Discovery of a second SALMFamide gene in the sea urchin Strongylocentrotus purpuratus reveals that L-type and F-type SALMFamide neuropeptides coexist in an echinoderm species.

The SALMFamides are a family of neuropeptides that act as muscle relaxants in the phylum Echinodermata. Two types of SALMFamides have been identified in echinoderms: firstly, the prototypical L-type SALMFamide peptides with the C-terminal sequence Leu-X-Phe-NH(2) (where X is variable), which have been identified in several starfish species and in the sea cucumber Holothuria glaberrima; secondly, F-type SALMFamide peptides with the C-terminal sequence Phe-X-Phe-NH(2), which have been identified in the sea cucumber Apostichopus japonicus. However, the genetic basis and functional significance of the occurrence of these two types of SALMFamides in echinoderms are unknown. Here we have obtained a new insight on this issue with the discovery that in the sea urchin Strongylocentrotus purpuratus there are two SALMFamide genes. In addition to a gene encoding seven putative F-type SALMFamide neuropeptides with the C-terminal sequence Phe-X-Phe-NH(2) (SpurS1-SpurS7), which has been reported previously (Elphick and Thorndyke, 2005; J. Exp. Biol., 208, 4273-4282), we have identified a gene that is expressed in the nervous system and that encodes a precursor of two putative L-type SALMFamide neuropeptides with the C-terminal sequences Ile-His-Phe-NH(2) (SpurS8) and Leu-Leu-Phe-NH(2) (SpurS9). Our discovery has revealed for the first time that L-type and F-type SALMFamide neuropeptides can coexist in an echinoderm species but are encoded by different genes. We speculate that this feature of S. purpuratus may apply to other echinoderms and further insights on this issue will be possible if genomic and/or neural cDNA sequence data are obtained for other echinoderm species.