The N terminus of Orai1 couples to the AKAP79 signaling complex to drive NFAT1 activation by local Ca2+ entry

Abstract

To avoid conflicting and deleterious outcomes, eukaryotic cells often confine second messengers to spatially restricted subcompartments. The smallest signaling unit is the Ca²⁺ nanodomain, which forms when Ca²⁺ channels open. Ca²⁺ nanodomains arising from store-operated Orai1 Ca²⁺ channels stimulate the protein phosphatase calcineurin to activate the transcription factor nuclear factor of activated T cells (NFAT). Here, we show that NFAT1 tethered directly to the scaffolding protein AKAP79 (A-kinase anchoring protein 79) is activated by local Ca²⁺ entry, providing a mechanism to selectively recruit a transcription factor. We identify the region on the N terminus of Orai1 that interacts with AKAP79 and demonstrate that this site is essential for physiological excitation-transcription coupling. NMR structural analysis of the AKAP binding domain reveals a compact shape with several proline-driven turns. Orai2 and Orai3, isoforms of Orai1, lack this region and therefore are less able to engage AKAP79 and activate NFAT. A shorter, naturally occurring Orai1 protein that arises from alternative translation initiation also lacks the AKAP79-interaction site and fails to activate NFAT1. Interfering with Orai1-AKAP79 interaction suppresses cytokine production, leaving other Ca²⁺ channel functions intact. Our results reveal the mechanistic basis for how a subtype of a widely expressed Ca²⁺ channel is able to activate a vital transcription pathway and identify an approach for generation of immunosuppressant drugs.

Keywords: AKAP79; calcium channel; store-operated; transcription factor.