Electron-microscopical approach to a structural model of intima collagen

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Abstract
Intima collagen was studied by EM (rotary shadowing and negative staining) and by analytical ultracentrifugation. The monomeric unit (Mr [relative MW] 170,000) consists of a 105 nm-long triple helix terminated by a small globular domain (Mr about 30,000) at 1 end and a large globular domain (Mr about 40,000) at the other end. The monomer was produced by selective reduction of interchain disulfide bridges. Before reduction, dimers, tetramers and larger filamentous structures were found. Dimers are lateral staggered aggregates of 2 monomers aligned in an anti-parallel fashion. This gives rise to an inner 75 nm-long region of 2 slightly intertwisted triple helices flanked by the large globular domains. The outer triple-helical segments (length 30 nm) with the small globular domains at their ends emerge at both sides of this structure. Interchain disulfide bridges are probably located in the vicinity of the large domains. Only the outer segments could be degraded by bacterial collagenase. In tetramers the outer segments of 2 dimers are covalently linked, forming a scissors-like structure. In the fibrous forms several tetramers are assembled end-to-end with an overlap between the outer segments. The molecular masses and sedimentation coefficients were calculated for these various forms from the EM observed dimensions and agreed with results obtained by ultracentrifugation. The unique structure of intima collagen suggests that it originates from a microfibrillar component and that it can be considered a unique collagenous protein, for which the designation type VI collagen is proposed.