Stage-Specific COPII-Mediated Cargo Selectivity in African Trypanosomes.

A hallmark of eukaryotic cells is the ability to form a secretory pathway connecting many intracellular compartments. In the early secretory pathway, coated protein complex II (COPII)-coated vesicles mediate the anterograde transport of newly synthesized secretory cargo from the endoplasmic reticulum to the Golgi apparatus. The COPII coat complex is comprised of an inner layer of Sec23/Sec24 heterodimers and an outer layer of Sec13/Sec31 heterotetramers. In African trypanosomes, there are two paralogues each of Sec23 and Sec24, that form obligate heterodimers (TbSec23.2/TbSec24.1, TbSec23.1/TbSec24.2). It is not known if these form distinct homotypic classes of vesicles or one heterotypic class, but it is known that TbSec23.2/TbSec24.1 specifically mediate forward trafficking of GPI-anchored proteins (GPI-APs) in bloodstream-form trypanosomes (BSF). Here, we showed that this selectivity was lost in insect procyclic stage parasites (PCF). All isoforms of TbSec23 and TbSec24 are essential in PCF parasites as judged by RNAi knockdowns. RNAi silencing of each subunit had equivalent effects on the trafficking of GPI-APs and p67, a transmembrane lysosomal protein. However, silencing of the TbSec23.2/TbSec24.1 had heterodimer had a significant impact on COPII mediated trafficking of soluble TbCatL from the ER to the lysosome. This finding suggests a model in which selectivity of COPII transport was altered between the BSF and PCF trypanosomes, possibly as an adaptation to a digenetic life cycle. African trypanosomes synthesize dense surface coats composed of stage-specific glycosylphosphatidylinositol lipid anchored proteins. We previously defined specific machinery in bloodstream stage parasites that mediate the exit of these proteins from the endoplasmic reticulum. Here, we performed similar analyses in the procyclic insect stage and found significant differences in this process. These findings contribute to our understanding of secretory processes in this unusual eukaryotic model system.