Abstract
The SALMFamides are a family of neuropeptides that act as muscle relaxants in echinoderms. Two types of SALMFamides have been identified: L-type (e.g. the starfish neuropeptides S1 and S2) with the C-terminal motif LxFamide (x is variable) and F-type with the C-terminal motif FxFamide. In the sea urchin Strongylocentrotus purpuratus (class Echinoidea) there are two SALMFamide genes, one encoding L-type SALMFamides and a second encoding F-type SALMFamides, but hitherto it was not known if this applies to other echinoderms. Here we report the identification of SALMFamide genes in the sea cucumber Apostichopus japonicus (class Holothuroidea) and the starfish Patiria miniata (class Asteroidea). In both species there are two SALMFamide genes: one gene encoding L-type SALMFamides (e.g. S1 in P. miniata) and a second gene encoding F-type SALMFamides plus one or more L-type SALMFamides (e.g. S2-like peptide in P. miniata). Thus, the ancestry of the two SALMFamide gene types traces back to the common ancestor of echinoids, holothurians and asteroids, although it is not clear if the occurrence of L-type peptides in F-type SALMFamide precursors is an ancestral or derived character. The gene sequences also reveal a remarkable diversity of SALMFamide neuropeptides. Originally just two peptides (S1 and S2) were isolated from starfish but now we find that in P. miniata, for example, there are sixteen putative SALMFamide neuropeptides. Thus, the SALMFamides would be a good model system for experimental analysis of the physiological significance of neuropeptide “cocktails” derived from the same precursor protein.
Highlights
Secreted neuropeptides mediate intercellular neuronal signalling and are involved in the regulation of diverse physiological processes and behaviours [1]
Apostichopus Japonicus To search for a second SALMFamide precursor in A. japonicus, the S. purpuratus L-type SALMFamide precursor was submitted as a query in a tBLASTn search of 29,666 A. japonicus contigs, which were downloaded from the supporting information section (Dataset S1) of an online paper by Du et al [21] available from the PloSONE website
The C-terminal sequence of the 11-residue peptide (MGFTGNTGILLamide) is ILLamide, which is unusual because a leucine (L) residue replaces the C-terminal phenylalanine (F) residue that is characteristic of both L-type and F-type SALMFamide neuropeptides
Summary
Secreted neuropeptides mediate intercellular neuronal signalling and are involved in the regulation of diverse physiological processes and behaviours [1]. To gain knowledge and understanding of how neuropeptide signalling systems have evolved it is necessary to analyse neuropeptide systems in a wide range of animal phyla. Much less is known about neuropeptide systems in deuterostomian invertebrates, which include invertebrate chordates (cephalochordates and urochordates), hemichordates and echinoderms (starfish, sea urchins, sea cucumbers). Research on these animals has recently begun to provide important insights on the evolution of neuropeptide signalling systems [4]. The pentaradial body plan of adult echinoderms provides a unique context for investigation of the physiological roles of neuropeptide signalling systems [4,5]
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