Glycosylated Hydroxytryptophan in a Mussel Adhesive Protein from Perna viridis

The 3,4-dihydroxyphenyl-l-alanine (Dopa)-containing proteins of mussel byssus play a critical role in wet adhesion and have inspired versatile new synthetic strategies for adhesives and coatings. Apparently, however, not all mussel adhesive proteins are beholden to Dopa chemistry. The cDNA-deduced sequence of Pvfp-1, a highly aromatic and redox active byssal coating protein in the green mussel Perna viridis, suggests that Dopa may be replaced by a post-translational modification of tryptophan. The N-terminal tryptophan-rich domain of Pvfp-1 contains 42 decapeptide repeats with the consensus sequences ATPKPW₁TAW₂K and APPPAW₁TAW₂K. A small collagen domain (18 Gly-X-Y repeats) is also present. Tandem mass spectrometry of isolated tryptic decapeptides has detected both C²-hexosylated tryptophan (W₁) and C²-hexosylated hydroxytryptophan (W₂), the latter of which is redox active. The UV absorbance spectrum of W₂ is consistent with 7-hydroxytryptophan, which represents an intriguing new theme for bioinspired opportunistic wet adhesion.The amino acid 3,4-dihydroxyphenyl-l-alanine (Dopa)³ occurs in many proteins of the mussel holdfast or byssus (1, 2) and has recently been incorporated into mussel-inspired synthetic polymers with versatile adhesive consequences (3–7). One byssal protein in particular, mussel foot protein-1 (Mfp-1), has been investigated from over 15 mussel byssi, where it protectively coats compliant collagen-like proteins in the thread core (8–11). Mfp-1s typically contain 10–15 mol % Dopa in a highly conserved repeating peptide structure (8). In the blue mussel Mytilus edulis fp-1 (Mefp-1), for example, the consensus decapeptide AKPSYPPTYK is repeated over 70 times in tandem, and much of the tyrosine (Y) is converted to Dopa (Y*) (Fig. 1). In stark contrast to this, only trace levels of Dopa were detected in Pvfp-1 from the green mussel Perna viridis (Linnaeus 1758) (12), a notoriously invasive fouling species originally from the Indo-Pacific region (13). Because P. viridis fp-1 (Pvfp-1) and its homologue, Mefp-1, are both strongly aromatic, quinogenic, and composed of highly polar decapeptide repeats (12), identifying the Dopa-mimetic substitutes in Pvfp-1 has been a matter of considerable interest.Open in a separate windowFIGURE 1.The common blue mussel M. edulis and green mussel P. viridis are shown with attached byssal threads. Distinct consensus decapeptide repeat sequences are associated with the repeat domains in fp-1 foot proteins of the two species. The amino acid Dopa (Y*, right panel) is prominent in mefp-1 repeats but absent from Pvfp-1. O denotes trans-4-hydroxyprolines. Residues denoted as X₁ and X₂ are shown by this study to be derived from tryptophan.