Store-operated Orai channels: structure and function

Abstract

In many animal cells, store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels function as an essential route for Ca(2+) entry. CRAC channels control many fundamental cellular functions including gene expression, motility, and cell proliferation, are involved in the etiology of several disease processes including a severe combined immunodeficiency syndrome, and have emerged as major targets for drug development. Although little was known of the molecular mechanisms of CRAC channel operation for several decades, the discovery of Orai1 as a prototypic CRAC channel protein and STIM1 as the endoplasmic reticulum (ER) Ca(2+) sensor has led to rapid progress in our understanding of the mechanisms and functions of CRAC channels. It is now known that activation of CRAC channels following ER Ca(2+) store depletion is governed by several events, which include the redistributions and accumulations of STIM1 and Orai1 into overlapping puncta at peripheral cellular sites, resulting in direct protein-protein interactions between the two proteins. In this chapter, I review the molecular features of the STIM and Orai proteins that regulate the gating and ion conduction mechanisms of CRAC channels.

Keywords: CRAC channel; Gating; Orai1; Permeation; Review; STIM1; Store-operated channel.

Figure 1.1

(A) A schematic representation of STIM1 and its key functional domains. These domains include: Sig, signal peptide; SAM, sterile alpha motif; TM, transmembrane domain; CC, coiled-coil domain; CAD, CRAC activation domain; ID_STIM1, inactivation domain of STIM1. (B) The predicted topology of Orai1. The STIM1-binding domains are shaded in pink. Key residues (mentioned in the text) are labeled.