Collagen XVI harbors an integrin alpha1 beta1 recognition site in its C-terminal domains

Collagen XVI is integrated tissue-dependently into distinct fibrillar aggregates, such as D-banded cartilage fibrils and fibrillin-1-containing microfibrils. In skin, the distribution of collagen XVI overlaps that of the collagen-binding integrins alpha1 beta1 and alpha2 beta1. Basal layer keratinocytes express integrin alpha2 beta1, whereas integrin alpha1 beta1 occurs in smooth muscle cells surrounding blood vessels, in hair follicles, and on adipocytes. Cells bearing the integrins alpha1 beta1 and alpha2 beta1 attach and spread on recombinant collagen XVI. Furthermore, collagen XVI induces the recruitment of these integrins into focal adhesion plaques, a principal step in integrin signaling. Of potential physiological relevance, these integrin-collagen XVI interactions may connect cells with specialized fibrils, thus contributing to the organization of fibrillar and cellular components within connective tissues. In cell-free binding assays, collagen XVI is more avidly bound by alpha1 beta1 integrin than by alpha2 beta1 integrin. Both integrins interact with collagen XVI via the A domain of their alpha subunits. A tryptic collagen XVI fragment comprising the collagenous domains 1-3 is recognized by alpha1 beta1 integrin. Electron microscopy of complexes of alpha1 beta1 integrin with this tryptic collagen XVI fragment or with full-length collagen XVI revealed a unique alpha1 beta1 integrin-binding site within collagen XVI located close to its C-terminal end.