Electron microscopic studies of calcifying vertebrate tissues reveal the locus of de novo mineral formation within matrix vesicles (MV). The direct involvement of MV in the initiation of mineral formation is supported by the fact that MV isolated from avian growth plate cartilage rapidly accumulate large amounts of Ca2+ and Pi and induce mineral formation. Exploration of the constituents of MV has revealed two major protein components, a 33 and a 36 kD protein, the former of which binds to cartilage‐specific collagens. These annexin‐like proteins bind to acidic phospholipids in the presence of submicromolar levels of Ca2+. Antibodies raised against both the purified 33 and the 36 kD MV annexin do not cross‐react with the other, indicating that they are distinct proteins. Reported here are studies elucidating the primary structure of both MV proteins using both conventional protein and molecular biologic methods. These studies establish that the 33 kD protein is nearly identical to anchorin CII (annexin V) and that the 36 kD protein is identical to avian annexin II. Immunolocalization studies show that hypertrophic chondrocytes at the calcification front of avian growth plate contain the highest level of these annexins. Further, immunogold labeling indicates that the annexins are localized within MV isolated from the growth plate. Recent studies indicate that annexin V is a new type of ion‐selective Ca2+ channel protein that possesses selective collagen binding properties. Since MV are tightly associated with the collagen‐ and proteoglycan‐rich matrix, it is tempting to speculate that this MV protein may be a component of stretch‐activated ion channels that enhance Ca2+ uptake during mechanical stress.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism
- Orthopedics and Sports Medicine