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BiP is a chaperone that prevents misfolding of proteins in the ER. The protein is also a homeostatic sensor in the unfolded-protein response (UPR), a signalling pathway activated by ER stress. Its levels must therefore be tightly controlled to avoid inappropriate UPR activation. Speculating that post-transcriptional mechanisms regulate BiP, Ingrid Haas and co-workers have used a clever trick to discriminate regulation of BiP mRNA and protein levels. They introduced a mouse BiP gene under the control of a tetracyclin-sensitive promoter into human cells and compared the levels of human and mouse BiP protein and mRNA under different conditions (see p. 2443). They show that, after removal of tetracyclin, the artificially elevated levels of BiP mRNA do not increase the level of BiP protein in unstressed cells. Under ER stress, by contrast, BiP protein levels do rise, which the authors demonstrate is due to increased translational efficiency. Since the effects are independent of the 5' and 3' UTRs of BiP mRNA, Haas and co-workers conclude that a novel translational feedback mechanism is involved.
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