ANNEXIN-V INTERACTIONS WITH COLLAGEN

Annexin V was originally identified as a collagen-binding protein call ed anchorin CII and was isolated from chondrocyte membranes by affinit y chromatography on native type II collagen. The binding of annexin V to native collagen type II is stable at physiological ionic strength w hen annexin V is reconstituted in liposomes. The binding to native col lagen types II and X, and to some extent to type I as well, was confir med using recombinant annexin V. A physiological role for annexin V in teractions with extracellular collagen is consistent with the localiza tion of annexin V on the outer cell surface of chondrocytes, microvill i of hypertrophic chondrocytes, fibroblasts and osteoblasts. A breakth rough in our understanding of the function of annexin V was made with the discovery of its calcium channel activity. At least one of several putative functions of annexin V became obvious from studies on matrix vesicles derived from calcifying cartilage. It was found that calcium uptake by matrix vesicles depend on collagen type II and type X bindi ng to annexin V in the vesicles and was lost when collagens were diges ted with collagenase; calcium influx was reconstituted after adding ba ck native collagen II or V. These findings indicate that annexin V pla ys a major role in matrix vesicle-initiated cartilage calcification as a collagen-regulated calcium channel.