Examination of the skeletal proteome of the brittle star Ophiocoma wendtii reveals overall conservation of proteins but variation in spicule matrix proteins.

Background: While formation of mineralized tissue is characteristic of many animal taxa, the proteins that interact with mineral are diverse and appear in many cases to be of independent origin. Extracellular matrix proteins involved in mineralization do share some common features. They tend to be disordered, secreted proteins with repetitive, low complexity. The genes encoding these proteins are often duplicated and undergo concerted evolution, further diversifying the repetitive domains. This makes it difficult to identify mineralization genes and the proteins they encode using bioinformatics techniques. Here we describe the use of proteomics to identify mineralization genes in an ophiuroid echinoderm, Ophiocoma wendtii (O. wendtii).

Results: We have isolated the occluded proteins within the mineralized tissue of the brittle star Ophiocoma wendtii. The proteins were analyzed both unfractionated and separated on SDS-PAGE gels. Each slice was analyzed using mass spectroscopy and the amino acid sequence of the most prevalent peptides was obtained. This was compared to both an embryonic transcriptome from the gastrula stage when skeleton is being formed and a tube foot (an adult mineralized tissue) transcriptome. Thirty eight proteins were identified which matched known proteins or protein domains in the NCBI databases. These include C-type lectins, ECM proteins, Kazal-type protease inhibitors, matrix metalloproteases as well as more common cellular proteins. Many of these are similar to those found in the sea urchin Strongylocentrotus purpuratus (S. purpuratus) skeleton. We did not, however, identify clear homologs to the sea urchin spicule matrix proteins, and the number of C-type lectin containing genes was much reduced compared to sea urchins. Also notably absent was MSP-130.

Conclusions: Our results show an overall conservation of the types of proteins found in the mineralized tissues of two divergent groups of echinoderms, as well as in mineralized tissues in general. However, the extensive gene duplication and concerted evolution seen in the spicule matrix proteins found in the sea urchin skeleton was not observed in the brittle star.