The short N-terminal domains of STIM1 and STIM2 control the activation kinetics of Orai1 channels

STIM1 and STIM2 are dynamic transmembrane endoplasmic reticulum Ca²⁺ sensors, coupling directly to activate plasma membrane Orai Ca²⁺ 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 Ca²⁺ entry and drastically slows store-induced Orai1 channel activation. Conversely, the 55-amino-acid STIM₂ terminus substituted within STIM1 strikingly enhances both Orai1-mediated Ca²⁺ 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 Ca²⁺.