Abstract
STIM1 and STIM2 are dynamic transmembrane endoplasmic reticulum Ca(2+) sensors, coupling directly to activate plasma membrane Orai Ca(2+) entry channels. Despite extensive sequence homology, the STIM proteins are functionally distinct. We reveal that the short variable N-terminal random coil sequences of STIM1 and STIM2 confer profoundly different activation properties. Using Orai1-expressing HEK293 cells, chimeric replacement of the 43-amino-acid STIM1 N terminus with that of STIM2 attenuates Orai1-mediated Ca(2+) entry and drastically slows store-induced Orai1 channel activation. Conversely, the 55-amino-acid STIM2 terminus substituted within STIM1 strikingly enhances both Orai1-mediated Ca(2+) entry and constitutive coupling to activate Orai1 channels. Hence, STIM N termini are powerful coupling modifiers, functioning in STIM2 to "brake" the otherwise constitutive activation of Orai1 channels afforded by its high sensitivity to luminal Ca(2+).
Highlights
The transmembrane ER4 proteins STIM1 and STIM2 function as sensors of Ca2ϩ within ER stores [1, 2]
The lower affinity of the STIM2 EF-hand accounts for differences in the activation thresholds of STIM1 and STIM2 [16, 20, 21], it does not explain the slow kinetics of STIM2 nor its dominance over endogenous store-operated channels (SOCs) activation
To further characterize the effect of the N-terminal domains of STIM1 and STIM2 on the activation of Orai1, we examined the characteristics of Ca2ϩ release-activated Ca2ϩ (CRAC) current activation in stable Orai1expresssing HEK293 cells transfected with each of the holoproteins and chimeric constructs
Summary
The transmembrane ER4 proteins STIM1 and STIM2 function as sensors of Ca2ϩ within ER stores [1, 2]. Chimeric exchange of the short N-terminal generated using conventional whole cell recordings as flexible random coil domains had profound effects on Orai1 described previously [19].
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