Endothelial cells assemble two distinct alpha6beta4-containing vimentin-associated structures: roles for ligand binding and the beta4 cytoplasmic tail.

The alpha6beta4 laminin binding integrin functions in the assembly of type I hemidesmosomes, which are specialized cell-matrix adhesion sites found in stratified epithelial cells. Although endothelial cells do not express all the components of type I hemidesmosomes, endothelial cells can express the alpha6beta4 integrin. Because endothelial cells lose expression of alpha6beta4 in culture, we expressed recombinant alpha6beta4 in the dermal microvascular endothelial cell line, HMEC-1, to test whether endothelial cells can assemble adhesion structures containing alpha6beta4. Using immunofluorescence microscopy, we found that recombinant alpha6beta4 concentrates specifically in a novel fibrillar structure on the basal surface of endothelial cells in the absence of an exogenous laminin substrate. This localization is regulated by an intracellular mechanism, because the beta4 cytoplasmic domain is sufficient to direct a reporter domain (IL-2R) to the fibrillar structures independently of recombinant alpha6beta4. In addition, this IL-2R-beta4 chimera is sufficient to recruit the intermediate filament-associated protein HD1/plectin to these fibrillar structures and this also occurs in the absence of recombinant alpha6beta4. The fibrillar localization pattern, as well as the recruitment of HD1/plectin, requires the first and second fibronectin type III repeats and the connecting segment of the beta4 tail. In addition, when endothelial cells are provided a laminin 5-rich matrix, recombinant alpha6beta4 redistributes from the fibrillar structure to type I hemidesmosome-like structures. The beta4 cytoplasmic domain can also direct a reporter domain to these type I hemidesmosome-like structures; however, this process is dependent upon the expression of recombinant alpha6beta4 Biochemical analysis indicates that both the fibrillar and the type I hemidesmosome-like structures are associated with the vimentin intermediate filament cytoskeleton. Thus, the results illustrate that endothelial cells have the essential components necessary to assemble at least two distinct alpha6beta4-containing and vimentin-associated structures on their basal surface and that the alpha6beta4 cytoplasmic tail and the availability of specific alph6beta4 ligands regulate receptor localization to these structures.