosin rods and between paramyosin and myosin rods were calculated using a model successfully applied previously to the analysis of the myosin rod sequences. Myosin rods aligned in parallel show optimal charge-charge interactions at multiples of 98 residue staggers (i.e. at axial displacements of multiples of 143 Å). Paramyosin rods, in contrast, appear to interact optimally at parallel staggers of 493 residues (i.e. at axial displacements of 720 Å) but show only weak interaction peaks at 98 or 296 residues. Similar calculations suggest optimal interactions between paramyosin molecules and myosin rods and in their anti-parallel alignments. The implications of these results for the structure of the bare zone and the assembly of nematode thick filaments are discussed.