Regulatory Role of N-terminal Orai Domains in Current Activation

Abstract

STIM1 and Orai1 represent the key components of the CRAC (Ca2+ release-activated Ca2+) channel signalling machinery. The Orai family comprises three members designated Orai1, Orai2 and Orai3. While the C-termini of all three Orai proteins are involved in the coupling of STIM1 to Orai, the exact role of their N-termini in the communication of STIM1 and Orai is still unclear. All Orai N-termini display a highly conserved cluster of positively charged and hydrophobic amino acids located close to the first transmembrane region. In contrast, only the Orai1 N-terminus but not that of Orai2 and Orai3 contains an arginine- and proline-rich region.In this study we focused on the role of the N-termini of Orai 1/3 proteins for current activation. It has been shown that an Orai1 N-terminal deletion mutant lacking the first 73 amino acids is still sufficient for current activation. Accordingly, an Orai1 N-terminal deletion mutant Δ1-47 lacking the arginine- and proline-rich region displayed a similar STIM1-dependent store-operated activation but showed an altered reactivation profile compared to wild-type Orai1. Further, Orai1 and Orai3 mutants with corresponding N-terminal deletions exhibited distinct STIM1-dependent activation in response to store-depletion. Moreover, activation of Orai3 via STIM1 or 2-APB apparently involved separate N-terminal regions.In summary, conserved regions within the N-termini of Orai 1/3 proteins play a distinct role in STIM1-mediated channel activation, and separate domains contribute to STIM1 or 2-APB induced Orai3 activation. (supported by a OAW scholarship and FWF P21118)