Regulation of Store-Operated Ca2+ Entry by SARAF

Abstract

Calcium (Ca²⁺) signaling plays a dichotomous role in cellular biology, controlling cell survival and proliferation on the one hand and cellular toxicity and cell death on the other. Store-operated Ca²⁺ entry (SOCE) by CRAC channels represents a major pathway for Ca²⁺ entry in non-excitable cells. The CRAC channel has two key components, the endoplasmic reticulum Ca²⁺ sensor stromal interaction molecule (STIM) and the plasma-membrane Ca²⁺ channel Orai. Physical coupling between STIM and Orai opens the CRAC channel and the resulting Ca²⁺ flux is regulated by a negative feedback mechanism of slow Ca²⁺ dependent inactivation (SCDI). The identification of the SOCE-associated regulatory factor (SARAF) and investigations of its role in SCDI have led to new functional and molecular insights into how SOCE is controlled. In this review, we provide an overview of the functional and molecular mechanisms underlying SCDI and discuss how the interaction between SARAF, STIM1, and Orai1 shapes Ca²⁺ signaling in cells.

Keywords: CRAC channel; Orai1; SARAF; STIM1; slow calcium dependent inactivation (CDI); store operated calcium entry (SOCE).

Figure 1

Interaction between the CRAC channel and SARAF during channel activation. Rectangle and oval symbols indicate the different domains of STIM1. At resting conditions, a fraction of SARAF (green) interacts with STIM1. Following ER Ca²⁺ depletion (step #1), SARAF undergoes a transient disassociation from STIM1 but the interaction is quickly regained (step #2) and further augmented (step #3) following binding of STIM1 to Orai1.

Figure 2

Models for SCDI by SARAF. Rectangle and oval symbols indicate the different domains of STIM1 as in Figure 1. (A) Ca²⁺-dependent interaction between ALG-2 (yellow) and SARAF (green) promotes channel inactivation via stabilization of SARAF dimers. (B) EFHB (brown) binds to the STIM1-Orai1 channel complex. Increase in Ca²⁺ causes disassociation of EFHB from STIM1. Subsequent binding of SARAF to STIM1 promotes channel inactivation. (C) Binding of Ca²⁺ to the STIM1 ID induces persistent channel inactivation when SARAF interacts with STIM1. (D) Binding of SARAF to STIM1 facilitates translocation of the channel complex from PM microdomains poor with PtdIns(4,5)P2 (off-white) to domains enriched with PtdIns(4,5)P2 (cyan). Local increase in Ca²⁺ promotes channel inactivation by remodeling the interaction of STIM1 with the PM.

Figure 3

Structure and putative redox-dependent rearrangement of the SARAF luminal fold. (A) Structure of the SARAF domain-swapped dimer (PDB ID: 6O2U). Chains A and B are shown in green and grey, respectively, and the 9th and 10th β-strands from chain A are also labeled. (B) Magnification of the structure of b1 to b8 from chain A with cysteine residues forming disulfide bonds highlighted in red. (C) Cartoon representation of the SARAF luminal fold showing putative dimer to monomer reversible transitions mediated by oxidation-reduction of cysteine pairs indicated in (B). S–S denotes disulfide bonds.