Folding-competent and Folding-defective Forms of Ricin A Chain Have Different Fates after Retrotranslocation from the Endoplasmic Reticulum

Abstract

This study reveals that components of the yeast ERAD-L pathway can discriminate between two subtly different forms of the same toxin substrate. Although precytosolic requirements are similar for both toxin structures, there is a divergence in fate on the cytosolic face of the ER membrane.

Isoform-selective Oligomer Formation of Saccharomyces cerevisiae p24 Family Proteins

“…Although our results indicate that they have redundant functions, this does not exclude the possibility that they also have their specific cargo proteins. Li et al (75) identified erp2⌬ by screening for resistance to exogenously expressed ricin A-chain. When the toxin is expressed in yeast and targeted to the ER lumen, it is translocated back to the cytosol and exhibits cytotoxicity.…”

How Ricin and Shiga Toxin Reach the Cytosol of Target Cells: Retrotranslocation from the Endoplasmic Reticulum

“…RTA has relatively minor requirements for Usa1p, Cue1p and Ubx7p and shows no requirements for Yos9p functions. Furthermore, RTA dislocation is independent of the E3 activity of Hrd1p (Li et al 2010). ER dislocation of RTA in plant cells is also independent of canonical ubiquitylation (Marshall et al 2008).…”

“…The initial remodelling of RTA after subunit reduction may therefore explain why, when expressed in the yeast ER, maximal toxicity of RTA requires the co-chaperones of Kar2p, the yeast equivalent of BiP (Li et al 2010) and rationalise a proposed role for the Hsp90 family ER chaperone GRP94 in ricin toxicity to mammalian cells (Spooner et al 2008a;Taylor et al 2010). …”

“…When expressed in the yeast ER, RTA dislocation is preceded by engagement with a specific COPII-interacting ER transmembrane p24 protein, leading to possible Golgi trafficking and subsequent return to the ER (Li et al 2010). This may be a universal requirement for ricin toxicity, since partial screening of an RNAi library in Drosophila melanogaster S2 cells for sensitivity changes to ricin challenge also identifies the fly equivalent of Erp2p and its interacting proteins as important in ricin toxicity (Pawar et al 2011).…”

“…However, when expressed exogenously in the yeast ER lumen RTA dislocates and folds to an active conformation in the cytosol, reducing yeast protein synthesis activity and causing a severe growth defect (Li et al 2010). This has allowed us to examine the requirements for RTA dislocation ( Figure 5).…”