Regulated cleavage of intracellular glycosylphosphatidylinositol in a trypanosome

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.