ENZYMATIC CONTROL OF COLLAGEN FIBRIL SHAPE

The shape of collagen fibrils growing in vitro in a cell-free enzyme/s ubstrate system is shown to be dependent on the enzyme/substrate (E/S) ratio. Long fibrils with tapered ends were generated by exposing pCco llagen (procollagen from which the N-propeptides had been removed) to procollagen C-proteinase (which acts by cleaving the C-propeptides fro m the pCcollagen, converting it to insoluble fibril-forming collagen). Tip shape profiles, established quantitatively by scanning transmissi on electron microscopy, depended critically on the C-proteinase/pCcoll agen ratio. The finest tips occurred at low ratios, the coarsest at hi gh ratios. All fibrils had molecules oriented with amino termini close st to the pointed ends, i.e. N,N-bipolar fibrils in which molecules ch ange orientation abruptly at one location along the fibril. Fibrils ha d maximal diameter at this molecular switch region. Shape asymmetric f ibrils occurred at low E/S ratios, near-shape symmetric fibrils occurr ed at high ratios. Fibrils generated at low E/S ratios bore the closes t resemblance to those formed in vivo except that the central shaft re gions of fibrils formed in vitro showed no tendency to be limited to a uniform diameter. (C) 1996 Academic Press Limited