A Glycosylated Byssal Precursor Protein from the Green Mussel Perna viridis with Modified Dopa Side-chains

Foot tissue of the green mussel Perna viridis contains a variety of byssal precursor proteins with the unusual redox-active amino acid, Dopa (L-β-3,4-dihydroxyphenyl-α-alanine). Eight proteins were detectable in acidic extracts of the Perna foot by a redox cycling assay with nitroblue tetrazolium. In one of these, however, P. viridis foot protein-1 (Pvfp-1), activity was not due to Dopa, but to another redox-active derivative. Based on specific colorimetric derivatization with Arnow's reagent, ninhydrin and phenylisothiocyanate (Edman), mass spectrometry, the redox-active derivative in Pvfp-1 is not consistent with any known modification. Another uncommon modification of Pvfp-1 involves O-glycosylation of threonine by mannose, glucose or fucose. As in previously characterized fp-1s, the primary sequence of the Pvfp-1 (apparent mass 89?kDa) has two consensus decapeptide motifs; one is APPKPX₁TAX₂K and the other is APPPAX₁TAX₂K, where P is Pro/Hyp, and X₁ and X₂ are difucosylated threonine and a redox sensitive derivative of tyrosine or Dopa, respectively. Of these two unusual residues, X₂ is unique to Pvfp-1, whereas O-glycosylated Thr has been previously detected in freshwater mussel fp-1. The sequence homology of Pvfp-1 with the common structural motifs of the fp-1 protein family strongly suggests that the Pvfp-1 functions as the byssal coating (lacquer) protein.