Tyrosine phosphorylation is required for ehrlichial internalization and replication in P388D1 cells.

Replication of Ehrlichia risticii was inhibited in P388D1 cells when a protein tyrosine kinase inhibitor (genistein or herbimycin A) was added after internalization of the organism at 3 h postinfection. Upon addition of genistein at day 1, 2, 3, or 4 postinfection, further proliferation of E. risticii was prevented. The inhibition was reversible, since regrowth of E. risticii occurred upon the removal of genistein. Genistein prevented spreading of E. risticii from P388D1 cells to THP-1 cells. Genistein did not prevent binding of [35S]methionine-labeled E. risticii to P388D1 cells but did prevent internalization of [35S]methionine-labeled E. risticii. 14CO2 production from L-[14C]glutamine in Percoll density gradient-purified E. risticii was not inhibited by genistein or herbimycin A, which suggests that these reagents did not directly inhibit ehrlichial energy metabolism. Double indirect immunofluorescence labeling with antiphosphotyrosine antibody and anti-E. risticii antibody revealed colocalization of tyrosine phosphoproteins with ehrlichial inclusions. There was, however, no colocalization of phosphotyrosine with phagosomes containing 0.5-microm-diameter fluorescent beads. Western immunoblot analysis revealed that 52- and 54-kDa proteins were tyrosine phosphorylated only in infected cells and that phosphorylation of these two proteins was reduced when infected cells were treated with genistein for 6 h. These results suggest that protein tyrosine phosphorylation is specific and essential for ehrlichial internalization, replication, and spreading in macrophages but not for binding.