Calcium signaling by STIM and Orai: intimate coupling details revealed

Abstract

STIM (stromal interaction molecule) and Orai, two recently identified protein families, mediate cellular Ca(2+) signals through a remarkably dynamic interaction. STIM proteins are sensors of Ca(2+) stored within the endoplasmic reticulum (ER). Orai proteins are highly selective plasma membrane (PM) channels that allow only Ca(2+) ions to flow into cells. Although present in separate membranes, the two proteins undergo profound reorganization culminating in an exquisite pas de deux within small junctional regions between the ER and PM. Before these proteins can embrace, STIM undergoes an activation process triggered by depletion of Ca(2+) stores. During its union with Orai, STIM induces the channel pore within Orai to open, allowing Ca(2+) ions to flow through the PM and provide crucial intracellular signals. Recent studies on the activation of STIM and its coupling to Orai provide valuable new insights into the nature of the liaison between these two proteins and the intricate mechanism through which activation of Ca(2+) signals occurs.

Fig. 1

Molecular architecture of STIM-Orai interactions. In its resting state, acidic residues of the cytosolic coiled-coil domain (CC1) of STIM1 electrostatically bind to and mask the basic residues of the active site of the CAD/SOAR domain. The transition to the activated state of STIM1 involves conformational changes on both the luminal and cytosolic sides of the protein. Decreased ER luminal Ca²⁺ causes dissociation of Ca²⁺ from the tight complex formed by the canonical (cEF) and hidden (hEF) EF-hands and the sterile α motif (SAM) on the luminal side of STIM1. This causes interactions between STIM1 molecules (see Fig. 2) and alteration in the cytosolic domain of STIM1 such that the intramolecular electrostatic interactions between CC1 and CAD/SOAR are broken and the CAD/SOAR domain positive charges are free to interact with the acidic domain within the C-terminal domain of Orai1 and activate the channel (“A”). The STIM1 protein also appears to interact with the N-terminal domain of Orai1 (“B”), although the nature of this interaction is not known. In addition, the far C-terminal polybasic sequence of STIM1 interacts with the plasma membrane (“C”), likely with negatively charged phospholipids.

Fig. 2

Store-operated oligomerization of STIM1 and formation of the active STIM/Orai complex. In its “resting” state, STIM1 may exist as a dimer maintained by interactions between the first coiled-coil (CC1) segment of STIM1. In the dimeric state, electrostatic interactions between basic residues in the CAD/SOAR domain and acidic residues within STIM1-CC1 mask the CAD/SOAR domain, making it incapable of interacting with Orai1. Dissociation of Ca²⁺ from the luminal side of STIM1 causes oligomerization of STIM1 through the EF-SAM domains. Although initiated within the ER lumen, STIM1 oligomer formation also involves interactions between CAD/SOAR domains, which may release this domain from its association with CC1. This “activated” oligomer of STIM1 interacts with the PM by virtue of exposed C-terminal polybasic domains and forms ER-PM junctions. The activated oligomeric STIM1 can then tether and trap Orai1 channels diffusing in the PM. At the same time, an electrostatic interaction between the available acidic domains in CAD/SOAR and basic domains in the C terminus of Orai1 results in formation of the activated channel complex, thereby allowing Ca²⁺ entry.