MULTIPLE FUNCTIONAL FORMS OF THE INTEGRIN VLA-2 CAN BE DERIVED FROM A SINGLE ALPHA(2) CDNA CLONE - INTERCONVERSION OF FORMS INDUCED BY AN ANTI-BETA(1) ANTIBODY

The integrin VLA-2 was previously found to bind to either collagen alone, or collagen plus laminin, but the mechanism for this cell-specific functional difference was unknown. Here we transfected VLA-2 alpha2 subunit cDNA into K562 cells and obtained VLA-2 (called Form-O) which bound to neither collagen nor laminin. We then used a Matrigel selection procedure to enrich for a minor subpopulation of K562 cells stably expressing a form of VLA-2 (Form-C) that bound to collagen but not laminin. In contrast, the same alpha2 cDNA transfected into RD cells yielded VLA-2 (Form-CL) which bound to both collagen and laminin. These Form-O, -C, and -CL activities were stably expressed during extended cell culture, and could not be qualitatively altered by adding phorbol esters or by exchanging the resident divalent cations. However, addition of stimulatory anti-beta1 antibodies (TS2/16, A-1A5) rapidly converted VLA-2 Form-O and Form-C into Form-CL. Anti-beta1 antibody stimulation of VLA-2 activity was observed not only on whole cells, but also with solubilized receptors. These results suggest (a) that the ligand binding specificity of VLA-2 can be determined by its cellular environment, rather than by variations in the primary sequence of the alpha2 subunit, (b) that stably inactive or partly active VLA-2 can be rapidly converted to a fully active form through conformational changes initiated at a nonligand binding site on the beta1 subunit, and (c) that the mechanisms for VLA-2 stimulation by phorbol ester and by antibody are quite distinct, because the latter does not require an intact cell.