Proteomic Identification of Novel Proteins from the Calcifying Shell Matrix of the Manila Clam Venerupis Philippinarum

The shell of the Manila clam Venerupis philippinarum is composed of more than 99% calcium carbonate and of a small amount of organic matrix (around 0.2%). In this study, we developed one of the first proteomic approaches applied to mollusc shell in order to characterise the matrix proteins that are believed to be essential for the formation of the biomineral. The insoluble organic matrix, purified after demineralisation of the shell powder with cold acetic acid (5%), was digested with trypsin enzyme and then separated on nano-LC prior to nanospray/quadrupole time-of-flight analysis. MS/MS spectra were searched against the above 11,000 EST sequences available on the NCBI public database for Venerupis. Using this approach, we were able to identify partial or full-length sequence transcripts that encode for shell matrix proteins. These include three novel shell proteins whose sequences do not present any homologous proteins or already described domains, two putative protease inhibitor proteins containing Kazal-type domains, and a putative Ca2+-binding protein containing two EF-hand domains. Biomineral formation and evolutionary implications are discussed.