LMP2A deletion and point mutants, with mutations in phosphotyrosine-containing protein-protein interaction motifs, were transiently expressed in 293 cells and their phosphorylation was examined in immune complex kinase assays as well as in vivo. In vitro LMP2A phosphorylation depended on tyrosine 112. In vivo, mutations of single tyrosines did not eliminate LMP2 phosphorylation, although mutation of the LMP2A ITAM decreased LMP2A phosphorylation. The relationship between LMP2A in vitro phosphorylation and that induced by cell-extracellular matrix (ECM) interactions was also investigated. While LMP2A was phosphorylated to higher levels in whole-cell extracts of stimulated cells, a difference in in vitro kinase assays with extracts from stimulated and unstimulated cells was not detected, indicating that the ECM-mediated regulation of LMP2A phosphorylation is lost in vitro. In the presence of LMP2A, several cellular proteins with molecular weights between 70 and 80 kDa were phosphorylated on tyrosine. This increase in cellular protein phosphorylation depended on the LMP2A ITAM motif and suggests that the ITAM may participate in signal-transduction events in epithelial cells.
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