GLYCOSYLPHOSPHATIDYLINOSITOL-DEPENDENT SECRETORY TRANSPORT IN TRYPANOSOMA-BRUCEI

We have investigated the role of glycosylphosphatidylinositol (GPI) an chors in forward secretory trafficking using African trypanosomes as a model system. Soluble GPI-minus forms of variant surface glycoprotein (VSG), in which the C-terminal GPI-addition peptide signal is deleted , are secreted from transformed procyclic trypanosomes with 5-fold red uced kinetics, relative to matched GPI-anchored constructs. Cell fract ionation and immunofluorescence localization studies indicate that the GPI-minus VSG reporters accumulate in the endoplasmic reticulum (ER). This transport defect is specific, since overexpression of GPI-minus VSG has no effect on the rate of transport of a second soluble secreto ry reporter (BiPN) when co-expressed in the same cells. Two results su ggest that delayed forward transport cannot be accounted for by failur e to fold/assemble in the absence of a GPI anchor, thereby leading to prolonged association with ER quality-control machinery. First, no evi dence was found for elevated association of GPI-minus VSG with the ER molecular chaperone, BiP. Secondly, newly synthesized GPI-minus VSG is dimerized efficiently, as judged by velocity-sedimentation analysis. GPI-dependent transport is not confined to the VSG reporters, because a similar dependence is found with another trypanosomal GPI-anchored p rotein, trans-sialidase. These findings suggest that GPI structures ac t in a positive manner to mediate efficient forward transport of some, and perhaps all, GPI-anchored proteins in the early secretory pathway of trypanosomes. Possible mechanisms for GPI-dependent transport are discussed with respect to current models of vesicular trafficking.