# Routes towards Novel Collagen-Like Biomaterials
## 3 Mussel Byssus-Silk, Collagen or Both?
### Hierarchical Assembly and Structure of preCols in the Byssus  
During biosynthesis, the preCol monomers form homotrimers of their unfolded \(\alpha\)-chains before being transported into storage vesicles. During this stage, the rod-like trimers further assemble into hexagonal higher-order 7 + 1 structures, in which they are stored until the byssus neogenesis triggers secretion (Figure 3) [90,91]. The morphology of these pre-secreted structures suggest that the collagen domain has a large influence on the early stages of assembly, whereas the other functional groups, such as the flanking domains and the His/DOPA-rich termini are kept inactive until they get molded into the final byssus thread within the mussel foot groove. This activation is most likely a result of the preCols coming into contact with a different chemical environment (oxidizing, high pH), the sudden presence of matrix proteins which have been shown to interact with preCols [92], and the influence of mechanical stimuli introduced by contractions within the mussel foot.  
As mentioned above, the proximal section of the mussel byssus differs from classical silk in that it comprises a dedicated mix of matrix proteins which embed the load-bearing, fibrous preCols. In this portion of the byssus, matrix proteins, in particular the proximal thread matrix protein (PTMP)1 with two von [PERSON] factor-like domains (which makes up about 30% of the proximal byssus), strongly interact with the preCol-assemblates and is thought to have a great influence on the higher order assembly of preColP and NG [93]. The current understanding is that PTMP1 builds a soft matrix with the ability to bind and lubricate the rod-like preCol during deformation, thereby forming a classical fiber-reinforced polymer with the ability to mitigate shear-induced damage to the comparably stiff collagen fibrils, while still allowing enough movement for the flanking domains to exercise their elastic behavior (Figure 4).  
Given that the distal portion of the fiber contains more than 90% (\(w/w\)) preCols, however, many of the physical attributes of the distal byssus must stem from the intrinsic ability of preColD to form stable assemblies without requiring the interaction with other mussel foot proteins. Most of its properties can therefore be explained by the hierarchical arrangement of preCols, which is a result of the block-like character of the protein. Instead of being a fiber reinforced matrix, the distal byssus thread is more  
Figure 4: The proximal part of the mussel byssus is elastic and contains fibrils of preColP embedded in a matrix of PTMP1 (proximal thread matrix protein 1). This matrix protein allows the fibrils to slide freely and act as within a fiber-reinforced composite material. The distal portion of the mussel byssus has a much higher stiffness, in addition to the ability to dissipate large amounts of mechanical energy without taking irreversible damage. The content of matrix protein in the distal thread is much lower and the collagen domains, instead of reinforcing an amorphous matrix, are tightly packed around silk-like crystalline \(\beta\)-sheets. Modified with permission from [89](Elsevier, 2014).  
like a particle reinforced block-copolymer, with the \(\beta\)-sheet crystals formed by the preColD-flanks embedded in a densely packed collagen matrix (Figure 3).  
This would explain the threefold higher prevalence of defects in the collagen sequence (\"nicks\") of preColD compared to preColP: Instead of being the ultimate load-bearing structure within a soft matrix (proximal byssus thread), the distal collagen domain of preColD is the flexible component in a particle reinforced system during low-stress situations and therefore absorbs small amounts of force before stronger mechanical loads result in the gradual unfolding of sacrificial bonds within the His/DOPA-termini and, subsequently, the \(\beta\)-crystalline flanks [94].