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.