Phosphorylation of a Novel Site on the beta 4 Integrin at the Trailing Edge of Migrating Cells Promotes Hemidesmosome Disassembly

Hemidesmosomes (HDs) are multiprotein structures that anchor epithelial cells to the basement membrane. HD components include the alpha 6 beta 4 integrin, plectin, and BPAGs (bullous pemphigoid antigens). HD disassembly in keratinocytes is necessary for cells to migrate and can be induced by EGF through beta 4 integrin phosphorylation. We have identified a novel phosphorylation site on the beta 4 integrin: S-1424. Preventing phosphorylation by mutating S -> A(1424) results in increased incorporation of beta 4 into HDs and resistance to EGF-induced disassembly. In contrast, mutating S -> D-1424 (mimicking phosphorylation) partially mobilizes beta 4 from HDs and potentiates the disassembly effects of other phosphorylation sites. In contrast to previously described sites that are phosphorylated upon growth factor stimulation, S-1424 already exhibits high constitutive phosphorylation, suggesting additional functions. Constitutive phosphorylation of S-1424 is distinctively enriched at the trailing edge of migrating keratinocytes where HDs are disassembled. Although most of this S-1424-phosphorylated beta 4 is found dissociated from HDs, a substantial amount can be associated with HDs near the cell margins, colocalizing with plectin but always excluding BPAGs, suggesting that phospho-S-1424 might be a mechanism to dissociate beta 4 from BPAGs. S-1424 phosphorylation is PKC dependent. These data suggest an important role for S-1424 in the gradual disassembly of HDs induced by cell retraction.