Permeation, selectivity and gating in store-operated CRAC channels

Store-operated Ca²⁺ release-activated Ca²⁺ (CRAC) channels are a widespread mechanism for generating cellular Ca²⁺ signals and regulate many Ca²⁺-dependent functions, including transcription, motility and proliferation. The opening of CRAC channels in response to depletion of intracellular Ca²⁺ stores involves a cascade of cellular events that culminate in direct interactions between STIM1, the endoplasmic reticulum Ca²⁺ sensor, and the channels composed of Orai proteins. Evidence gathered over the last two decades indicates that CRAC channels display a unique functional pore fingerprint characterized by exquisite Ca²⁺ selectivity, low unitary conductance, and low permeability to large cations. Here, we review the key pore properties of CRAC channels and discuss recent progress in addressing the molecular foundations of these properties. Structure-function and cysteine-scanning studies have revealed the identity and organization of pore-lining residues, including those that form the selectivity filter, providing a structural framework for understanding CRAC channel pore properties. Recent studies in pore mutants that produce STIM1-independent constitutive channel activation indicate that exquisite Ca²⁺ selectivity in CRAC channels is not hardwired into Orai proteins, but is instead manifested only following the binding of STIM1 to the intrinsically poorly Ca²⁺-selective Orai channels. These findings reveal new functional aspects of CRAC channels and suggest that the selectivity filter of the CRAC channel is a dynamic structure whose conformation and functional properties are powerfully regulated by the channel activation stimulus.