Jd. Bangs et al., A SOLUBLE SECRETORY REPORTER SYSTEM IN TRYPANOSOMA-BRUCEI - STUDIES ON ENDOPLASMIC-RETICULUM TARGETING, The Journal of biological chemistry, 271(31), 1996, pp. 18387-18393
A homolog of the endoplasmic reticulum (ER) hsp70 protein, binding pro
tein (BiP), from the parasitic protozoan Trypanosoma brucei (Bangs, J.
D., Uyetake, L., Brickman, M. J., Balber, A. E., and Boothroyd, J. C.
(1993) J. Cell Sci, 105, 1101-1113) is further characterized, In co-p
recipitation experiments, BiP transiently associates with newly synthe
sized secretory proteins, including variant surface glycoprotein (VSG)
, confirming its role as a molecular chaperone, To study the molecular
signals targeting BiP to the ER, we have developed soluble secretory
reporters for expression in transformed procyclic trypanosomes. Deleti
on of the BiP C-terminal tetrapeptide (MDDL) and the glycosylphosphati
dylinositol-anchor addition sequence of VSG converts these proteins to
secreted forms, Attachment of MDDL to VSG results in intracellular re
tention confirming that MDDL is a trypanosomal ER localization signal,
Secretion of both reporters is inefficient, but further truncation of
the BiP C-terminal peptide binding domain allows quantitative export
(t1/2 similar to 1 h) of the N-terminal ATPase domain (BiPN), consiste
nt with the conserved domain structure of hsp70 proteins, This is the
first demonstration of soluble protein secretion in African trypanosom
es. Using the BiPN reporter, the sequence specificity of C-terminal te
trapeptide retention signals in trypanosomes is analyzed and found to
be similar to higher eukaryotes. These results indicate that the basic
signals mediating protein targeting to the ER lumen are conserved thr
oughout the wide range of eukaryotic evolution.