STIM2 Is an Inhibitor of STIM1-Mediated Store-Operated Ca2+ Entry

Abstract

The coupling mechanism between endoplasmic reticulum (ER) Ca(2+) stores and plasma membrane (PM) store-operated channels (SOCs) remains elusive [1-3]. STIM1 was shown to play a crucial role in this coupling process [4-7]; however, the role of the closely related STIM2 protein remains undetermined. We reveal that STIM2 is a powerful SOC inhibitor when expressed in HEK293, PC12, A7r5, and Jurkat T cells. This contrasts with gain of SOC function in STIM1-expressing cells. While STIM1 is expressed in both the ER and plasma membrane, STIM2 is expressed only intracellularly. Store depletion induces redistribution of STIM1 into distinct "puncta." STIM2 translocates into puncta upon store depletion only when coexpressed with STIM1. Double labeling shows coincidence of STIM1 and STIM2 within puncta, and immunoprecipitation reveals direct interactions between STIM1 and STIM2. Independent of store depletion, STIM2 colocalizes with and blocks the function of a STIM1 EF-hand mutant that preexists in puncta and is constitutively coupled to activate SOCs. Thus, whereas STIM1 is a required mediator of SOC activation, STIM2 is a powerful inhibitor of this process, interfering with STIM1-mediated SOC activation at a point downstream of puncta formation. The opposing functions of STIM1 and STIM2 suggest they may play a coordinated role in controlling SOC-mediated Ca(2+) entry signals.

ORAI1-Mediated Calcium Influx in Lactation and in Breast Cancer

“…Indeed, basal calcium levels declined in the presence of Stim2 siRNA-mediated inhibition in HC11 mammary cells. A modest, rather than an order or magnitude increase, in STIM2 levels during lactation would avoid the potential inhibitory effects of STIM2 on ORAI1 activity (38), which occurs with sustained high STIM2 overexpression (35). It is likely that the relative levels of STIM1, STIM2, and ORAI1 are important in the context of the physiological environment, as the expression levels of STIM1 and ORAI1 dictate the nature of the channel current (49).…”

“…5A). Given the report of opposing effects of STIM1 and STIM2 on ORAI1-mediated calcium influx (38), the ratio of the expression of these 2 genes was compared in breast cancer cell lines. The comparison between the relative levels of STIM1 and STIM2 indicated that the relative levels of STIM1 to STIM2 were similar in the breast cancer cell lines, with the exception of the ERBB2 overexpressing cell line SK-BR-3 (Fig.…”

“…The altered relative levels of STIM1 and STIM2 in SK-BR-3 cells did not seem to be a consequence of ERBB2 activity or expression, as neither treatment with the ERBB2 inhibitor AG825 (10 mmol/L) in SK-BR-3 cells nor the activation of ERBB2 in MCF-7 cells with heregulin b1 (10 ng/mL) had a significant effect on the STIM1/STIM2 ratio (data not shown). Given the different roles and properties of STIM1 and STIM2 (35), and the report that overexpression of STIM2 may have inhibitory effects on the STIM1 activation of ORAI1 (38), and our own finding that some breast cancer cell lines are characterized by an alteration in the STIM1/ STIM2 ratio, we explored further the significance of STIM1 and STIM2 expression levels in breast cancer.…”

Alternative splicing converts STIM2 from an activator to an inhibitor of store-operated calcium channels

“…However, STIM1 and STIM2 are known to form heterodimers (Williams et al, 2001;Soboloff et al, 2006;Darbellay et al, 2010), which could provide a mechanism to recruit STIM2β to Orai channels and facilitate inhibition. We used a FRET assay to assess the ability of STIM2β to heterodimerize with STIM1.…”

Calcium signaling in lymphocytes

“…STIM1 is an acknowledged activator of store-operated Ca 2+ entry and CRAC channel function as discussed below, but the role of STIM2 was initially controversial, with conflicting results reported by different groups [36,37,43]. An important clue as to the possible distinction between the two proteins came when STIM2 showed up as a major hit in an RNAi screen designed to find regulators of basal Ca 2+ concentration, which is kept within very narrow limits in cells [••44].…”