RESUMO
Madin-Darby canine kidney (MDCK) cells have been extensively used as a model for the study of epithelial polarization. The contacts between the cell and extra-cellular matrix (ECM) provide a signal for the polarization of apical membrane markers. In order to study the molecular basis of these contacts, MDCK cells extracts in Triton X-100 were affinity-purified on laminin, yielding polypeptides of 100-110 and 36 kDa, but only the second one could be enzymatically iodinated from the cell surface. This protein was also recognized by an antibody against the 37/67-kDa laminin/elastin family of proteins. Different polypeptides were purified by the same method on type I collagen. An antibody developed against the polypeptides purified on laminin recognized also a 67-kDa protein, blocked 125I-laminin binding to a population of high affinity (1.5 nM KD) binding sites and caused a significant decrease in cell attachment and spreading to laminin or endogenous ECM. This antibody did not interfere with MDCK cell attachment to fibronectin or collagen matrices, but still impaired cell spreading. An apical MDCK plasma membrane protein (184 kDa), fully polarized in untreated cells, was partially mispolarized after treatment with anti-36 kDa antibody. These results are consistent with a model of various ECM receptors operating together in these cells, and show an important role of a non-integrin 36-kDa laminin binding protein related to the 67-kDa laminin receptor family in cell attachment, spreading and polarization.
