Karl E. Kadler

Pleomorphism in type I collagen fibrils produced by persistence of the procollagen N-propeptide

The assembly of type I collagen and type I pN-collagen was studied in vitro using a system for generating these molecules enzymatically from their immediate biosynthetic precursors. Collagen generated by C-proteinase digestion of pC-collagen formed D-periodically banded fibrils that were essentially cylindrical (i.e. circular in cross-section). In contrast, pN-collagen generated by C-proteinase digestion of procollagen formed thin, sheet-like structures that were axially D-periodic in longitudinal section, of varying lateral widths (up to several microns) and uniform in thickness (approximately 8 nm). Mixtures of collagen and pN-collagen assembled to form a variety of pleomorphic fibrils. With increasing pN-collagen content, fibril cross-sections were progressively distorted from circular to lobulated to thin and branched structures. Some of these structures were similar to fibrils observed in certain heritable disorders of connective tissue where N-terminal procollagen processing is defective. The observations are considered in terms of the hypothesis that the N-propeptides are preferentially located on the surface of a growing assembly. The implications for normal diameter control of collagen fibrils in vivo are discussed.

Procollagen Processing Control of Type I Collagen Fibril Assembly

Vertebrate collagens constitute a family of at least twelve genetic types that shows remarkable diversity in molecular structure and supramolecular assembly (Mayne & Burgeson, 1987). Types I, II and III collagens assemble in vivo to form fibrils of uniform diameter, near circular cross-section and with a characteristic axial periodicity of 65 to 67 nm (D). Fibrils in vivo are long (several µm) and diameters range from 8 nm to 500 nm, depending on collagen type, species, age and tissue of origin (Parry & Craig, 1984). The mechanisms that control fibril shape and diameter in vivo are poorly understood.

Effects of Mutations that Change Primary Structure of Collagen on the Self-Assembly of the Protein into Fibrils

We have recently observed that a single base mutation in a gene for type I procollagen converts a glycine residue to cysteine and that the substitution for the glycyl residue has a remarkable effect both on the conformation of the molecule and the morphology of the fibrils that are formed as the mutated procollagen molecule is processed to collagen (Vogel et al., 1987; 1988; Kadler et al., 1988b). The observations have largely been made possible through the development of a new system for examining the self-assembly of collagen de novo (Kadler et al., 1987; 1988a).