Efficient binding of native, soluble 125I-labeled type I rat collagen to mouse 3T3 fibroblast monolayers requires prior warming of the ligand to 35-37 degrees C for 10-30 min. Decreased binding at high ligand concentrations is ascribed to ligand-ligand interactions rather than to negative cooperativity. Addition of bacterial collagenase to monolayers labeled with the 125I-ligand releases a constant fraction (80%) of the bound ligand over a 2-h interval at 37 degrees C, indicating that little of the ligand becomes inaccessible by pinocytosis. Colchicine (10(-7) M) and vinblastine (5 X 10(-8) M) do not inhibit binding by morphologically intact monolayers. Cytochalasins and concanavalin A show dose-related inhibition of binding by intact monolayers that is due to a reduction in the number of available binding sites rather than to a change in binding site affinity. The collagen binding site on the fibroblast surface is proposed as an organizing center for the assembly of periodic type I collagen fibrils.

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