Mechanism of different spatial distributions of Caenorhabditis elegans and human STIM1 at resting state.

Recent studies have identified STIM1 and Orai1 as essential and conserved components of the Ca2+ release-activated Ca2+ (CRAC) channel. However, the reason STIM1 exhibits different distributions in nematode Caenorhabditis elegans and in human cells before endoplasmic reticulum (ER) calcium store depletion has not been clarified. Compared to the diffuse ER distribution of human STIM1 (H.STIM1), we found that C. elegans STIM1 (C.STIM1) was pre-oligomerized in puncta at the cell periphery before Ca2+ store depletion when expressed in HEK293 cells. Interestingly, these C.STIM1 puncta failed to induce aggregation of C. elegans Orai1 (C.Orai1), and no CRAC current was detected in quiescent cells. However, upon store depletion, C.Orai1 and C.STIM1 functioned as a pair to locally sense the store depletion signal and to activate the CRAC channel. We substituted the N-terminus of H.STIM1 for the N-terminus of C.STIM1 (H_C.STIM1), which resulted in pre-puncta resting localization. In contrast, by replacing the C-terminus of C.STIM1 with that of H.STIM1, we made a chimeric protein (C.STIM1_H) that exhibited two distribution profiles at resting state, a diffuse ER pattern like H.STIM1, and large aggregates. Taken together, our results suggest that (1) despite highly conserved functional domains, C. elegans STIM1 and human STIM1 display different spatial distributions in HEK293 cells before store depletion; (2) the C.STIM1 puncta at peripheral sites are not sufficient for the aggregation and activation of C.Orai1 in the absence of store depletion; (3) the distinct distributions of C.STIM1 and H.STIM1 at resting state are determined by the cytoplasmic region of STIM1.