Collagen XI nucleates self-assembly and limits lateral growth of cartilagefibrils

Fibrils of embryonic cartilage are heterotypic alloys formed by collagens I I, IX, and XI and have a uniform diameter of similar to 20 nm, The molecula r basis of this lateral growth control is poorly understood. Collagen II su bjected to fibril formation in vitro produced short and tapered tactoids wi th strong D-periodic banding. The maximal width of these tactoids varied ov er a broad range. By contrast, authentic mixtures of collagens II, ni and X I yielded long and weakly banded fibrils, which, strikingly, had a uniform width of about 20 nm, The same was true for mixtures of collagens II and XI lacking collagen IX as long as the molar excess of collagen II was less th an 8-fold. At higher ratios, the proteins assembled into tactoids coexistin g with cartilage-like fibrils, Therefore, diameter control is an inherent p roperty of appropriate mixtures of collagens II and XI, Collagen IX is not essential for this feature but strongly increases the efficiency of fibril formation. Therefore, this protein may be an important stabilizing factor o f cartilage fibrils.