For these fibril experiments, atelo-collagen was prepared as described above

For these fibril experiments, atelo-collagen was prepared as described above. polymers associating to establish chemical equilibrium among higher-order species shows trends inG andG consistent with our experimental observations, including a concentration-dependent crossover inG/caround 300 Hz. This work suggests that telopeptides facilitate transient intermolecular interactions between collagen proteins, even in the acidic conditions used here. == Introduction == Collagen is the predominant structural protein in vertebrates, where it represents more than one-quarter of the protein in our bodies. Collagens supramolecular structure as ordered fibrils provides connective tissues their ability to withstand stress and confers mechanical properties to the extracellular matrix that play a role Rabbit Polyclonal to ZNF134 in influencing cellular development (1, 2, 3). Given its preponderance and easy extraction from tissues, it is not surprising that PF-04217903 collagen has found use in a wide variety of materials applications (4). The majority of these physiological and materials functions rely on the ability of collagen to form hierarchically-structured assemblies. From isolated triple-helical collagen proteins, collagen forms highly ordered fibrils, whose striking degree of ordering is seen in its so-called D-banding, a striped pattern that reflects the differential molecular density that repeats along the fibril axis (Fig. 1a) (5). Self-assembly of collagen can be replicated in vitro, and leads to PF-04217903 fibrils exhibiting the same D-banding as observed in tissue-extracted fibrils. For this reason, in vitro manipulations are widely used to study collagen self-assembly. == Figure 1 . == (a) Hierarchical organization pathway of fibrillar collagen. A collagen protein consists of an extended triple helix, flanked at both ends by short nonhelical domains called telopeptides that are indicated within the dashed circles. Collagens associate laterally to form well-ordered fibrils exhibiting a characteristic D-banding pattern, associated with overlap and gap regions of higher and lower collagen density along the fibril, and indicated by the white/dark striped pattern in the bottom schematic. (b) Schematic of a bead trapped via optical tweezers in a solution of collagen molecules (not to scale). Short-range thermal fluctuations of the bead are used to determine the viscoelastic properties of the surrounding collagen solution. To see this figure in color, go online. Fibril assembly occurs in three phases: nucleation, in which a critical number of collagen molecules form a core association; growth, representing the lateral and longitudinal assembly into fibrils; and saturation, in which the solution is depleted of free collagen proteins and fibril growth terminates. Kinetics of this assembly process are affected by many different parameters, including solution conditions such as pH, ionic strength, identity of ions, and temperature (5, 6, 7, 8, 9). They are also influenced by collagens molecular composition. Most strikingly, the removal of collagens ends, called telopeptides, drastically slows fibril assembly (10, 11, 12). Telopeptides contribute less than 5% of the overall molecular weight and length to the 300 kDa, 300-nm-long collagen protein, and unlike the rest of collagen, are not triple helical. Because they are not bound in a triple helix, they can be proteolytically cleaved by noncollagenolytic proteases, a process that is performed using pepsin during high-yield extraction of collagen from tissue. Although telopeptides influence the kinetics of fibril formation, they do not strongly affect fibrillar structure: the information required for assembly is encoded within the sequence and structure PF-04217903 of the triple helix itself (10, 13). In vivo, telopeptides serve a vital role by forming intermolecular cross-links that stabilize fibrillar organization and contribute to tensile strength (14, 15). X-ray studies have revealed that the C-terminal telopeptides of one collagen in a fibril lie directly adjacent to the key matrix metalloprotease (MMP) cleavage site of a neighboring collagen, suggesting that their presence may play a role in regulating collagen fibril degradation (16). Supporting the physiological role of telopeptides, telopeptide fragments have been used as markers for disease progression, for example , in osteoporosis, arthritis, and cancer (17, 18, 19, 20). Despite their specific locations within PF-04217903 fibrils, the presence or absence of telopeptides does not influence the lateral forces between collagens within a fibril, and telopeptides.