The groups studied different pathogens, but both came to similar conclusions. Both bacteria, Shigella and Salmonella, inject several invasion-promoting proteins into the host, including a protein with similarity to a mammalian inositol phosphatase. Mauricio Terebiznik, Sergio Grinstein (Hospital for Sick Children, Toronto, ON), and colleagues watched how phosphoinositides were affected during Salmonella infection. They found that one PIP2 species, PtdIns(4,5)P2, was depleted from the base of membrane ruffles that formed where bacteria were pushing to get in, thanks to the phosphatase activity of the SigD effector protein. Like SigD, the Shigella effector IpgD studied by Kirsten Nieburh (Institut Pasteur, Paris, France), Bernard Payrastre (INSERM, Toulouse, France), and colleagues also removed PIP2 from target cells; in this case, the product was identified as PtdIns(5)P.
The result in both cases was a softer plasma membrane. Expression of either phosphatase in mammalian cells caused membrane blebbing and relaxed attachment between the actin cytoskeleton and the membrane. Vesicles were released from the membrane into the SigD-expressing cells. The loosened membrane sped up bacterial invasion—Salmonella mutants lacking SigD were slower to gain entry.
Although PtdIns(5)P may affect membrane-cytoskeleton interactions, both groups imagine that the loss of PIP2 is at the heart of it all. “Actin cross-linkers are bound by PIP2,” says Grinstein. “Bacteria have taken advantage of this. By chewing away PIP2, they weaken the interaction.” The weakening of the plasma membrane environment may assist the fission of vesicles from the membrane. It remains to be seen whether phosphatase conversion of PIP2 into PtdIns(5)P is a general mechanism to control membrane properties during other processes, such as motility or endocytosis. ▪