Cross-linking in adhesive quinoproteins: Studies with model decapeptides

Mytilus edulis foot protein-1 (mefp1) is a major component of the byssus, a n adhesive holdfast in mussels. The recent report of 5,5'-di(dihydroxypheny l-L-alanine) (diDOPA) cross-links in byssus [McDowell et al. (1999) J. Biol . Chem. 274, 20293] has raised questions about the relationship of these to mefp1. About 80% of the primary structure of mefp1 consists of a tandemly repeated consensus sequence Ala(1)-Lys(2)-Pro(3)-Ser(4)-Tyr(5)-Pro(6)-Pro(7 )-Thr(8)-Tyr(9)-Lys(10) with varying degrees of posttranslational hydroxyla tion to hydroxyprolines in positions 3, 6, and 7 and to DOPA in positions 5 and 9. Six natural or synthetic variants of this decapeptide were subjecte d to oxidation by tyrosinase or periodate. DOPA is the only residue to suff er losses in all oxidized peptides. Moreover, using MALDI TOF mass spectrom etry, oxidized decapeptides all showed evidence of multimer formation and a mass loss of 6 Da per coupled pair of peptides. Multimer formation was inh ibited by addition of DOPA-like o-diphenols, but addition of simple amines such as free Lys had no effect. The results are consistent with aryloxy cou pling to diDOPA followed by reoxidation to diDOPA quinone. There are subtle but noteworthy variations, however, in multimer formation among the peptid e congeners. Decapeptides with Pro(3) modified to trans-4-hydroxyproline do not form multimers beyond dimers; they also exhibit significant Lys losses following oxidation of DOPA. Moreover, in Ala-Lys-Hyp-Ser-Tyr-DiHyp-Hyp-Th r-DOPA-Lys, Tyr appears to be protected from oxidation by tyrosinase.