Protein folding, quality control and degradation in the endoplasmic reticulum: Assessing the function of ER-resident molecular chaperones and folding factors
About 30% of the mammalian gene products are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER) membrane. These polypeptides enter co-translationally in the ER lumen, which contains resident molecular chaperones and folding factors that assist their maturation. Native proteins are released from the ER lumen and are transported through the secretory pathway at their final intra- or extra-cellular destination. Polypeptides that have failed acquisition of the native structure are translocated across the ER membrane and are destroyed by cytosolic proteasomes. Cell and organism homeostasis relies on a balanced activity of the ER folding, quality control and degradation machineries as shown by the dozens of human diseases related to defective maturation or disposal of individual polypeptides generated in the ER. The aim of our work is to understand the molecular mechanisms regulating- chaperone-assisted protein folding in the mammalian ER;- chaperone-assisted protein disposal from the mammalian ER;- a novel post-translational process determining intralumenal chaperone content that we define as EDEM tuning;- the intralumenal content of calnexin in cells from a mouse model of combined lipoprotein lipase deficiency. We are in the best position to address and answer at least part of the open questions remaining in the field because of our extensive knowledge of the basic mechanisms underlying protein folding and quality control in mammalian cells and because of the several tools and model systems generated and available in our lab (e.g. cell lines lacking individual chaperones and folding factors and a series of folding-competent and folding-defective membrane-bound, soluble, glycosylated and non-glycosylated model proteins). We believe that a thorough knowledge of the processes and of the factors determining the fate of newly synthesized polypeptides in the ER lumen is essential. It will eventually allow manipulation of protein folding, quality control and degradation events. This will be instrumental to delay progression or even to cure diseases caused by inefficient functioning of the cellular protein factory. It will also ameliorate our capacity to produce high amounts of active recombinant proteins to be employed in the clinics and in the industry.