Abstract
The byssal attachment of California mussels Mytilus californianus provides secure adhesion in the presence of moisture, a feat that still eludes most synthetic polymers. Matrix-assisted laser desorption ionization mass spectrometry was used to probe the footprints of byssal attachment plaques on glass cover slips for adhesive proteins. Besides the abundant mcfp-3 protein family (Zhao, H., Robertson, N. B., Jewhurst, S. A., and Waite, J. H. (2006) J. Biol. Chem. 281, 11090-11096), two new proteins, mcfp-5 and mcfp-6, with masses of 8.9 kDa and 11.6 kDa, respectively, were identified in footprints, partially characterized and completely sequenced from a cDNA library. mcfp-5 resembles mcfp-3 in its basic pI and abundant 3,4-dihydroxyphenyl-L-alanine (Dopa; 30 mol %), but is distinct in two respects: it is more homogeneous in primary sequence and is polyphosphorylated. mcfp-6 is basic and contains a small amount of Dopa (<5 mol %). In contrast to mcfp-3 and -5, tyrosine prevails at 20 mol %, and cysteine is present at 11 mol %, one-third of which remains thiolate. Given the oxidative instability of Dopa and cysteine at pH 8.2 (seawater), we tested the hypothesis that thiols serve to scavenge dopaquinones by adduct formation. Plaque footprints were hydrolyzed and screened for cysteine dopaquinone adducts using phenylboronate affinity chromatography. 5-S-Cysteinyldopa was detected at nearly 1 mol %. The results suggest that mcfp-6 may provide a cohesive link between the surface-coupling Dopa-rich proteins and the bulk of the plaque proteins.
Highlights
Given that engineering durable adhesive bonds between minerals and organic polymers in the presence of moisture remains a serious technological challenge, fundamental insights into the mechanism of holdfast adhesion in mussels and other sessile marine organisms represent a potential data base of bio-inspired solutions to the moisture problem [1]
If Dopa moieties of adsorbed proteins interact with the mineral surface, what interactions define their binding to other proteins in the adhesive plaque? Of the two new proteins detected in the footprints, mcfp-6 is a thiol-rich protein that may mediate coupling of the surface proteins with those in the plaque by cysteinyldopa cross-links
The adhesive footprints of M. californianus contain several distinct proteins that are detected in situ by MALDI-TOF mass spectrometry
Summary
Given that engineering durable adhesive bonds between minerals and organic polymers in the presence of moisture remains a serious technological challenge, fundamental insights into the mechanism of holdfast adhesion in mussels and other sessile marine organisms represent a potential data base of bio-inspired solutions to the moisture problem [1]. One popular technique for improving “wet” adhesion on siliceous substrates involves the application of surface-coupling agents or adhesion promoters [2]. The use of surface-coupling agents to promote adhesion resonates with the adhesive biochemistry of byssal plaques made by mussels. With its many Dopa residues, mcfp-3 has been compared with a multifunctional surface-coupling agent [4]. If multiple Dopa side chains represent the surface-ligating moiety in mcfp-3, there must be another moiety that is specialized for reactivity with other plaque proteins. The present investigation was undertaken to determine whether there is a sidedness to the reactivity of mussel adhesive proteins with surfaces. If Dopa moieties of adsorbed proteins interact with the mineral surface, what interactions define their binding to other proteins in the adhesive plaque? If Dopa moieties of adsorbed proteins interact with the mineral surface, what interactions define their binding to other proteins in the adhesive plaque? Of the two new proteins detected in the footprints (mcfp-5 and mcfp-6), mcfp-6 is a thiol-rich protein that may mediate coupling of the surface proteins with those in the plaque by cysteinyldopa cross-links
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