The location of the epitopes for monoclonal antibodies against chicken type IV and type V collagens were directly determined in the electron microscope after rotary shadowing of antibody/collagen mixtures. Three monoclonal antibodies against type IV collagen were examined, each one of which was previously demonstrated to be specific for only one of the three pepsin-resistant fragments of the molecule. The three native fragments were designated (F1)2F2, F3, and 7S, and the antibodies that specifically recognize each fragment were called, respectively, IA8 , IIB12 , and ID2 . By electron microscopy, monoclonal antibody IA8 recognized an epitope located in the center of fragment (F1)2F2 and in tetramers of type IV collagen at a distance of 288 nm from the 7S domain, the region of overlap of four type IV molecules. Monoclonal antibody IIB12 , in contrast, recognized an epitope located only 73 nm from the 7S domain. This result therefore provides direct visual evidence that the F3 fragment is located closest to the 7S domain and the order of the fragments must be 7S-F3-(F1)2F2. The epitope for antibody ID2 was located in the overlap region of the 7S domain, and often several antibody molecules were observed to binding to a single 7S domain. The high frequency with which antibody molecules were observed to bind to fragments of type IV collagen suggests that there is a single population of type IV molecules of chain organization [alpha 1(IV)]2 alpha 2(IV), and that four identical molecules must form a tetramer that is joined in an antiparallel manner at the 7S domain. The monoclonal antibodies against type V collagen, called AB12 and DH2 , were both found to recognize epitopes close to one another, the epitopes being located 45-48 nm from one end of the type V collagen molecule. The significance of this result still remains uncertain, but suggests that this site is probably highly immunoreactive. It may also be related to the specific cleavage site of type V collagen by selected metalloproteinases and by alpha-thrombin. This cleavage site is also known to be located close to one end of the type V molecule.

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