Distinct roles of class I and class III phosphatidylinositol 3-kinases in phagosome formation and maturation

Phagosomes acquire their microbicidal properties by fusion with lysosomes. Products of phosphatidylinositol 3-kinase (PI 3-kinase) are required for ph agosome formation, but their role in maturation is unknown. Using chimeric fluorescent proteins encoding tandem FYVE domains, we found that phosphatid ylinositol 3-phosphate (PIMP) accumulates greatly but transiently on the ph agosomal membrane. Unlike the 3'-phosphoinositides generated by class I PI 3-kinases which are evident in the nascent phagosomal cup, PI(3)P is only d etectable after the phagosome has sealed. The class III PI 3-kinase VPS34 w as found to be responsible for PI(3)P synthesis and essential for phagolyso some formation. In contrast, selective ablation of class I PI 3-kinase reve aled that optimal phagocytosis, but not maturation, requires this type of e nzyme. These results highlight the differential functional role of the two families of kinases, and raise the possibility that PI(3)P production by VP S34 may be targeted during the maturation arrest induced by some intracellu lar parasites.