Comparative electron-microscope studies on type-III and type-I collagens

Long-spacing-segment crystallites prepared from type III collagen with the chain composition [alpha1(III)]3 and type I collagen with the chain composition [alpha(I)]i12alpha2 have been compared in the electron microscope after positive. staining with phosphotungstic acid and uranyl acetate. The comparison revealed several differences in intensities of the cross-striation bands as well as significant differences in band positions. The latter occur most prominently in three distinct regions of the crystallites. Further, crystallites prepared from type III collagen contain an additional intensely staining band in an area corresponding to the carboxy-terminal end of the molecule. The latter band is still observed following limited pepsin digestion and presumably represents a slight elongation of the helical portion of the type III molecule when compared to the type I molecule. In spite of the somewhat altered distribution of charged groups as indicated in studies on the long-spacing-segment crystallites, type III molecules are capable of forming fibrils of the native type with a cross-striation pattern and periodicity virtually identical to that observed when type I molecules are precipitated as native fibers.