Functional partitioning of beta1 integrins revealed by activating and inhibitory mAbs

Integrins exist in different activation states on the surfaces of cells. Addition of the proper signal, ligand, or antibody can alter the activation state of these molecules. We report here the identification of two immunocytochemically distinct populations of beta1 integrins on fixed embryonic chick dermal fibroblasts. One population, recognized by the integrin activating mAb TASC, localizes to discrete regions of the cell, most likely focal contacts. These integrins co-localize with other proteins, such as vinculin and F-actin, and their retention at these sites is dependent on the actin cytoskeleton. The other population, identified with the inhibitory mAb W1B10, is more evenly distributed throughout the cell surface, and its pattern remains unchanged after disruption of the actin cytoskeleton. Double labeling experiments using Fab fragments of TASC alongside whole W1B10 IgG revealed non-overlapping staining patterns. These results show that it is possible to visualize and study discrete populations of integrins on cell surfaces using two different antibodies. We hypothesize that these antibodies report differences in the distribution of receptors in two different states. A model is proposed describing the ligand independent recruitment of integrins based on these findings and results from other labs.