Regulated cleavage of intracellular glycosylphosphatidylinositol in a trypanosome. Peroxisome-to-endoplasmic reticulum translocation of a phospholipase C.
Cell exposure to hypo-osmolarity and alkalinity triggers a spectrum of responses including activation of phospholipases. Glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) is expressed in Trypanosoma brucei, a protozoan parasite that causes human African trypanosomiasis. We examined possible contributions of GPI-PLC to the response of T. brucei to hypo-osmotic or mildly alkaline conditions. GPIs were detected at the endoplasmic reticulum (ER). They were cleaved after exposure of T. brucei to hypo-osmolarity or mild alkalinity, which also, strikingly, caused translocation of GPI-PLC from glycosomes (peroxisomes) to the ER. A catalytically inactive Gln81Leu mutant of GPI-PLC failed to cleave GPIs despite being transported from glycosomes to the ER after hypo-osmotic or mild alkaline treatment of the parasites. In contrast, a Cys347Ser mutant of the enzyme could not exit glycosomes after treatment of cells expressing the protein with mild base or hypo-osmotic buffer. We conclude that: (a) GPI-PLC contributes to loss of GPIs in T. brucei treated with hypo-osmotic or mildly alkaline buffer; (b) access of GPI-PLC to its substrate in vivo can be regulated post-translationally; (c) translocation of GPI-PLC from glycosomes to the ER is important for in vivo cleavage of GPIs; (d) Cys347 is part of a peptide motif required for post-translational targeting of GPI-PLC to the ER. Glycosome-to-ER movement of GPI-PLC reveals a novel pathway for intracellular protein traffic. The physiological significance of GPI digestion in cells exposed to mildly alkalinity or hypo-osmolarity is discussed.
Subramanya
Sandesh
S
Department of Cellular Biology, University of Georgia, Athens, GA 30602, USA.
Mensa-Wilmot
Kojo
K
D000818:Animals; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism*; D017261:Glycosylphosphatidylinositols / Q000235:genetics / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D009154:Mutation; D009995:Osmosis; D020675:Peroxisomes / Q000737:chemistry / Q000378:metabolism*; D021381:Protein Transport; D014346:Trypanosoma brucei brucei / Q000737:chemistry / Q000378:metabolism*; D010738:Type C Phospholipases / Q000235:genetics / Q000378:metabolism*
D017261:Glycosylphosphatidylinositols; D010738:Type C Phospholipases