The plasma membrane protein "Orai" (ORAI1 and ORAI2 in humans) forms the pore of the CRAC channel. The protein ORAI1 is a structural component of the CRAC calcium channel. ORAI1 interacts with the STIM1 protein. STIM1 is a transmembrane protein of the endoplasmic reticulum (ER). STIM1 can sense the concentration of Ca2+ inside the ER. When the concentration of Ca2+ inside the ER becomes low, STIM1 proteins aggregate and interact with ORAI1 located in the cell surface membrane. When the concentration of Ca2+ inside the ER approaches an upper set point, another protein, SARAF (TMEM66) associates with STIM1 to inactivate the store-operated calcium channel (SOCE).