Buccalin-like and myomodulin-like peptides in the stomatogastric ganglion of the crab Cancer borealis.

The buccalins and myomodulins are families of peptides originally isolated and purified from the accessory radula closer (ARC) nerve–muscle system of the marine mollusc Aplysia californica (Cropper et al. 1987a,b, 1988). In A. californica, the buccalin family consists of 19 related peptides (buccalins A–S), each of which exhibits a homologous carboxyl terminal sequence, Phe-X-Gly-Gly-X-NH2 (Cropper et al. 1988; Miller et al. 1993a). A cDNA clone encoding a buccalin precursor polypeptide has recently been isolated (Miller et al. 1993a). This buccalin gene appears to be present as a single copy in the A. californica genome. The precursor peptide predicted from this gene encodes all 19 buccalin-like molecules, several in multiple copies. The myomodulin family consists of nine related peptides (myomodulins A–I), all possessing a common Met-Leu-Arg-Leu-NH2 carboxyl terminal sequence (Cropper et al. 1987a; Miller et al. 1993b). As with buccalin, a cDNA clone encoding a myomodulin prohormone has been isolated from A. californica (Miller et al. 1993b). This cDNA clone encodes seven of the nine myomodulins (A, B, D and F–I), with multiple copies of myomodulin A. Because myomodulins C and E are not found in the prohormone, but have been biochemically isolated, additional myomodulin transcripts must be present. Both the buccalins and myomodulins have pronounced physiological actions on nerve and muscle preparations in A. californica. For example, buccalin A decreases the size of contractions and excitatory junction potentials in several of the ARC muscles (Cropper et al. 1990) as well as increasing the excitability and duration of action potentials in a number of mechanosensory neurons (Raymond et al. 1989; Rosen et al. 1989). Myomodulin enhances the contractility of the ARC complex (Cropper et al. 1987a,b) in addition to producing varied effects in a number of peripherally located neurons (Cleary