Structural basis for apparent heterogeneity of collagens in human basement membranes: type IV procollagen contains two distinct chains.

Fetal cells isolated from human amniotic fluid synthesize type IV procollagen when grown in monolayer culture. The procollagen, which contains two biochemically distinct chains, was found to be structurally and immunologically related to type IV collagen chains and collagenous fragments isolated from human placenta. Limited pepsin digestion of the intact procollagen that was deposited in the cell layer during culture produced a heterogeneous population of collagenous peptides comparable to that obtained during isolation of type IV collagens from human tissues. These studies support the hypothesis that basement membranes contain at least two genetically distinct type IV procollagen chains and suggest that the heterogeneity of collagenous components obtained after pepsin digestion of tissues and isolated basement membranes can result from degradative cleavage of the procollagen at a limited number of protease-sensitive sites.