COLLAGEN FIBRILS INVITRO GROW FROM POINTED TIPS IN THE C-TERMINAL TO N-TERMINAL DIRECTION

Growth of collagen fibrils was examined in a system in which collagen monomers are generated by specific enzymic cleavage of type I pCcollagen with procollagen C-proteinase. Fibrils formed at 37°C had highly tapered and symmetrical pointed tips. The pattern of cross-striations in the pointed tips indicated that all the molecules were oriented so that the N-termini were directed towards the tip. At 29°C and 32°C, the fibrils formed were thicker. One end of fibrils formed at 29°C was blunt, and the other was pointed. The spear-like projection then became a new pointed tip for growth in the opposite direction. The results suggested a model for fibril growth with at least three distinct binding sites for monomers. In the model, the pointed tip is the site with the highest affinity for the binding of monomers and most probably defines the critical concentration for fibril assembly. The main shaft of the fibril is a site with very low affinity for binding. The blunted end defines a low-affinity binding site where monomers can bind in opposite orientation to produce growth from a new pointed end.