Life Stage-Specific Cargo Receptors Facilitate Glycosylphosphatidylinositol-Anchored Surface Coat Protein Transport in Trypanosoma brucei

Abstract

African trypanosomes are protozoan parasites that cause African sleeping sickness. Critical to the success of the parasite is the variant surface glycoprotein (VSG), which covers the parasite cell surface and which is essential for evasion of the host immune system. VSG is membrane bound by a glycolipid (GPI) anchor that is attached in the earliest compartment of the secretory pathway, the endoplasmic reticulum (ER). We have previously shown that the anchor acts as a positive forward trafficking signal for ER exit, implying a cognate receptor mechanism for GPI recognition and loading in coated cargo vesicles leaving the ER. Here, we characterize a family of small transmembrane proteins that act at adaptors for this process. This work adds to our understanding of general GPI function in eukaryotic cells and specifically in the synthesis and transport of the critical virulence factor of pathogenic African trypanosomes.

Processing and targeting of cathepsin L (TbCatL) to the lysosome inTrypanosoma brucei

“…It is possible that the C‐terminal domain subtly influences post‐Golgi sorting, but another possibility is that ER exit is delayed. Export of TbCatL from the ER is dependent on transmembrane p24 receptors for loading into departing COPII vesicles (Kruzel, Zimmett, & Bangs, ), and the half‐time for transport of TbCatLΔ to the lysosome is the same as for secretion of the default bulk flow reporter BiPN (Bangs et al, ), as would be predicted if motifs for recognition by the export machinery resided in the deleted domain. When overexpressed, either in the depletion or in the presence of endogenous TbCatL, ~20% of the reporter was secreted in the mature M′ form.…”

Endoplasmic reticulum–associated degradation and disposal of misfolded GPI-anchored proteins inTrypanosoma brucei

“…We have generated considerable evidence that GPI anchors also act as forward trafficking signals for ER exit in trypanosomes: 1) deletion of the GPI signal sequence reduces rates of ER exit ( Triggs and Bangs, 2003 ); 2) conversely, addition of a GPI signal accelerates forward trafficking of soluble cargo ( Kruzel et al , 2017 ); and 3) GPI-dependent ER exit is mediated by a specific subset of COPII Sec23:Sec24 heterodimers ( Sevova and Bangs, 2009 ) and 4) also by a subset of p24 orthologues ( Kruzel et al , 2017 ). One difference from yeast and mammals is that the trimannosyl core of the newly attached GPI anchor is undecorated with phosphoethanolamine ( Krakow et al , 1986 ; Menon et al , 1988 ).…”

“…This situation would likely be lethal without rapid clearance, and we proposed ERAD as the mechanism for maintaining short-term viability. However, given that ERAD is not favored for disposal of misfolded GPI-APs in yeast and mammals, and that GPI anchors are also forward trafficking signals for ER exit in trypanosomes ( Triggs and Bangs, 2003 ; Sevova and Bangs, 2009 ; Kruzel et al , 2017 ), it is not certain that ERAD can actually serve this purpose. We now ask whether a misfolded HA-tagged E6 reporter (HA:E6) is also degraded by ERAD, or by preferential transport to the lysosome.…”

Evolution of Antigenic Variation in African Trypanosomes: Variant Surface Glycoprotein Expression, Structure, and Function

“…packaging into secretory cargo transport vesicles - in these model organisms ER exit is the winner. GPI anchors are also ER exit signals in trypanosomes [42,48,49] , but in this case it is retention and disposal by ERAD that dominates. [29,50] Whether trypanosomes have the capacity to cope with VSG levels of misfolded protein remains to be tested [To Do #6].…”