Elastomeric gradients: a hedge against stress concentration in marine holdfasts?

Abstract

The byssal threads of marine mussels are elastomeric fibres with a great capacity for absorbing and dissipating energy. Up to 70% of the total absorbed energy can be dissipated in the byssus. Because byssal threads attach the mussel to hard inert surfaces in its habitat, they must combine the need to be good shock absorbers with appropriate matching of Young's modulus between living tissue and a hard substratum such as stone-stiffnesses that can differ by five orders of magnitude. Recent data suggest that improved modulus matching and decreased stress concentration between different portions of the byssus is achieved by the use of protein gradients. Protein gradients in byssal threads are constructed using natural macromolecular chimeras having a central collagenous domain, variable flanking modules and histidine-rich amino and carboxy termini. Stiff silk-like flanking modules prevail distally, while at the animal end, rubbery modules resembling elastin predominate. In between the two thread ends there is a mix of both module types. The histidine-rich termini provide metal binding/cross-linking sites, while collagen domains may confer self-assembly on all parts of the structure. A graded axial distribution of flanking modules is expected to moderate stress concentration in joined materials having disparate moduli.

Structural and functional features of a collagen-binding matrix protein from the mussel byssus

“…Interestingly, the C termini of A1 and A2 are coupled in PTMP1, suggesting that a mechanism regulating their ligand affinity could affect both domains concomitantly. However, in light of lack of shear dependent collagen binding in our SPR experiments (data not shown), it remains enigmatic if PTMP1 could be a shear dependent bi-functional cross-linker as postulated previously 9 . Furthermore, other than with integrins 14 , collagen binding is not strictly dependent on the presence of bound metal, with collagen binding being slightly enhanced in the presence of zinc ions in PTMP1 (Fig.…”

“…It has previously been hypothesized that PTMP1, like other VWA domain containing proteins, binds with and interconnects byssal collagens 9,11 . Due to the lack of available PreCol proteins, we analysed binding of heterologous collagens to PTMP1 and its isolated VWA domains using surface plasmon resonance spectroscopy (SPR) 13,27,28 , since all fibrous collagens show similar structural features.…”

“…1c). The overall collagenous content in the proximal section is much lower, with matrix proteins representing 34% of the dry weight 9 ( Fig. 1d).…”

“…1d). Some of these matrix proteins have been identified previously, but only assumptions exist concerning their structure and function [9][10][11] .…”

Secretion of byssal threads in Mytilus galloprovincialis: quantitative and qualitative values after spawning stress

“…Specifically for the case of histidine, 19 it has been reported a functionality with a pronounced effect on metal chelation and/or cross-20 link ability (Waite et al 1998) as well as the capacity to form a significant part (up to 22 mol% 21 in protein mcfp-4) of the junction between collagen fibres and foam-like adhesive plaques in 22 the mussel Mytilus californianus (Zhao and Waite 2006). Whenever histidine-rich domains 23 occur in proteins, they usually bind with metal and, byssal collagen of Mytilusdomains that can help to utilise more metal chelate cross-link for byssal stability and integrity 1 (Lucas et al 2002). According to its functionality in the cross-link potential, M. 2 galloprovincialis would be expected to produce stronger and stiffer threads by virtue of having 3 more histidine in the flanking domains of all its precols that might help to counteract lower 4 byssus secreted in those mussels recently spawned.…”