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 pr esent study we investigated cell surface proteins involved in the inte raction of cells with chondroadherin in cell adhesion and by affinity purification. Adhesion of bovine articular chondrocytes to chondroadhe rin-coated dishes was dependent on Mg2+ or Mn2+ but not Ca2+. Adhesion was partially inhibited by an antibody recognizing beta 1 integrin su bunit. Chondroadherin-binding proteins from chondrocyte lysates were a ffinity purified on chondroadherin-Sepharose. The beta(1) integrin ant ibody immunoprecipitated two proteins with molecular mass similar to 1 10 and 140 kD (nonreduced) from the EDTA-eluted material. These result s indicate that a beta 1 integrin on chondrocytes interacts with chond roadherin. To identify the alpha integrin subunit(s) involved in inter action 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 ag reement with cell adhesion experiments where antibodies against the in tegrin subunit alpha 2 partially inhibited adhesion of human fibroblas t 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 ce lls to collagen type II and chondroadherin. The results suggested that adhesion to collagen type II and chondroadherin involves similar or n earby sites on the alpha 2 beta 1 integrin. Although alpha 2 beta 1 is a receptor for both collagen type II and chondroadherin, only adhesio n of cells to collagen type II was found to mediate spreading.