Integrin alpha 2 beta 1 is a receptor for the cartilage matrix protein chondroadherin

Chondroadherin (the 36-kD protein) is a leucine-rich, cartilage matrix protein known to mediate adhesion of isolated chondrocytes. In the present study we investigated cell surface proteins involved in the interaction of cells with chondroadherin in cell adhesion and by affinity purification. Adhesion of bovine articular chondrocytes to chondroadherin-coated dishes was dependent on Mg2+ or Mn2+ but not Ca2+. Adhesion was partially inhibited by an antibody recognizing beta 1 integrin subunit. Chondroadherin-binding proteins from chondrocyte lysates were affinity purified on chondroadherin-Sepharose. The beta(1) integrin antibody immunoprecipitated two proteins with molecular mass similar to 110 and 140 kD (nonreduced) from the EDTA-eluted material. These results indicate that a beta 1 integrin on chondrocytes interacts with chondroadherin. To identify the alpha integrin subunit(s) involved in interaction of cells with the protein, we affinity purified chondroadherin-binding membrane proteins from human fibroblasts. Immunoprecipitation of the EDTA-eluted material from the affinity column identified alpha 2 beta 1 as a chondroadherin-binding integrin. These results are in agreement with cell adhesion experiments where antibodies against the integrin subunit alpha 2 partially inhibited adhesion of human fibroblast and human chondrocytes to chondroadherin. Since alpha 2 beta 1 also is a receptor for collagen type II, we tested the ability of different antibodies against the alpha 2 subunit to inhibit adhesion of T47D cells to collagen type II and chondroadherin. The results suggested that adhesion to collagen type II and chondroadherin involves similar or nearby sites on the alpha 2 beta 1 integrin. Although alpha 2 beta 1 is a receptor for both collagen type II and chondroadherin, only adhesion of cells to collagen type II was found to mediate spreading.