Infection of macrophages with Legionella pneumophila induces phosphorylation of a 76-kilodalton protein.

Infection of peritoneal macrophages from susceptible A/J mice with Legionella pneumophila induced phosphorylation of a 76-kDa protein. The phosphorylation occurred when macrophages were infected with a virulent strain of L. pneumophila but did not occur when they were infected with an avirulent strain or with other bacteria such as either Pseudomonas aeruginosa or Salmonella typhimurium. Also, no phosphorylation of this protein was observed when macrophages were stimulated with either lipopolysaccharide or phorbol myristate acetate. However, phosphorylation did occur in macrophages infected with a virulent strain of L. pneumophila and treated with either erythromycin to inhibit growth or with cytochalasin D to inhibit uptake of L. pneumophila by macrophages. These results support the view that phosphorylation of this protein occurs during the early phases of interaction between L. pneumophila and macrophages. The role of this specific protein in the recognition, intracellular uptake, and growth of L. pneumophila in permissive macrophages remains to be clarified.