Segregation of Type I Collagen Homo- and Heterotrimers in Fibrils

Normal type I collagen is a heterotrimer of two alpha1(I) and one alpha2(I) chains, but various genetic and environmental factors result in synthesis of homotrimers that consist of three alpha1(I) chains. The homotrimers completely replace the heterotrimers only in rare recessive disorders. In the general population, they may compose just a small fraction of type I collagen. Nevertheless, they may play a significant role in pathology; for example, synthesis of 10-15% homotrimers due to a polymorphism in the alpha1(I) gene may contribute to osteoporosis. Homotrimer triple helices have different stability and less efficient fibrillogenesis than heterotrimers. Their fibrils have different mechanical properties. However, very little is known about their molecular interactions and fibrillogenesis in mixtures with normal heterotrimers. Here we studied the kinetics and thermodynamics of fibril formation in such mixtures by combining traditional approaches with 3D confocal imaging of fibrils, in which homo- and heterotrimers were labeled with different fluorescent colors. In a mixture, following a temperature jump from 4 to 32 degrees C, we observed a rapid increase in turbidity most likely caused by formation of homotrimer aggregates. The aggregates promoted nucleation of homotrimer fibrils that served as seeds for mixed and heterotrimer fibrils. The separation of colors in confocal images indicated segregation of homo- and heterotrimers at a subfibrillar level throughout the process. The fibril color patterns continued to change slowly after the fibrillogenesis appeared to be complete, due to dissociation and reassociation of the pepsin-treated homo- and heterotrimers, but this remixing did not significantly reduce the segregation even after several days. Independent homo- and heterotrimer solubility measurements in mixtures confirmed that the subfibrillar segregation was an equilibrium property of intermolecular interactions and not just a kinetic phenomenon. We argue that the subfibrillar segregation may exacerbate effects of a small fraction of alpha1(I) homotrimers on formation, properties, and remodeling of collagen fibers.