A RECOMBINANT TAIL-LESS INTEGRIN BETA(4) SUBUNIT DISRUPTS HEMIDESMOSOMES, BUT DOES NOT SUPPRESS ALPHA(6)BETA(4)-MEDIATED CELL-ADHESION TO LAMININS

To examine the function of the alpha(6) beta(4) integrin we have determined its ligand-binding ability and overexpressed two potentially dominant negative mutant beta(4) subunits, lacking either the cytoplasmic or extracellular domain, in bladder epithelial 804G cells. The results of cell adhesion and radioligand-binding assays showed that alpha(6) beta(4) is a receptor for several laminin isoforms, including laminin 1, 2, 4, and 5. Over-expression of the tail-less or head-less mutant beta(4) subunit did not suppress alpha(6) beta(4)-mediated adhesion to laminins, as both types of transfectants adhered to these ligands in the presence of blocking anti-beta(1) antibodies as well as the controls. However, immunofluorescence experiments indicated that the endogenous alpha(6) beta(4) integrin and other hemidesmosomal markers were not concentrated in hemidesmosomes in cells overexpressing tail-less beta(4), while the distribution of these molecules was not altered in cells overexpressing the head-less subunit. Electron microscopic studies confirmed that cells overexpressing tail-less beta(4) had a drastically reduced number of hemidesmosomes, while cells expressing the head-less subunit had a normal number of these structures. Thus, expression of a tail-less, but not a head-less mutant beta(4) subunit leads to a dominant negative effect on hemidesmosome assembly without suppressing initial adhesion to laminins. We conclude that the alpha(6) beta(4) integrin binds to several laminins and plays an essential role in the assembly and/or stability of hemidesmosomes, that alpha(6) beta(4)-mediated adhesion and hemidesmosome assembly have distinct requirements, and that it is possible to use a dominant negative approach to selectively interfere with a specific function of an integrin.