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4 Integrin Subunit
Division of Cell Biology, The Netherlands Cancer Institute, NL-1066 CX Amsterdam; and * Department of Dermatology,
University of Pennsylvania, Philadelphia, Pennsylvania 19104
Bullous pemphigoid antigen 180 (BP180) is a
component of hemidesmosomes, i.e., cell-substrate adhesion complexes. To determine the function of specific
sequences of BP180 to its incorporation in hemidesmosomes, we have transfected 804G cells with cDNA-constructs encoding wild-type and deletion mutant forms
of human BP180. The results show that the cytoplasmic
domain of BP180 contains sufficient information for
the recruitment of the protein into hemidesmosomes because removal of the extracellular and transmembrane domains does not abolish targeting. Expression
of chimeric proteins, which consist of the membrane
targeting sequence of K-Ras fused to the cytoplasmic
domain of BP180 with increasing internal deletions or
lacking the NH2 terminus, indicates that the localization of BP180 in hemidesmosomes is mediated by a segment that spans 265 amino acids. This segment comprises two important regions located within the central
part and at the NH2 terminus of the cytoplasmic domain of BP180.
To investigate the effect of the 
6
4 integrin on the
subcellular distribution of BP180, we have transfected
COS-7 cells, which lack 64 and BP180, with cDNAs
for BP180 as well as for human 6A and 4. We provide evidence that a mutant form of BP180 lacking
the collagenous extracellular domain as well as a chimeric protein, which contains the entire cytoplasmic
domain of BP180, are colocalized with 64. In contrast, when cells were transfected with cDNAs for
6A and mutant forms of 4, either lacking the cytoplasmic COOH-terminal half or carrying phenylalanine
substitutions in the tyrosine activation motif of the
cytoplasmic domain, the recombinant BP180 molecules
were mostly not colocalized with 64, but remained
diffusely distributed at the cell surface. Moreover, in
cells transfected with cDNAs for 6A and a 4/1
chimera, in which the cytoplasmic domain of 4 was
replaced by that of the 1 integrin subunit, BP180 was
not colocalized with the 64/1 chimera in focal adhesions, but remained again diffusely distributed. These
results indicate that sequences within the cytoplasmic
domain of 4 determine the subcellular distribution of
BP180.
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