The adhesive plaques of Mytilus byssus are investigated increasingly to determine the molecular requirements for wet adhesion. Mfp-2 is the most abundant protein in the plaques, but little is known about its function. Analysis of Mfp-2 films using the surface forces apparatus detected no interaction between films or between a film and bare mica; however, addition of Ca2+ and Fe3+ induced significant reversible bridging (work of adhesion Wad ≈ 0.3 mJ/m2 to 2.2 mJ/m2) between two films at 0.35 m salinity. The strongest observed Fe3+-mediated bridging approaches the adhesion of oriented avidin-biotin complexes. Raman microscopy of plaque sections supports the co-localization of Mfp-2 and iron, which interact by forming bis- or tris-DOPA-iron complexes. Mfp-2 adhered strongly to Mfp-5, a DOPA-rich interfacial adhesive protein, but not to another interfacial protein, Mfp-3, which may in fact displace Mfp-2 from mica. In the presence of metal ions or Mfp-5, Mfp-2 adhesion was fully reversible. These results suggest that plaque cohesiveness depends on Mfp-2 complexation of metal ions, particularly Fe3+ and also by Mfp-2 interaction with Mfp-5 at the plaque-substratum interface.

Mussel Fibers While it is possible to make strong fibers or threads from organic materials, most suffer from high wear abrasion. Marine mussels attach themselves to rocky seashores using a series of byssal threads. Despite the constant rubbing caused by the motion of the tides, the threads show high wear resistance. Harrington et al. (p. 216, published online 4 March; see the Perspective by Messersmith) now find that the threads are protected by a proteinaceous outer cuticle that is rich in the amino acid 3,4-dihydroxyphenylalanine (dopa), which is known to be a strong adhesive. The cuticle is also rich in metal ions, primarily Fe3+. The dopa-metal crosslinks helped to form the tough outer coating. Marine mussel byssal threads have an outer coating in which proteins are linked to metal ions. The extensible byssal threads of marine mussels are shielded from abrasion in wave-swept habitats by an outer cuticle that is largely proteinaceous and approximately fivefold harder than the thread core. Threads from several species exhibit granular cuticles containing a protein that is rich in the catecholic amino acid 3,4-dihydroxyphenylalanine (dopa) as well as inorganic ions, notably Fe3+. Granular cuticles exhibit a remarkable combination of high hardness and high extensibility. We explored byssus cuticle chemistry by means of in situ resonance Raman spectroscopy and demonstrated that the cuticle is a polymeric scaffold stabilized by catecholato-iron chelate complexes having an unusual clustered distribution. Consistent with byssal cuticle chemistry and mechanics, we present a model in which dense cross-linking in the granules provides hardness, whereas the less cross-linked matrix provides extensibility.