Co-chaperone Specificity in Gating of the Polypeptide Conducting Channel in the Membrane of the Human Endoplasmic Reticulum.

In mammalian cells, signal peptide-dependent protein transport into the endoplasmic reticulum (ER) is mediated by a dynamic polypeptide-conducting channel, the heterotrimeric Sec61 complex. Previous work has characterized the Sec61 complex as a potential ER Ca(2+) leak channel in HeLa cells and identified ER lumenal molecular chaperone immunoglobulin heavy-chain-binding protein (BiP) as limiting Ca(2+) leakage via the open Sec61 channel by facilitating channel closing. This BiP activity involves binding of BiP to the ER lumenal loop 7 of Sec61α in the vicinity of tyrosine 344. Of note, the Y344H mutation destroys the BiP binding site and causes pancreatic ß-cell apoptosis and diabetes in mice. Here, we systematically depleted HeLa cells of the BiP co-chaperones by siRNA-mediated gene silencing and used live cell Ca(2+) imaging to monitor the effects on ER Ca(2+) leakage. Depletion of either one of the ER lumenal BiP co-chaperones, ERj3 and ERj6, but not the ER membrane-resident co-chaperones (such as Sec63 protein, which assists BiP in Sec61 channel opening) led to increased Ca(2+) leakage via Sec6 complex, thereby phenocopying the effect of BiP depletion. Thus, BiP facilitates Sec61 channel closure (i.e. limits ER Ca(2+) leakage) via the Sec61 channel with the help of ERj3 and ERj6. Interestingly, deletion of ERj6 causes pancreatic ß-cell failure and diabetes in mice and humans. We suggest that co-chaperone-controlled gating of the Sec61 channel by BiP is particularly important for cells, which are highly active in protein secretion, and that breakdown of this regulatory mechanism can cause apoptosis and disease.

A non-canonical mechanism for Crm1-export cargo complex assembly.

The transport receptor Crm1 mediates the export of diverse cargos containing leucine-rich nuclear export signals (NESs) through complex formation with RanGTP. To ensure efficient cargo release in the cytoplasm, NESs have evolved to display low affinity for Crm1. However, mechanisms that overcome low affinity to assemble Crm1-export complexes in the nucleus remain poorly understood. In this study, we reveal a new type of RanGTP-binding protein, Slx9, which facilitates Crm1 recruitment to the 40S pre-ribosome-associated NES-containing adaptor Rio2. In vitro, Slx9 binds Rio2 and RanGTP, forming a complex. This complex directly loads Crm1, unveiling a non-canonical stepwise mechanism to assemble a Crm1-export complex. A mutation in Slx9 that impairs Crm1-export complex assembly inhibits 40S pre-ribosome export. Thus, Slx9 functions as a scaffold to optimally present RanGTP and the NES to Crm1, therefore, triggering 40S pre-ribosome export. This mechanism could represent one solution to the paradox of weak binding events underlying rapid Crm1-mediated export.

Interaction of Pseudomonas aeruginosa Exotoxin A with the human Sec61 complex suppresses passive calcium efflux from the endoplasmic reticulum.

According to live-cell calcium-imaging experiments, the Sec61 complex is a passive calcium-leak channel in the human endoplasmic reticulum (ER) membrane that is regulated by ER luminal immunoglobulin heavy chain binding protein (BiP) and cytosolic Ca(2+)-calmodulin. In single channel measurements, the open Sec61 complex is Ca(2+) permeable. It can be closed not only by interaction with BiP or Ca(2+)-calmodulin, but also with Pseudomonas aeruginosa Exotoxin A which can enter human cells by retrograde transport. Exotoxin A has been shown to interact with the Sec61 complex and, thereby, inhibit ER export of immunogenic peptides into the cytosol. Here, we show that Exotoxin A also inhibits passive Ca(2+) leakage from the ER in human cells, and we characterized the N-terminus of the Sec61 α-subunit as the relevant binding site for Exotoxin A.

Targeting cell migration and the endoplasmic reticulum stress response with calmodulin antagonists: a clinically tested small molecule phenocopy of SEC62 gene silencing in human tumor cells.

BACKGROUND: Tumor cells benefit from their ability to avoid apoptosis and invade other tissues. The endoplasmic reticulum (ER) membrane protein Sec62 is a key player in these processes. Sec62 is essential for cell migration and protects tumor cells against thapsigargin-induced ER stress, which are both linked to cytosolic Ca²âº. SEC62 silencing leads to elevated cytosolic Ca²âº and increased ER Ca²âº leakage after thapsigargin treatment. Sec62 protein levels are significantly increased in different tumors, including prostate, lung and thyroid cancer. METHODS: In lung cancer, the influence of Sec62 protein levels on patient survival was analyzed using the Kaplan-Meier method and log-rank test. To elucidate the underlying pathophysiological functions of Sec62, Ca²âº imaging techniques, real-time cell analysis and cell migration assays were performed. The effects of treatment with the calmodulin antagonists, trifluoperazine (TFP) and ophiobolin A, on cellular Ca²âº homeostasis, cell growth and cell migration were compared with the effects of siRNA-mediated Sec62 depletion or the expression of a mutated SEC62 variant in vitro. Using Biacore analysis we examined the Ca²âº-sensitive interaction of Sec62 with the Sec61 complex. RESULTS: Sec62 overproduction significantly correlated with reduced patient survival. Therefore, Sec62 is not only a predictive marker for this type of tumor, but also an interesting therapeutic target. The present study suggests a regulatory function for Sec62 in the major Ca²âº leakage channel in the ER, Sec61, by a direct and Ca²âº-sensitive interaction. A Ca²âº-binding motif in Sec62 is essential for its molecular function. Treatment of cells with calmodulin antagonists mimicked Sec62 depletion by inhibiting cell migration and rendering the cells sensitive to thapsigargin treatment. CONCLUSIONS: Targeting tumors that overproduce Sec62 with calmodulin antagonists in combination with targeted thapsigargin analogues may offer novel personalized therapeutic options.

BiP-mediated closing of the Sec61 channel limits Ca2+ leakage from the ER.

In mammalian cells, signal peptide-dependent protein transport into the endoplasmic reticulum (ER) is mediated by a dynamic protein-conducting channel, the Sec61 complex. Previous work has characterized the Sec61 channel as a potential ER Ca(2+) leak channel and identified calmodulin as limiting Ca(2+) leakage in a Ca(2+)-dependent manner by binding to an IQ motif in the cytosolic aminoterminus of Sec61α. Here, we manipulated the concentration of the ER lumenal chaperone BiP in cells in different ways and used live cell Ca(2+) imaging to monitor the effects of reduced levels of BiP on ER Ca(2+) leakage. Regardless of how the BiP concentration was lowered, the absence of available BiP led to increased Ca(2+) leakage via the Sec61 complex. When we replaced wild-type Sec61α with mutant Sec61αY344H in the same model cell, however, Ca(2+) leakage from the ER increased and was no longer affected by manipulation of the BiP concentration. Thus, BiP limits ER Ca(2+) leakage through the Sec61 complex by binding to the ER lumenal loop 7 of Sec61α in the vicinity of tyrosine 344.

Different effects of Sec61α, Sec62 and Sec63 depletion on transport of polypeptides into the endoplasmic reticulum of mammalian cells.

Co-translational transport of polypeptides into the endoplasmic reticulum (ER) involves the Sec61 channel and additional components such as the ER lumenal Hsp70 BiP and its membrane-resident co-chaperone Sec63p in yeast. We investigated whether silencing the SEC61A1 gene in human cells affects co- and post-translational transport of presecretory proteins into the ER and post-translational membrane integration of tail-anchored proteins. Although silencing the SEC61A1 gene in HeLa cells inhibited co- and post-translational transport of signal-peptide-containing precursor proteins into the ER of semi-permeabilized cells, silencing the SEC61A1 gene did not affect transport of various types of tail-anchored protein. Furthermore, we demonstrated, with a similar knockdown approach, a precursor-specific involvement of mammalian Sec63 in the initial phase of co-translational protein transport into the ER. By contrast, silencing the SEC62 gene inhibited only post-translational transport of a signal-peptide-containing precursor protein.

Ca2+-calmodulin inhibits tail-anchored protein insertion into the mammalian endoplasmic reticulum membrane.

Cytosolic components and pathways have been identified that are involved in inserting tail-anchored (TA) membrane proteins into the yeast or mammalian endoplasmic reticulum (ER) membrane. Searching for regulatory mechanisms of TA protein biogenesis, we found that Ca(2+)-calmodulin (CaM) inhibits the insertion of TA proteins into mammalian ER membranes and that this inhibition is prevented by trifluoperazine, a CaM antagonist that interferes with substrate binding of Ca(2+)-CaM. The effects of Ca(2+)-CaM on cytochrome b(5) and Synaptobrevin 2 suggest a direct interaction between Ca(2+)-CaM and TA proteins. Thus, CaM appears to regulate TA insertion into the ER membrane in a Ca(2+) dependent manner.

Live cell calcium imaging combined with siRNA mediated gene silencing identifies Ca²âº leak channels in the ER membrane and their regulatory mechanisms.

In mammalian cells, the endoplasmic reticulum (ER) plays a key role in protein biogenesis as well as in calcium signalling. The heterotrimeric Sec61 complex in the ER membrane provides an aqueous path for newly-synthesized polypeptides into the lumen of the ER. Recent work from various laboratories suggested that this heterotrimeric complex may also form transient Ca(2+) leak channels. The key observation for this notion was that release of nascent polypeptides from the ribosome and Sec61 complex by puromycin leads to transient release of Ca(2+) from the ER. Furthermore, it had been observed in vitro that the ER luminal protein BiP is involved in preventing ion permeability at the level of the Sec61 complex. We have established an experimental system that allows us to directly address the role of the Sec61 complex as potential Ca(2+) leak channel and to characterize its putative regulatory mechanisms. This system combines siRNA mediated gene silencing and live cell Ca(2+) imaging. Cells are treated with siRNAs that are directed against the coding and untranslated region (UTR), respectively, of the SEC61A1 gene or a negative control siRNA. In complementation analysis, the cells are co-transfected with an IRES-GFP vector that allows the siRNA-resistant expression of the wildtype SEC61A1 gene. Then the cells are loaded with the ratiometric Ca(2+)-indicator FURA-2 to monitor simultaneously changes in the cytosolic Ca(2+) concentration in a number of cells via a fluorescence microscope. The continuous measurement of cytosolic Ca(2+) also allows the evaluation of the impact of various agents, such as puromycin, small molecule inhibitors, and thapsigargin on Ca(2+) leakage. This experimental system gives us the unique opportunities to i) evaluate the contribution of different ER membrane proteins to passive Ca(2+) efflux from the ER in various cell types, ii) characterize the proteins and mechanisms that limit this passive Ca(2+) efflux, and iii) study the effects of disease linked mutations in the relevant components.

Interaction of calmodulin with Sec61α limits Ca2+ leakage from the endoplasmic reticulum.

In eukaryotes, protein transport into the endoplasmic reticulum (ER) is facilitated by a protein-conducting channel, the Sec61 complex. The presence of large, water-filled pores with uncontrolled ion permeability, as formed by Sec61 complexes in the ER membrane, would seriously interfere with the regulated release of calcium from the ER lumen into the cytosol, an essential mechanism for intracellular signalling. We identified a calmodulin (CaM)-binding motif in the cytosolic N-terminus of mammalian Sec61α that bound CaM but not Ca2+-free apocalmodulin with nanomolar affinity and sequence specificity. In single-channel measurements, CaM potently mediated Sec61-channel closure in Ca2+-dependent manner. At the cellular level, two different CaM antagonists stimulated calcium release from the ER through Sec61 channels. However, protein transport into microsomes was not modulated by Ca2+-CaM. Molecular modelling of the ribosome/Sec61/CaM complexes supports the view that simultaneous ribosome and CaM binding to the Sec61 complex may be possible. Overall, CaM is involved in limiting Ca2+ leakage from the ER.