Hemidesmosomes (HDs) are multiprotein structures that anchor epithelial cells to the basement membrane. HD components include the α6β4 integrin, plectin, and BPAGs (bullous pemphigoid antigens). HD disassembly in keratinocytes is necessary for cells to migrate and can be induced by EGF through β4 integrin phosphorylation. We have identified a novel phosphorylation site on the β4 integrin: S1424. Preventing phosphorylation by mutating S→A1424 results in increased incorporation of β4 into HDs and resistance to EGF-induced disassembly. In contrast, mutating S→D1424 (mimicking phosphorylation) partially mobilizes β4 from HDs and potentiates the disassembly effects of other phosphorylation sites. In contrast to previously described sites that are phosphorylated upon growth factor stimulation, S1424 already exhibits high constitutive phosphorylation, suggesting additional functions. Constitutive phosphorylation of S1424 is distinctively enriched at the trailing edge of migrating keratinocytes where HDs are disassembled. Although most of this S1424-phosphorylated β4 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-S1424 might be a mechanism to dissociate β4 from BPAGs. S1424 phosphorylation is PKC dependent. These data suggest an important role for S1424 in the gradual disassembly of HDs induced by cell retraction.
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