Molecular structure of short-chain (SC) cartilage collagen by electron microscopy

We have recently observed that aged and/or hypertrophying chondrocytes in culture synthesize a small collagen molecule termed short-chain (SC) collagen. Our previous biochemical studies have suggested that this molecule is slightly less than half the length of "typical" interstitial collagens and should have both a helical, collagenous domain and a nonhelical, globular one. In the present study we have examined the structure of this molecule by electron microscopy of rotary-shadowed preparations and segment-long-spacing crystallites. Rotary-shadowed SC collagen molecules appear as rods with a length of 132 nm and a knob at one end. Preparations of native molecules that have been treated by limited pepsin digestion show only the rod-like domain. These results are consistent with the rod-like domain having the molecular structure of a collagen helix, which is refractory to pepsin digestion, and the knob representing a globular, nonhelical domain. Segment-long-spacing crystallites of pepsin-digested molecules confirm the length of the helical domain to be 132 nm. Positively stained crystallites show a banding pattern different from other collagens.