Sequential assembly of collagen revealed by atomic force microscopy.

Most polymers which comprise biological filaments assemble by two mechanisms: nucleation and elongation or a sequential, stepwise process involving a hierarchy of intermediate species. We report the application of atomic force microscopy (AFM) to the study of the early events in the sequential or stepwise mode of assembly of a macromolecular filament. Collagen monomers were assembled in vitro and the early structural intermediates of the assembly process were examined by AFM and correlated with turbidimetric alterations in the assembly mixture. The assembly of collagen involved a sequence of distinctive filamentous species which increased in both diameter and length over the time course of assembly. The first discrete population of collagen oligomers were 1-2 nm in diameter (300-500 nm in length); at later time points, filaments approximately 2-6 nm in diameter (> 10 microns in length) many with a conspicuous approximately 67-nm axial period were observed. Occasional mature collagen fibrils with a approximately 67-nm axial repeat were found late in the course of assembly. Our results are consistent with initial end-to-end axial association of monomers to form oligomers followed by lateral association into higher-order filaments. On this basis, there appears to be at least two distinctive types of structural interactions (axial and lateral) which are operative at different levels in the assembly hierarchy of collagen.