Hyperunstable matrix proteins in the byssus of Mytilus galloprovincialis

Abstract

SUMMARYThe marine mussel Mytilus galloprovincialis is tethered to rocks in the intertidal zone by a holdfast known as the byssus. Functioning as a shock absorber, the byssus is composed of threads, the primary molecular components of which are collagencontaining proteins (preCOLs) that largely dictate the higher order self-assembly and mechanical properties of byssal threads. The threads contain additional matrix components that separate and perhaps lubricate the collagenous microfibrils during deformation in tension. In this study, the thread matrix proteins (TMPs), a glycine-, tyrosine-and asparagine-rich protein family, were shown to possess unique repeated sequence motifs, significant transcriptional heterogeneity and were distributed throughout the byssal thread. Deamidation was shown to occur at a significant rate in a recombinant TMP and in the byssal thread as a function of time. Furthermore, charge heterogeneity presumably due to deamidation was observed in TMPs extracted from threads. The TMPs were localized to the preCOL-containing secretory granules in the collagen gland of the foot and are assumed to provide a viscoelastic matrix around the collagenous fibers in byssal threads. Supplementary material available online at

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

“…In static ELISA-like assays 28 , PTMP1 bound to a broad range of immobilized soluble heterologous collagens (type I, II, III, IV, V), as well as the collagen-resembling triple-helical peptide (PPG) 10 , with affinities in the sub-micromolar range (Fig. 5c,d), as calculated from curve fits according to a one-site binding model.…”

“…Since PTMP1 is also capable of binding triple-helical structures without pronounced electrostatic interactions, like the collagenresembling triple-helical peptide (PPG) 10 , and the isolated VWA domains bound collagen with much less affinity than when combined, the collagen-binding mode of PTMP1 differs from that of integrins. In addition, the binding differs to that of von Willebrand factor since PTMP1 lacks nearly all of the critical interacting residues of the vWFA3 domain 12 .…”

“…The combination of a high structural stability and rigidity of the PTMP1 molecule mediated by its inter-domain linker, its affinity for collagens, and its abundance in the proximal thread section certainly is indicative of a structural and mechanical contribution of PTMP1 to the byssus thread. Furthermore, the impact of matrix proteins like PTMP1 on collagen fibril formation and assembly is obvious when fibres are artificially drawn from purified PreCol proteins 40,41 , since these fibres were different from the natural byssal threads with respect to size and mechanical characteristics 10 . Concerning the establishment of collagenous materials with new properties, the in vitro function of PTMP1 might inspire material scientists to adjust morphologies and also mechanical features of reconstituted collagen fibres to be used in different applications, for example, in the generation of gradient materials inspired by the mussel byssus 38 .…”

“…Insoluble contents were removed by centrifugation, and the concentration was adjusted to 1 mg ml À 1 in 1% acetic acid. The collagen-resembling triple-helical peptide (PPG) 10 (PeptaNova) was diluted in 10 mM HCl (30 mg ml À 1 ). A polyclonal antiserum against fulllength PTMP1 was created in a rabbit (Pineda antibody service, Berlin) according to the manufacturer's standard immunization protocol and tested for specific detection of PTMP1 and its variants in immunoblots.…”

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