A PtdIns(3)P-specific probe cycles on and off host cell membranes during Salmonella invasion of mammalian cells

Salmonella invade nonphagocytic cells by eliciting their own internalizatio n; upon contact with the host cell, the bacteria induce membrane ruffles hi ghly localized to the point of contact between the invading bacterium and t he host cell [1, 2]. The bacterium is then internalized into an unusual cyt osolic organelle, the Salmonella-containing vacuole (SCV). Early endosomal markers (including EEA1) have recently been shown to be associated with the SCV shortly after invasion [3]. EEA1, a protein involved in early endosome fusion, is recruited to early endosomal membranes in part by the interacti on between its FYVE finger and phosphatidylinositol 3-phosphate [PtdIns(3)P ] [4], a characteristic lipid of early endosomes. This suggests a possible role for PtdIns(3)P during Salmonella infection. To investigate this, we ge nerated a highly specific probe for PtdIns(3)P that was used to follow inva sion of Salmonella in nonphagocytic cells. Here, we show that PtdIns(3)P is present on the membranes of SCVs shortly after invasion and also that it i s present on the membrane ruffles produced immediately prior to invasion. W e also show that this specific probe cycles on and off the membranes of nas cent SCVs even when PtdIns 3-kinase activity is inhibited, demonstrating th at invading Salmonella influence the composition of the membranes that enve lop them during invasion.