In vitro reconstitution of phagosome-endosome fusion: evidence for regulation by heterotrimeric GTPases.

We have assessed the role of heterotrimeric GTPases on in vitro fusion of phagosomes and endosomes. Highly purified phagosomes were found to contain G alpha s, G alpha i1, G alpha i2, G alpha i3, and G beta subunits of heterotrimeric GTP-binding proteins. A functional role for G proteins was established using an in vitro phagosome-endosome fusion assay. First, addition of AlF4- and purified G beta gamma subunits to the in vitro assay blocked fusion, indicating that heterotrimeric G proteins may play a role, either direct or indirect, in phagosome maturation. Second, a striking inhibitory effect was observed when the vesicles were incubated with peptides that preferentially activate G alpha s. A similar effect on phagosome-endosome fusion was observed with cholera toxin, a reagent known to activate G alpha s. Our results suggest that one or more heterotrimeric G proteins, including Gs, mediate and/or regulate phagosome-endosome fusion.