THE INTEGRIN-ALPHA-9-BETA-1 MEDIATES CELL ATTACHMENT TO A NON-RGD SITE IN THE 3RD FIBRONECTIN TYPE-III REPEAT OF TENASCIN

We have previously reported the sequence of the integrin alpha 9 subun it, a partner of the beta 1 subunit that is expressed in basal keratin ocytes, hepatocytes, airway epithelial cells, and smooth and skeletal muscle. In the present study, we have stably expressed alpha 9 beta 1 on the surface of the human embryonic kidney cell line 293 and the hum an colon carcinoma cell line SW480 and used these transfected cell lin es to identify ligand(s) for this integrin. Transfected cells did not appear to utilize alpha 9 beta 1 for attachment to the extracellular m atrix proteins fibronectin, laminin, vitronectin, fibrinogen, thrombos pondin, or type I or IV collagen. However, in contrast to mock transfe ctants, both 293 cells and SW480 cells expressing alpha 9 beta 1 adher ed to intact chicken tenascin. By utilizing a variety of recombinant f ragments of tenascin, we were able to localize the binding site for al pha 9 beta 1 to the third type III repeat. This repeat contains the ar ginine-glycine-aspartic acid (RGD) tripeptide that has been shown to s erve as a binding site in tenascin for alpha v-integrins. However, the RGD site does not appear to be the binding site for alpha 9 beta 1, a s the attachment of alpha 9 transfectants to this fragment was not inh ibited by RGD peptide, nor by changing the RGD site to RAD or RAA.