The skeletal proteome of the sea star Patiria miniata and evolution of biomineralization in echinoderms.

Background: Proteomic studies of skeletal proteins have revealed large, complex mixtures of proteins occluded within the mineral. Many skeletal proteomes contain rapidly evolving proteins with repetitive domains, further complicating our understanding. In echinoderms, proteomic analysis of the skeletal proteomes of mineralized tissues of the sea urchin Strongylocentrotus purpuratus prominently featured spicule matrix proteins with repetitive sequences linked to a C-type lectin domain. A comparative study of the brittle star Ophiocoma wendtii skeletal proteome revealed an order of magnitude fewer proteins containing C-type lectin domains. A number of other proteins conserved in the skeletons of the two groups were identified. Here we report the complete skeletal proteome of the sea star Patiria miniata and compare it to that of the other echinoderm groups.

Results: We have identified eighty-five proteins in the P. miniata skeletal proteome. Forty-two percent of the proteins were determined to be homologous to proteins found in the S. purpuratus skeletal proteomes. An additional 34 % were from similar functional classes as proteins in the urchin proteomes. Thirteen percent of the P. miniata proteins had homologues in the O. wendtii skeletal proteome with an additional 29% showing similarity to brittle star skeletal proteins. The P. miniata skeletal proteome did not contain any proteins with C-lectin domains or with acidic repetitive regions similar to the sea urchin or brittle star spicule matrix proteins. MSP130 proteins were also not found. We did identify a number of proteins homologous between the three groups. Some of the highly conserved proteins found in echinoderm skeletons have also been identified in vertebrate skeletons.

Conclusions: The presence of proteins conserved in the skeleton in three different echinoderm groups indicates these proteins are important in skeleton formation. That a number of these proteins are involved in skeleton formation in vertebrates suggests a common origin for some of the fundamental processes co-opted for skeleton formation in deuterostomes. The proteins we identify suggest transport of proteins and calcium via endosomes was co-opted to this function in a convergent fashion. Our data also indicate that modifications to the process of skeleton formation can occur through independent co-option of proteins following species divergence as well as through domain shuffling.