Ultrastructural morphology and domain structure of a unique collagenous component of basement membranes

A disulfide cross-linked collagenous fragment (7 S) has been isolated by pepsin solubilization from several tissues rich in basement membranes including bovine lung, human placenta, and the murine EHS tumor. Examination of this material by the rotary shadowing technique indicates that these fragments are similar to but not identical with the 7S collagen described recently [Risteli, J., Bächinger, H.P., Engel, J., Furthmayr, H., & Timpl, R. (1980) Eur. J. Biochem. 108, 239-250]. The central rodlike portion of the particles was found to be similar in length; however, the peripheral four arms of 7S particles from bovine and murine sources are 10 nm longer in comparison to those from human sources. In addition, about 5-7% of all the particles contain a fifth arm. Specific antibodies to bovine 7 S cross-react with murine 7 S but only to a rather limited extent with human 7 S. These antibodies react with antigenic sites located at the ends of the peripheral arms of the fragment as visualized directly with rotary shadowing techniques. The data are consistent with a structural difference in type IV collagens from bovine, human, and mouse which leads to pepsin cleavage at different sites in a particular noncollagenous region adjacent to 7 S. However, since bovine 7S antibodies cross-react with human and murine tissues by immunofluorescence despite the lack of complete serological cross-reactivity, it is suggested that type IV collagens from all three species have some degree of homology in this region.