Palmitoylation is the Switch that Assigns Calnexin to Quality Control or ER Calcium Signaling

The palmitoylation of calnexin serves to enrich calnexin on the mitochondria-associated membrane (MAM). Given a lack of information on the significance of this finding, we have investigated how this endoplasmic reticulum (ER)-internal sorting signal affects the functions of calnexin. Our results demonstrate that palmitoylated calnexin interacts with sarcoendoplasmic reticulum (SR) calcium transport ATPase (SERCA) 2b and that this interaction determines ER calcium content and the regulation of ER-mitochondria calcium crosstalk. In contrast, non-palmitoylated calnexin interacts with the oxidoreductase ERp57 and performs its well-known function in quality control. Interestingly, our results also show that calnexin palmitoylation is an ER stress-dependent mechanism. Following a short term ER stress, calnexin quickly becomes less palmitoylated, which shifts its function from the regulation of calcium signaling towards chaperoning and quality control of known substrates. These changes also correlate with a preferential distribution of calnexin to the MAM under resting conditions or the rough ER and ER quality control compartment (ERQC) following ER stress. Our results have therefore identified the switch that assigns calnexin either to calcium signaling or to protein chaperoning.