Fig 1: Both E102Q and L95fs Sig1R variants are unstable and lose their abilities to control Ca2+ flux in Neuro2a cells AImmunoblotting analysis of Neuro2a (N2a) cells expressing Sig1R‐FLAG variants.BCycloheximide chase analysis on N2a cells expressing Sig1R‐FLAG. Quantitative data of immunoblotting for the levels of Sig1R‐FLAG protein and its variants during the cycloheximide chase were plotted as mean ± SEM of three independent experiments (upper panel). Representative immunoblots for the levels of Sig1R‐FLAG proteins were shown (lower panel). **P < 0.01, *P = 0.0216 in E102Q versus wild type (WT); ## P < 0.01, # P < 0.05 in L95fs versus WT. A one‐way ANOVA with subsequent post hoc Tukey's test.CN2a 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). Representative immunoblots were shown (lower panel). *P < 0.05 versus no inhibitor control. A one‐way ANOVA with subsequent post hoc Tukey's test.DSchematic representation of subcellular fractionation used in this study. Details are described in the Materials and Methods section (Cyto, cytosol; P1, nucleus and debris; Mito, mitochondria; MAM, mitochondria‐associated membrane; P3, microsomal fraction).E, FSubcellular 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) or 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.GInteraction 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.HSuppression 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, JCytoplasmic (I) or mitochondrial (J) calcium (Ca2+) flux in N2a or N2a cells stably expressing human IP3R3 (N2a‐IP3R3). siCtrl or siSig1R was transfected into N2a or N2a‐IP3R3 cells, then cytoplasmic and mitochondrial Ca2+ flux were determined with fluo‐4 and Case12‐mito, respectively. The fluorescence intensity was normalized to the intensity in resting state at 0 s and plotted as mean ± SD. **P < 0.01, *P < 0.05. Two‐way ANOVA with subsequent post hoc Tukey's test. n = 10 each.K, LCytoplasmic (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 in (I and J). n = 10 each. **P < 0.01, *P < 0.05. Two‐way ANOVA with subsequent post hoc Tukey's test. Source data are available online for this figure.