Introduction 2004 2004 −1 −1 2004 2004 2004 2007 1999 1997 2005 2005 1989 1991 Protein folding in the ER cis-trans 2+ 2+ 2+ 2+ 1 Fig. 1  d i \documentclass[12pt]{minimal}
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\begin{document}$$ {\text{HPO}}^{{{\text{2}} - }}_{{\text{4}}} $$\end{document}  N C 1 2004 1990 2004 1 2007 K M 2004 N C 2007 3 9 2 d  d N d d d 1 Saccharomyces cerevisiae N 2005 2006 2 2004 2004 2004 2001 Fig. 2 Disulfide bond formation and isomerization reactions catalyzed by PDI  2007 3 2006a 2006b N 2003 2001 IPK 2006 Fig. 3 ER associated protein degradation. The exact nature of the retrotranslocation channel is unknown. The list of E2 and E3 enzymes catalyzing ubiquitination of ERAD substrates is not exhaustive. Abbreviations: NEF = HSP70 co-chaperone, Ub = ubiquitin  Engineering of chaperone machineries 2004 2006b 2004 2002 2005 2003 2003 2003 2006 2006 2005 2007 2007 2006b 1995 2007 2004 2005 2003 2006 2007 2007 2004 2006 1992 1997 2003 2007 2002 2006b 1992 2007 Unfolded protein response (UPR) 2005 4 2004 2005 2+ 2007 Fig. 4 Signal transduction pathway in the UPR. In the human UPR caspase 4 substitutes for caspase 12. In yeast and filamentous fungi, the IRE1-XBP-1 (Hac1p/HACA) pathway is the only known UPR signal transduction pathway. [Reprinted in modified form with permission from Bentham Science Publishers from Schröder M, Kaufman, RJ (2006) Divergent roles of IRE1α and PERK in the unfolded protein response. Curr Mol Med 6:5–36]  4 2007 2005 2006a 2007 4 2003 1 via BCL 2001 tribbles-related protein TRB 2005 in trans 2007 4 HAC1 2000 2003 2007 4 N L 2006 2005 XBP-1 2006 Engineering of the UPR Saccharomyces cerevisiae Aspergillus niger 2003a b 2007 2006 2006 2005 2003a 2007 2006 2004 Conclusions Engineering of chaperone systems by overexpressing a single component of the ER-resident protein folding machinery has overall yielded mixed results. Our basic understanding of protein folding in the ER is still incomplete. Addressing these open questions should underpin engineering approaches to improve the performance of chaperone systems. A more detailed understanding of chaperone function and regulation should inform future work to improve chaperone systems. Co-expression of different holdases or targeting of heterologous or cytosolic holdases to the ER may yield more consistent improvements for different heterologous proteins. The function of overexpressed chaperones may not be the same as at their normal physiological concentrations, because of the lack of a corresponding increase in co-chaperones and co-factors. To improve foldase function, concomitant elevation of BiP, its co-chaperones, and ATP levels should be attempted. Engineering of the UPR suggests that mimicking an UPR by expression of its activated signaling molecules does not consistently improve productivities. Dissection of UPR signaling activities may be necessary to improve heterologous protein secretion. To place engineering of the UPR on an informed basis, we need to understand the UPR in more detail. It is still not clear what the most upstream events in activation of the UPR are, how UPR signaling integrates into cellular signaling, and how the UPR decides between a prosurvival and an apoptotic response to ER stress. Cell engineering also needs to address the potential problem that increased oxidative protein folding may be inherently toxic to cells, because of elevated cellular ROS levels.