Ehlers-Danlos syndrome type VIIB. Morphology of type I collagen fibrils formed in vivo and in vitro is determined by the conformation of the retained N-propeptide.

Previously we showed that fibrils generated from collagen and pNcollagen-ex6 from fibroblasts of an individual with Ehlers-Danlos syndrome (EDS) type VIIB were hieroglyphic in cross-section and all N-propeptides were located at the fibril surface. Hieroglyphs were resolved to near-cylindrical fibrils (that were similar in appearance to the fibrils seen in the tissues of individuals with EDS type VIIB) by treatment with N-proteinase which cleaved the pN alpha 1(I) chains but not the pN alpha 2(I)-ex6 chains (Watson, R. B., Wallis, G. A., Holmes, D. F., Viljoen, D., Byers, P. H., and Kadler, K. E. (1992) J. Biol. Chem. 267, 9093-9100). Here, quantitative scanning transmission electron microscopy (STEM) showed that N-propeptides in hieroglyphs were in a "bent-back" conformation and thus located exclusively in the overlap zone of the fibril D-period (D = 67 nm). In contrast, STEM of fibrils from the dermis of an individual with EDS type VIIB showed that partially cleaved N-propeptides (in which cleaved pN alpha 1(I) remained in noncovalent association with pN alpha 2(I)-ex6 chains) were distributed equally between the gap and overlap zones of the fibrils. Comparison of experimental data with theoretical mass distributions of the fibril based on amino acid sequence data gave a consistent value of 33 nm for the total axial extent for the N-propeptides in hieroglyphic and tissue fibrils irrespective of the location of N-propeptides to the gap or overlap zone. These data exclude the possibility that N-propeptides adopt a random configuration, but rather, that they locate to specific sites in the gap and overlap zones. The results demonstrated that cleavage of pN alpha 1(I) chains in vivo releases the N-propeptides from the constraints of the bent-back conformation. Co-distribution of partially cleaved N-propeptides between gap and overlap zones allows a higher surface packing density of N-propeptides and explains how circularity of large diameter fibrils can be achieved despite the retention of N-propeptides in tissues of individuals with EDS type VIIB.