Differential distribution of two 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 variants of the alpha 6 subunit, alpha 6A and alpha 6B, characterized by different cytoplasmic domains. By using indirect immunofluorescence with isoform-specific polyclonal antibodies, we showed that the two isoforms of the alpha 6 subunit distribute differently on the ventral plasma membrane of these cells cultured on laminin-coated substrates. In fact, while the alpha 6A subunit was found codistributing with vinculin in focal contacts, the alpha 6B subunit showed a homogeneously distributed punctate pattern. This difference was particularly evident when preparations of ventral plasma membranes were used for the immunolocalization. Furthermore, when cells were cultured on fibronectin, a substrate not recognized by the alpha 6 beta 1 laminin receptor, the distribution of the two alpha 6 isoforms was similar to that observed on laminin, with alpha 6A still colocalizing with vinculin in focal adhesions. 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.