Interferon-gamma selectively induces Rab5a synthesis and processing in mononuclear cells

Abstract

Macrophage activation by interferon (IFN)-gamma is characterized by enhanced phagocytosis and killing of internalized pathogens. We studied the effects of IFN-gamma on Rab5a, a GTPase involved in both endocytosis and phagocytosis. IFN-gamma induced the synthesis of Rab5a in mononuclear cells as detected by immunoprecipitation and by Western blotting. Rab5a messenger RNA levels were also increased. Elevated protein expression was detected as early as 6 h following IFN-gamma and was maximal at 24 h. Following IFN-gamma, membrane association of Rab5a:GTP was substantially increased. Rab5b and Rab5c, as well as Rab7 and Rab11, Rab GTPases localized in the endosomal-lysosomal pathway, were unaffected by IFN-gamma. Moreover, Rab5a expression in non-macrophages was unaltered by IFN-gamma. Rab5a is a prenylated protein, and newly synthesized Rab5a was rapidly processed following IFN-gamma. However, elevated geranylgeranylation was not Rab5a-specific since all the Rab5 isoforms were more rapidly prenylated in vitro using cytosol from IFN-gamma-treated cells. Last, guanine nucleotide exchange on Rab5a was elevated about 3-fold in the presence of cytosol from IFN-gamma-treated cells. The selective effect of IFN-gamma on Rab5a, synthesis, processing, and nucleotide exchange suggests that Rab isoforms have closely associated but not identical functions and that selective enhancement of membrane trafficking may play a key role in intracellular killing.