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The SALMFamides are a family of neuropeptides that act as muscle relaxants in echinoderms. Analysis of genome/transcriptome sequence data from the sea urchin Strongylocentrotus purpuratus (Echinoidea), the sea cucumber Apostichopus japonicus (Holothuroidea), and the starfish Patiria miniata (Asteroidea) reveals that in each species there are two types of SALMFamide precursor: an L-type precursor comprising peptides with a C-terminal LxFamide-type motif and an F-type precursor solely or largely comprising peptides with a C-terminal FxFamide-type motif. Here, we have identified transcripts encoding SALMFamide precursors in the brittle star Ophionotus victoriae (Ophiuroidea) and the feather star Antedon mediterranea (Crinoidea). We have also identified SALMFamide precursors in other species belonging to each of the five echinoderm classes. As in S. purpuratus, A. japonicus, and P. miniata, in O. victoriae there is one L-type precursor and one F-type precursor. However, in A. mediterranea only a single SALMFamide precursor was found, comprising two peptides with a LxFamide-type motif, one with a FxFamide-type motif, five with a FxLamide-type motif, and four with a LxLamide-type motif. As crinoids are basal to the Echinozoa (Holothuroidea + Echinoidea) and Asterozoa (Asteroidea + Ophiuroidea) in echinoderm phylogeny, one model of SALMFamide precursor evolution would be that ancestrally there was a single SALMFamide gene encoding a variety of SALMFamides (as in crinoids), which duplicated in a common ancestor of the Echinozoa and Asterozoa and then specialized to encode L-type SALMFamides or F-type SALMFamides. Alternatively, a second SALMFamide precursor may remain to be discovered or may have been lost in crinoids. Further insights will be obtained if SALMFamide receptors are identified, which would provide a molecular basis for experimental analysis of the functional significance of the "cocktails" of SALMFamides that exist in echinoderms.
Figure 1. Patiria miniata SALMFamide precursors. (A) L-type SALMFamide precursor. (B) F-type SALMFamide precursor. The predicted signal peptide of the precursor proteins is shown in blue and the putative SALMFamide neuropeptides are shown in red, with C-terminal glycine residues that are potential substrates for amidation shown in orange. Putative cleavage sites are shown in green.
Figure 2. Comparison of the structure of SALMFamide genes in the sea urchin S. purpuratus and the starfish P. miniata. (A). Diagrammatic representation of the structure of the L-type SALMFamide precursor gene in S. purpuratus and P. miniata. (B). Diagrammatic representation of the structure of the F-type SALMFamide precursor gene in S. purpuratus and P. miniata. Exons are drawn to scale as rectangles and introns are shown as lines but not to scale (intron lengths are shown in parentheses). Non-coding exons or regions of exons are shown in white; note that the 5′ and 3′ of the genes are shown as a dashed line because it is possible that additional non-coding bases are transcribed in vivo but are not present in transcripts that have been sequenced. The protein-coding exons or regions of exons are shaded (gray) or colored, with the N-terminal signal peptide shown in blue and the predicted neuropeptide products of the precursors shown in red (L-type SALMFamides) or yellow (F-type SALMFamides). The numbers above the protein-coding exons are amino-acid residue positions in the protein products of the genes. A feature that appears to distinguish L-type and F-type SALMFamide genes is the structure of the first protein-coding exons. Thus, in the two L-type SALMFamide genes the first protein-coding exon also encodes a 5′ non-coding region, whilst in the two F-type SALMFamide genes the first protein-coding exon does not have a 5′ non-coding region. These differences in gene structure provide important evidence that the L-type SALMFamide precursor genes in S. purpuratus and P. miniata are orthologs and the F-type SALMFamide precursor genes in S. purpuratus and P. miniata are orthologs.
Figure 3. Ophionotus victoriae and Antedon mediterranea SALMFamide precursors. (A)
O. victoriae L-type SALMFamide precursor. (B)
O. victoriae F-type SALMFamide precursor. (C)
A. mediterranea SALMFamide precursor. The predicted signal peptide of the precursor proteins is shown in blue, putative neuropeptides derived from the precursors are shown in red with C-terminal glycine residues that likely substrates for amidation shown in orange. Putative cleavage sites are shown in green.
Figure 4. The occurrence and properties of SALMFamide precursors (A) and putative SALMFamide peptides (B) in species representing each of the five extant echinoderm classes. (A) SALMFamide precursors are shown in a phylogenetic diagram according to the phylogeny of Telford et al. (18) and O’Hara et al. (21), with crinoids basal to the Echinozoa (Holothuroidea and Echinoidea) and the Asterozoa (Asteriodea + Ophiuroidea). The estimated divergence times for the nodes (labeled with numbers in pentagons) according to O’Hara et al. (21) are: 1. 501-542 Ma, 2. 482-421 Ma, 3. 464-485 Ma, and 4. at least 479 Ma. S. pur is the sea urchin Strongylocentrotus purpuratus (Echinoidea), A. jap is the sea cucumber Apostichopus japonicus (Holothuroidea), P. min is the starfish Patiria miniata (Asteroidea), O. vic is the brittle star Ophionotus victoriae (Ophiuroidea), and A. med is the feather star Antedon mediterranea (Crinoidea). Signal peptides are shown in blue and dibasic or monobasic cleavage sites are shown in green. L-type SALMFamides with a C-terminal LxFamide motif or with an L-type-like motif (e.g., IxFamide) are shown in red. F-type SALMFamides with a FxFamide motif or with an F-type-like motif (e.g., YxFamide) are shown in yellow. SALMFamides with a FxLamide-type motif are shown in orange and SALMFamides with a LxLamide motif are shown in dark red. Peptides that do not conform with any of the four color-coded categories are shown in white (e.g., GVPPYVVKVTYamide in A. japonicus and SRLPFHSGLMQamide in O. victoriae). The diagram shows how in a presumed ancestral-type precursor in crinoids the majority of the putative peptides have a FxLamide-type motif or a LxLamide-type motif and there is only one L-type SALMFamide and one F-type SALMFamide. However, as a consequence of specialization following a presumed duplication of the ancestral-type gene in a common ancestor of the Echinozoa and Asterozoa, two types of SALMFamide precursor have evolved: one that is predominantly comprised of L-type SALMFamides (red) and another that is exclusively or predominantly comprised of F-type SALMFamides (yellow). (B) C-terminal alignments of SALMFamide neuropeptides derived from the precursor proteins shown in (A). The C-terminal regions of each peptide are color-coded according to the key shown in (A).
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