Figure 2A. Immunoblotting analysis of Neuro2a (N2a) cells expressing Sig1R-FLAG variants. B. Cycloheximide chase analysis on N2a cells expressing Sig1R-FLAG. Quantitative data of immunoblotting for the levels of Sig1R-FLAGprotein and its variants during the cyclohexamide chase were plotted as mean ± SEM of three independent experiments (upper panel). Representative immunoblots for the levels of Sig1R-FLAGproteins were shown (lower panel). **: p < 0.01, *: p = 0.0216 in E102Q vs. wild-type (WT); ##: p < 0.01, #: p < 0.05 in L95fs vs. WT. A one-way ANOVA with subsequent post hoc Tukey's test. C. N2a cells transfected with Sig1R-FLAG variants were incubated with MG-132 (10 µM) or the combination of E64d and pepstatin A (5 µg/mL each) (E64d/PepA) for 8 h. The relative mean levels of Sig1R-FLAG variants determined by immunoblotting from three independent experiments were plotted as mean ± SEM (upper panel). The representative immunoblots were shown (lower panel). *: p < 0.05 vs. no inhibitor control. A one-way ANOVA with subsequent post hoc Tukey's test. D. Schematic representation of subcellular fractionation used in this study. Details are described in Materials and methods section (Cyto: cytosol, P1: nucleus and debris, Mito: mitochondria, MAM: mitochondria-associated membrane, P3: microsomal fraction). E and F. Subcellular localization of inositol 1,4,5-triphosphate receptor type 1 (IP3R1) and type 3 (IP3R3) in N2a cells stably expressing human IP3R3 (N2a-IP3R3 cells, E). IP3R1 and IP3R3 in the isolated fractions were identified by immunoblotting with the specific antibodies as indicated. Proper fractionation of these samples was confirmed by the fraction specific markers as indicated. E and F. Subcellular localization of inositol 1,4,5-triphosphate receptor type 1 (IP3R1) and type 3 (IP3R3) in HeLa cells (F). IP3R1 and IP3R3 in the isolated fractions were identified by immunoblotting with the specific antibodies as indicated. Proper fractionation of these samples was confirmed by the fraction specific markers as indicated. G. Interaction of IP3R3 with wild-type Sig1R or E102Q ALS-linked Sig1R mutant. Sig1R-FLAG variants were transfected in N2a-IP3R3 cells, and IP3R3 was co-immunoprecipitated using an anti-FLAG antibody and identified by immunoblotting with the specific antibodies as indicated. H. Suppression of Sig1R by siRNA. Lysates of N2a cells transfected with control siRNA (siCtrl) or siRNA against Sig1R (siSig1R) for 24 h were blotted. Similar results were obtained from three independent experiments. Paired t-test was used for statistical analysis. I and J. Cytoplasmic calcium (Ca2+) flux in N2a or human IP3R3 stably expressing N2a (N2a-IP3R3) cells. siCtrl or siSig1R was transfected to N2a or N2a-IP3R3 cells, then cytoplasmicCa2+ flux were determined with fluo-4 and Case12-mito, respectively. The fluorescent intensity was normalized to the intensity in resting state at 0 s, and plotted as mean ± SD. I and J. mitochondrial (J) calcium (Ca2+) flux in N2a or human IP3R3 stably expressing N2a (N2a-IP3R3) cells. siCtrl or siSig1R was transfected to N2a or N2a-IP3R3 cells, then mitochondrial Ca2+ flux were determined with fluo-4 and Case12-mito, respectively. The fluorescent intensity was normalized to the intensity in resting state at 0 s, and plotted as mean ± SD. K and L. Cytoplasmic (K) or mitochondrial (L) Ca2+ flux in N2a-IP3R3 cells. siCtrl or siSig1R was transfected with or without the Sig1R-FLAG variants. The data are obtained and plotted as described above. n = 10 each. **: p < 0.01, *: p < 0.05 (I-L). Two-way ANOVA with subsequent post hoc Tukey's test (I-J).