Characterization of a Cystine-Rich Polyphenolic Protein Family from the Blue Mussel Mytilus edulis L.

Marine bivalve mollusks synthesize, in the phenol and accessory glands of the foot, proteins that integrate the post-translationally hydroxylated amino acid 3,4-dihydroxyphenylalanine (DOPA) into their primary sequence. These polyphenolic proteins serve as structural and adhesive components of the byssal threads which form the extraorganismic holdfast. One family of byssal precursors, previously characterized in a number of mytiloid species, consists of proteins between 70-130 kDa containing 8-18 mol % DOPA. The high molecular weight precursor isolated from the foot of the blue mussel (Mytilus edulis Linnaeus, 1758) is here designated as Mefp-1. We now present evidence for the occurrence, in M. edulis, of a second, structurally unrelated, family of DOPA proteins (Mefp-2) of about 42-47 kDa. These novel proteins contain 2-3 mol % DOPA and, in startling contrast to Mefp-1, are also enriched in the disulphide-containing amino acid cystine (6-7 mol %). Consideration of the amino acid compositions of Mefp-1 and 2 and of the terminal adhesive plaques of byssal threads suggests that Mefp-2 makes up about 25% of plaque protein, whereas Mefp-1 content is about 5%. The Mefp-2 family exhibits electrophoretic microheterogeneity, but members share similar N- and C-terminal amino acid sequences. Analysis of peptides isolated after tryptic hydrolysis suggests that the primary sequence of Mefp-2 is tandemly repetitive, with at least three types of motif. The sequence degeneracy of the motifs is greater than in Mefp-1. Mefp-2 has minimal sequence homology with known structural proteins and may be a structural element of the plaque matrix.