A Transport Form of Collagen from Embryonic Tendon: Electron Microscopic Demonstration of an NH2-Terminal Extension and Evidence Suggesting the Presence of Cystine in the Molecule

When cells were isolated from chickembryo tendons and incubated in vitro for 2-6 hr, essentially all the newly-synthesized collagen was recovered from the incubation medium as a transport form larger than tropocollagen. Experiments in which cells were incubated with [ 14 C]cystine suggested that the transport form contained cystine and that it was, in part, stabilized by disulfide bonds. Electron microscopy of segment-long-spacing aggregates prepared from the transport form of collagen showed that the native molecule differed from tropocollagen in that it had an extension of about 13 nm (130 Å) at the NH 2 -terminal end.