Toxin Instability and Its Role in Toxin Translocation from the Endoplasmic Reticulum to the Cytosol

Abstract

AB toxins enter a host cell by receptor-mediated endocytosis. The catalytic A chain then crosses the endosome or endoplasmic reticulum (ER) membrane to reach its cytosolic target. Dissociation of the A chain from the cell-binding B chain occurs before or during translocation to the cytosol, and only the A chain enters the cytosol. In some cases, AB subunit dissociation is facilitated by the unique physiology and function of the ER. The A chains of these ER-translocating toxins are stable within the architecture of the AB holotoxin, but toxin disassembly results in spontaneous or assisted unfolding of the isolated A chain. This unfolding event places the A chain in a translocation-competent conformation that promotes its export to the cytosol through the quality control mechanism of ER-associated degradation. A lack of lysine residues for ubiquitin conjugation protects the exported A chain from degradation by the ubiquitin-proteasome system, and an interaction with host factors allows the cytosolic toxin to regain a folded, active state. The intrinsic instability of the toxin A chain thus influences multiple steps of the intoxication process. This review will focus on the host–toxin interactions involved with A chain unfolding in the ER and A chain refolding in the cytosol.

Inhibition of Cholera Toxin and Other AB Toxins by Polyphenolic Compounds

“…Reduction of the CTA1/CTA2 disulfide bond occurs in the ER and facilitates the subsequent chaperone-assisted separation of CTA1 from its holotoxin [9–12]. The dissociated CTA1 subunit shifts to a disordered state that activates its ERAD-directed export to the cytosol; interaction with several host factors in the cytosol allows CTA1 to regain an active, folded conformation that elevates intracellular cAMP through the ADP-ribosylation of Gsα [5,13]. …”

Co- and Post-translocation Roles for HSP90 in Cholera Intoxication

“…The conformational instability of free CTA1 affects many steps of the intoxication process (14,15). For example, CTA1 spontaneously unfolds after its PDI-mediated release from the holotoxin in the ER (6).…”

“…Exported ERAD substrates move through protein-conducting channels in the ER membrane and are targeted for cytosolic degradation by the 26 S proteasome in a ubiquitin-dependent manner (21,22). A number of AB toxins exploit this system for A chain passage into the cytosol (15,23,24). CTA1 and A chains from other ER-translocating toxins contain an arginine over lysine bias in their amino acid sequences.…”

Thermal Unfolding of the Pertussis Toxin S1 Subunit Facilitates Toxin Translocation to the Cytosol by the Mechanism of Endoplasmic Reticulum-Associated Degradation

“…Instability in the toxin A chain appears to be a common property of ERAD-exploiting toxins (2). The A chain maintains a stable conformation during transit from the cell surface to the ER because of its covalent and/or noncovalent interactions with the holotoxin (27,49,50).…”

“…Some AB toxins move by retrograde vesicular transport from the cell surface to the endosomes, from the endosomes to the Golgi apparatus, and from the Golgi apparatus to the endoplasmic reticulum (ER), where A/B subunit dissociation occurs (1). Unfolding of the dissociated A chain places the toxin in a translocation-competent conformation and activates the host mechanism of ER-associated degradation (ERAD) (2). This quality control system exports misfolded proteins from the ER to the cytosol through protein-conducting channels in the ER membrane (3).…”