Generating an unfoldase from thioredoxin-like domains.

Protein-disulfide isomerase (PDI), an endoplasmic reticulum (ER)-resident protein, is primarily known as a catalyst of oxidative protein folding but also has a protein unfolding activity. We showed previously that PDI unfolds the cholera toxin A1 (CTA1) polypeptide to facilitate the ER-to-cytosol retrotranslocation of the toxin during intoxication. We now provide insight into the mechanism of this unfoldase activity. PDI includes two redox-active (a and a') and two redox-inactive (b and b') thioredoxin-like domains, a linker (x), and a C-terminal domain (c) arranged as abb'xa'c. Using recombinant PDI fragments, we show that binding of CTA1 by the continuous PDIbb'xa' fragment is necessary and sufficient to trigger unfolding. The specific linear arrangement of bb'xa' and the type a domain (a' versus a) C-terminal to bb'x are additional determinants of activity. These data suggest a general mechanism for the unfoldase activity of PDI: the concurrent and specific binding of bb'xa' to particular regions along the CTA1 molecule triggers its unfolding. Furthermore, we show the bb' domains of PDI are indispensable to the unfolding reaction, whereas the function of its a' domain can be substituted partially by the a' domain from ERp57 (abb'xa'c) or ERp72 (ca degrees abb'xa'), PDI-like proteins that do not unfold CTA1 normally. However, the bb' domains of PDI were insufficient to convert full-length ERp57 into an unfoldase because the a domain of ERp57 inhibited toxin binding. Thus, we propose that generating an unfoldase from thioredoxin-like domains requires the bb'(x) domains of PDI followed by an a' domain but not preceded by an inhibitory a domain.

Derlin-1 facilitates the retro-translocation of cholera toxin.

Cholera toxin (CT) intoxicates cells by using its receptor-binding B subunit (CTB) to traffic from the plasma membrane to the endoplasmic reticulum (ER). In this compartment, the catalytic A1 subunit (CTA1) is unfolded by protein disulfide isomerase (PDI) and retro-translocated to the cytosol where it triggers a signaling cascade, leading to secretory diarrhea. How CT is targeted to the site of retro-translocation in the ER membrane to initiate translocation is unclear. Using a semipermeabilized-cell retro-translocation assay, we demonstrate that a dominant-negative Derlin-1-YFP fusion protein attenuates the ER-to-cytosol transport of CTA1. Derlin-1 interacts with CTB and the ER chaperone PDI as assessed by coimmunoprecipitation experiments. An in vitro membrane-binding assay showed that CTB stimulated the unfolded CTA1 chain to bind to the ER membrane. Moreover, intoxication of intact cells with CTB stabilized the degradation of a Derlin-1-dependent substrate, suggesting that CT uses the Derlin-1 pathway. These findings indicate that Derlin-1 facilitates the retro-translocation of CT. CTB may play a role in this process by targeting the holotoxin to Derlin-1, enabling the Derlin-1-bound PDI to unfold the A1 subunit and prepare it for transport.

Protein disulfide isomerase-like proteins play opposing roles during retrotranslocation.

Misfolded proteins in the endoplasmic reticulum (ER) are retained in the organelle or retrotranslocated to the cytosol for proteasomal degradation. ER chaperones that guide these opposing processes are largely unknown. We developed a semipermeabilized cell system to study the retrotranslocation of cholera toxin (CT), a toxic agent that crosses the ER membrane to reach the cytosol during intoxication. We found that protein disulfide isomerase (PDI) facilitates CT retrotranslocation, whereas ERp72, a PDI-like protein, mediates its ER retention. In vitro analysis revealed that PDI and ERp72 alter CT's conformation in a manner consistent with their roles in retrotranslocation and ER retention. Moreover, we found that PDI's and ERp72's opposing functions operate on endogenous ER misfolded proteins. Thus, our data identify PDI family proteins that play opposing roles in ER quality control and establish an assay to further delineate the mechanism of CT retrotranslocation.

Trak1 mutation disrupts GABA(A) receptor homeostasis in hypertonic mice.

Hypertonia, which results from motor pathway defects in the central nervous system (CNS), is observed in numerous neurological conditions, including cerebral palsy, stroke, spinal cord injury, stiff-person syndrome, spastic paraplegia, dystonia and Parkinson disease. Mice with mutation in the hypertonic (hyrt) gene exhibit severe hypertonia as their primary symptom. Here we show that hyrt mutant mice have much lower levels of gamma-aminobutyric acid type A (GABA(A)) receptors in their CNS, particularly the lower motor neurons, than do wild-type mice, indicating that the hypertonicity of the mutants is likely to be caused by deficits in GABA-mediated motor neuron inhibition. We cloned the responsible gene, trafficking protein, kinesin binding 1 (Trak1), and showed that its protein product interacts with GABA(A) receptors. Our data implicate Trak1 as a crucial regulator of GABA(A) receptor homeostasis and underscore the importance of hyrt mice as a model for studying the molecular etiology of hypertonia associated with human neurological diseases.

The CDY-related gene family: coordinated evolution in copy number, expression profile and protein sequence.

Theories predict that the long-term survival of duplicated genes requires their functional diversification, which can be accomplished by either subfunctionalization (the partitioning of ancestral functions among duplicates) or neofunctionalization (the acquisition of novel function). Here, we characterize the CDY-related mammalian gene family, focusing on three aspects of its evolution: gene copy number, tissue expression profile and amino acid sequence. We show that the progenitor of this gene family arose de novo in the mammalian ancestor via domain accretion. This progenitor later duplicated to generate CDYL and CDYL2, two autosomal genes found in all extant mammals. Prior to human-mouse divergence (and perhaps preceding the eutherian radiation), a processed CDYL transcript retroposed onto the Y chromosome to create CDY, the Y-linked member of the family. In the simian lineage, CDY was retained and subsequently amplified on the Y. In non-simian mammals, however, CDY appears to have been lost. The retention of the Y-linked CDY genes in simians spurred the process of subfunctionalization and possibly neofunctionalization. Subfunctionalization is evidenced by the observation that simian CDYL and CDYL2 retained their somatic housekeeping transcripts but lost the spermatogenic transcripts to the newly arisen CDY. Neo-functionalization is suggested by the rapid evolution of the CDY protein sequence. Thus, the CDY-related family offers an instructive example of how duplicated genes undergo functional diversification in both expression profile and protein sequence. It also supports the previously postulated notion that there is a tendency for spermatogenic functions to transfer from autosomes to the Y chromosome.