We have investigated the role of glycosylphosphatidylinositol (GPI) anchors 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 reduced kinetics, relative to matched GPI-anchored constructs. Cell fractionation 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 secretory reporter (BiPN) when co-expressed in the same cells. Two results suggest that delayed forward transport cannot be accounted for by failure to fold/assemble in the absence of a GPI anchor, thereby leading to prolonged association with ER quality-control machinery. First, no evidence 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 protein, trans-sialidase. These findings suggest that GPI structures act 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.
Research Article| November 01 1998
Glycosylphosphatidylinositol-dependent secretory transport in Trypanosoma brucei
Mary Ann McDOWELL;
Dawn M. RANSOM;
James D. BANGS
James D. BANGS 2
1Department of Medical Microbiology and Immunology, University of Wisconsin-Madison Medical School, 1300 University Avenue, Madison, WI 53706, U.S.A.
2To whom correspondence should be addressed (e-mail firstname.lastname@example.org).
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Mary Ann McDOWELL, Dawn M. RANSOM, James D. BANGS; Glycosylphosphatidylinositol-dependent secretory transport in Trypanosoma brucei. Biochem J 1 November 1998; 335 (3): 681–689. doi: https://doi.org/10.1042/bj3350681
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