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Infection of epithelial cells by two biovars of Chlamydia trachomatis results in the tyrosine phosphorylation of several host proteins. The most prominent change in host protein tyrosine phosphorylation involves a complex of proteins with molecular masses of 75 to 85 kDa (pp75/85) and 100 kDa (pp100). The C. trachomatis-induced tyrosine phosphorylation of pp75/85 and pp100 is observed in several cell lines, including epithelial cells, fibroblasts, and macrophages. Subcellular fractionation and detergent solubility properties of pp75/85 are consistent with its association with the cytoskeleton. Phosphoamino acid analysis demonstrates that the pp75/85 complex is phosphorylated on both tyrosine and serine residues. Immunofluorescence studies of chlamydia-infected cells by using fluorescein isothiocyanate-phalloidin and antibodies to phosphotyrosine and cortactin demonstrate that tyrosine-phosphorylated proteins, as well as cortactin, are localized to the chlamydial vacuole and that this process is facilitated by actin.