DIFFERENTIAL DISTRIBUTION OF 2 CYTOPLASMIC VARIANTS OF THE ALPHA-6-BETA-1 INTEGRIN LAMININ RECEPTOR IN THE VENTRAL PLASMA-MEMBRANE OF EMBRYONIC FIBROBLASTS

The integrin alpha 6 beta 1 is a receptor involved in the adhesion of several cell types to laminin, By using function-blocking antibodies, we have shown that alpha 6 beta 1 is a functional laminin receptor in chick embryo fibroblasts. We also found that these cells express two v ariants of the alpha 6 subunit, alpha 6A and alpha 6B, characterized b y different cytoplasmic domains, By using indirect immunofluorescence with isoform-specific polyclonal antibodies, we showed that the two is oforms of the alpha 6 subunit distribute differently on the ventral pl asma membrane of these cells cultured on laminin-coated substrates, In fact, while the alpha 6A subunit was found codistributing with vincul in in focal contacts, the alpha 6B subunit showed a homogeneously dist ributed punctate pattern. This difference was particularly evident whe n preparations of ventral plasma membranes were used for the immunoloc alization. Furthermore, when cells were cultured on fibronectin, a sub strate not recognized by the alpha 6 beta 1 laminin receptor, the dist ribution of the two alpha 6 isoforms was similar to that observed on l aminin, with alpha 6A still colocalizing with vinculin in focal adhesi ons, Our results indicate that two forms of the alpha 6 beta 1 laminin receptor coexpressed in the same cells show distinctive distributions , and suggest that receptor occupancy by laminin is not essential for the accumulation of the alpha 6A beta 1 integrin in adhesion plaques.