Abstract
The variant surface glycoprotein (VSG) of bloodstream form Trypanosoma brucei (Tb) is a critical virulence factor. The VSG glycosylphosphatidylinositol (GPI)-anchor strongly influences passage through the early secretory pathway. Using a dominant-negative mutation of TbSar1, we show that endoplasmic reticulum (ER) exit of secretory cargo in trypanosomes is dependent on the coat protein complex II (COPII) machinery. Trypanosomes have two orthologues each of the Sec23 and Sec24 COPII subunits, which form specific heterodimeric pairs: TbSec23.1/TbSec24.2 and TbSec23.2/TbSec24.1. RNA interference silencing of each subunit is lethal but has minimal effects on trafficking of soluble and transmembrane proteins. However, silencing of the TbSec23.2/TbSec24.1 pair selectively impairs ER exit of GPI-anchored cargo. All four subunits colocalize to one or two ER exit sites (ERES), in close alignment with the postnuclear flagellar adherence zone (FAZ), and closely juxtaposed to corresponding Golgi clusters. These ERES are nucleated on the FAZ-associated ER. The Golgi matrix protein Tb Golgi reassembly stacking protein defines a region between the ERES and Golgi, suggesting a possible structural role in the ERES:Golgi junction. Our results confirm a selective mechanism for GPI-anchored cargo loading into COPII vesicles and a remarkable degree of streamlining in the early secretory pathway. This unusual architecture probably maximizes efficiency of VSG transport and fidelity in organellar segregation during cytokinesis.