Expression of multiple forms of an adhesive plaque protein in an individual mussel, Mytilus edulis

Abstract

Individual blue mussels, Mytilus edulis L., can express at least 20 variants of a small protein known as M. edulis foot protein 3 or Mefp3. Mefp3 has been shown to be a component of the adhesive plaque of the byssus, the structure securing mussels to solid substrata. The cDNAs and deduced fp3 protein sequences display more variation at the carboxy-terminus than at the N-terminus, although there is some variation present throughout the protein. This indicates that there most likely are multiple copies of the gene encoding this protein. Each protein sequence contains a signal peptide, 24 to 25 residues in length, and a mature protein sequence of 44 to 54 residues. Gly is the most common amino acid in the mature protein at 20 to 25 mol%. Tyr and Arg follow closely at 20 to 23 and 16 to 21 mol%, respectively. Both of these amino acids were previously shown to be post-translationally modified to 3,4-dihydroxyphenylalanine (Dopa) and 4-hydroxyarginine, respectively, in this protein. MALDI-TOF (matrix-assisted laser desorption ionization with time-of-flight) mass spectrometric analysis of the underside of adhesive plaques reveals the presence of Mefp3-like proteins. Curiously, only four or five out of 20 possible fp3 variants are detectable in plaques deposited on glass or plastic. This would suggest that selection of protein variants for deposition onto surfaces is determined at the level of translation.