Differential distribution of two cytoplasmic variants of the α6β1 integrin laminin receptor in the ventral plasma membrane of embryonic fibroblasts

The integrin α6β1 is a receptor involved in the adhesion of several cell types to laminin. By using function-blocking antibodies, we have shown that α6β1 is a functional laminin receptor in chick embryo fibroblasts. We also found that these cells express two variants of the α6 subunit, α6A and α6B, characterized by different cytoplasmic domains. By using indirect immunofluorescence with isoform-specific polyclonal antibodies, we showed that the two isoforms of the α6 subunit distribute differently on the ventral plasma membrane of these cells cultured on laminin-coated substrates. In fact, while the α6A subunit was found codistributing with vinculin in focal contacts, the α6B 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 α6β1 laminin receptor, the distribution of the two α6 isoforms was similar to that observed on laminin, with α6A still colocalizing with vinculin in focal adhesions. Our results indicate that two forms of the α6β1 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 α6Aβ1 integrin in adhesion plaques.