Transbilayer translocation of membrane phosphatidylserine and its role in macrophage invasion in Leishmania promastigotes.

Infectivity of Leishmania promastigotes has been shown to be growth cycle-dependent and restricted to the stationary phase. By using annexin V-FITC binding and procoagulant activity measurement assays, we show here that the promastigotes in the stationary phase contain significantly higher amounts of phosphatidylserine (PS) on their surface as compared to the log phase promastigotes. We also demonstrate that the infectivity of the promastigotes is determined by the presence of PS on their surface. In addition, by using NBD-labelled phospholipids, we show that the promastigote plasma membrane contains ATP-dependent out-to-in and ATP-independent in-to-out PS translocases which regulate the PS localisation in two-halves of the membrane bilayer, and that the greater amounts of external PS observed in the stationary phase promastigotes is perhaps due to the slower ATP-dependent out-to-in PS movements in these cells, as compared to the log phase promastigotes.