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Background
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Antigen stimulation of immune cells triggers Ca++ entry through Ca++ release-activated Ca++ (CRAC) channels. The ORAI family is a recently identified set of proteins that are essential components of these CRAC channels. A missense mutation in the ORAI1 protein in humans is the cause of one form of hereditary severe combined immune deficiency (SCID) which results in ablated T-cell Ca++ entry. It has been suggested that ORAI1 functions as a highly selective Ca++ plasma membrane channel that is gated through interactions with the stromal interaction molecule 1 (STIM1), the store-activated endoplasmic reticulum Ca++ sensor. Like ORAI1, ORAI2 also functions as a highly selective Ca++ plasma membrane channel that is gated through interactions with STIM1, although at a lesser efficacy than ORAI1. Although ORAI3 can also function as Ca++ plasma membrane channel, ORAI3 channels failed to produce detectable Ca++ selective currents in cells co-transfected with ORAI3 and STIM1, indicating that ORAI3 channels undergo a lesser degree of depotentiation than ORAI1 or ORAI2. Na+ currents through ORAI1, 2 and 3 channels were equally inhibited by extracellular Ca++, indicating that each have similar affinities for Ca++ within the selectivity filter. STIM1, in its function as a Ca++ sensor and an activator of CRAC channels, migrates to the plasma membrane from endoplasmic reticulum-like sites which act as cellular Ca++ stores. A related molecule, STIM2, inhibits the STIM1-mediated store-operated Ca++ entry, and can form complexes with STIM1, suggesting these two proteins may play a coordinated role in controlling Ca++ entry. The ORAI antibodies are predicted to have no cross-reactivity to the other ORAI proteins. Similarly, the STIM antibodies will not cross-react with the other STIM protein.
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