Glycosylphosphatidylinositol-specific phospholipase C regulates transferrin endocytosis in the African trypanosome.
GPI-PLC (glycosylphosphatidylinositol-specific phospholipase C) is expressed in bloodstream-form Trypanosoma brucei, a protozoan that causes human African trypanosomiasis. Loss of genes encoding GPI-PLC reduces the virulence of a pleomorphic strain of the parasite, for reasons that are not clear. In the present paper, we report that GPI-PLC stimulates endocytosis of transferrin by 300-500%. Surprisingly, GPI-PLC is not detected at endosomes, suggesting that the enzyme does not interact directly with the endosomal machinery. We therefore hypothesized that a diffusible product of the GPI-PLC enzyme reaction [possibly DAG (diacylglycerol)] mediated the biological effects of the protein. Two sets of data support this assertion. First, a catalytically inactive Q81L mutant of GPI-PLC, expressed in a GPI-PLC-null background, had no effect on endocytosis, indicating that enzyme activity is essential for the protein to stimulate endocytosis. Secondly, the exogenous DAGs OAG (1-oleyl-2-acetyl-sn-glycerol) and DMG (dimyristoylglycerol) independently stimulated endocytosis of transferrin. Furthermore, the DAG mimic PMA, a phorbol ester, also activated endocytosis in T. brucei. DAG-stimulated endocytosis is a novel pathway in the trypanosome. We surmise that (i) GPI-PLC regulates transferrin endocytosis in T. brucei, (ii) GPI-PLC is a signalling enzyme, and (iii) DAG is a second messenger for GPI-PLC. We propose that regulation of endocytosis is a physiological function of GPI-PLC in bloodstream T. brucei.
Subramanya
Sandesh
S
Department of Cellular Biology, University of Georgia, 724 Biological Sciences Building, Athens, GA 30602, USA.
Hardin
C Frank
CF
Steverding
Dietmar
D
Mensa-Wilmot
Kojo
K
D000818:Animals; D004075:Diglycerides / Q000378:metabolism; D004705:Endocytosis / Q000502:physiology*; D054800:Glycosylphosphatidylinositol Diacylglycerol-Lyase / Q000235:genetics / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D010703:Phorbol Esters / Q000378:metabolism; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism*; D014168:Transferrin / Q000378:metabolism*; D014346:Trypanosoma brucei brucei / Q000201:enzymology* / Q000378:metabolism
D004075:Diglycerides; D010703:Phorbol Esters; D015800:Protozoan Proteins; D014168:Transferrin; D054800:Glycosylphosphatidylinositol Diacylglycerol-Lyase