Structural and stoichiometric determinants of Ca2+ release-activated Ca2+ (CRAC) channel Ca2+-dependent inactivation

Abstract

Depletion of intracellular Ca(2+) stores in mammalian cells results in Ca(2+) entry across the plasma membrane mediated primarily by Ca(2+) release-activated Ca(2+) (CRAC) channels. Ca(2+) influx through these channels is required for the maintenance of homeostasis and Ca(2+) signaling in most cell types. One of the main features of native CRAC channels is fast Ca(2+)-dependent inactivation (FCDI), where Ca(2+) entering through the channel binds to a site near its intracellular mouth and causes a conformational change, closing the channel and limiting further Ca(2+) entry. Early studies suggested that FCDI of CRAC channels was mediated by calmodulin. However, since the discovery of STIM1 and Orai1 proteins as the basic molecular components of the CRAC channel, it has become apparent that FCDI is a more complex phenomenon. Data obtained using heterologous overexpression of STIM1 and Orai1 suggest that, in addition to calmodulin, several cytoplasmic domains of STIM1 and Orai1 and the selectivity filter within the channel pore are required for FCDI. The stoichiometry of STIM1 binding to Orai1 also has emerged as an important determinant of FCDI. Consequently, STIM1 protein expression levels have the potential to be an endogenous regulator of CRAC channel Ca(2+) influx. This review discusses the current understanding of the molecular mechanisms governing the FCDI of CRAC channels, including an evaluation of further experiments that may delineate whether STIM1 and/or Orai1 protein expression is endogenously regulated to modulate CRAC channel function, or may be dysregulated in some pathophysiological states.

Molecular mechanisms of STIM/Orai communication

“…Several studies have reported that the activation of Orai1 depends on the number of interacting STIM1 proteins ( 59 , 79 , 145 ). Scrimgeour et al ( 145 , 146 ) have demonstrated that the extent of CRAC current inactivation depends on the number of STIM1 molecules that bind to the Orai1 channel. The fewer STIM1 proteins interact with Orai1, the fewer CRAC channel currents inactivate.…”

“…The fewer STIM1 proteins interact with Orai1, the fewer CRAC channel currents inactivate. Furthermore, an enhanced STIM1-to-Orai1 ratio increases selectivity for divalent cations as well as decreasing 2-aminoethoxydiphenyl borate (2-APB)-mediated current potentiation ( 145 , 146 ).…”

“…STIM1 has been suggested to define the extent of inactivation in dependence of the STIM1-to-Orai1 ratio ( 145 , 146 ). Furthermore, an acidic cluster (aa475–483) termed the CRAC modulatory domain (CMD) in STIM1 COOH terminus is indispensable for FCDI of Orai/CRAC channels ( 33 , 75 , 112 ).…”

Evidence for the interaction of peroxiredoxin-4 with the store-operated calcium channel activator STIM1 in liver cells

“…SOC activation is initiated by a decrease in the Ca 2+ concentration in the lumen of a sub-region of the ER which is devoid of ribosomes and located in ER-PM junctions at the periphery of the cell [1,5,6]. This leads to the dissociation of Ca 2+ from the luminal E, F hand-sterile alpha (EF-SAM) Ca 2+ binding motif in the N-terminal of STIM1, conformational changes in STIM1, movement of STIM1 in the plane of the ER membrane, interaction of the cytoplasmic CRAC activation domain (CAD) in the C-terminal of STIM1 with Orai1 located in the PM, and formation of an active functional SOC [1,7]. Functional SOCs are composed of a hexamer of Orai1 polypeptides and approximately 4 STIM1 polypeptides [1,7].…”

“…This leads to the dissociation of Ca 2+ from the luminal E, F hand-sterile alpha (EF-SAM) Ca 2+ binding motif in the N-terminal of STIM1, conformational changes in STIM1, movement of STIM1 in the plane of the ER membrane, interaction of the cytoplasmic CRAC activation domain (CAD) in the C-terminal of STIM1 with Orai1 located in the PM, and formation of an active functional SOC [1,7]. Functional SOCs are composed of a hexamer of Orai1 polypeptides and approximately 4 STIM1 polypeptides [1,7]. It is generally thought that in order to achieve SOCE activation, Orai1 must be present in the PM and STIM1 in the peripheral ER, located in close proximity to the plasma membrane [1].…”