Type I collagen fibrillogenesis in vitro. Additional evidence for the assembly mechanism.

The intensity of scattered light at angles between 30 degrees and 120 degrees has been measured during the heat gelation lag phase of rat tail tendon collagen. As previously reported, the intensity at a scattering angle of 90 degrees does not change during the lag phase of gelation, whereas during this period, the intensity extrapolated to zero degrees more than doubles. Based on measurement of the Rayleigh factor at low angles, it is concluded that the lag phase terminates when the molecular weight is greater than 930,000 which is consistent with the formation of a linear 4D staggered trimer as previously proposed. Once lateral growth begins, the molecular weight continues to increase approximately linearly with time until a molecular weight of 4 X 10(6) (5 +/- 1 trimers) is reached, at which time the rate of increase of molecular weight increases significantly. It is concluded that a trimer with about 5 strands forms during the early phases of lateral growth and appears similar to the microfibrillar unit proposed based on x-ray diffraction modeling. Further growth occurs by linear and lateral addition of the trimeric units in a manner still under investigation.