Although both the
calcium store-dependent
CRAC channels and the store-independent
ARC channels are regulated by the
protein STIM1,
CRAC channels are regulated by STIM1 in the endoplasmic reticulum, whilst
ARC channels are regulated by the STIM1 constitutively resident in the plasma membrane. We now demonstrate that activation of the
ARC channels, but not
CRAC channels, is uniquely dependent on phosphorylation of a single residue (T389) in the extensive cytosolic domain of STIM1 by
protein kinase A. We further demonstrate that the phosphorylation of the T389 residue by
protein kinase A is mediated by the association of plasma membrane STIM1 with the scaffolding
protein AKAP79. Together, these findings indicate that the phosphorylation status of this single residue in STIM1 represents a key molecular determinant of the relative activities of these two co-existing Ca(2+) entry channels that are known to play critical, but distinct, roles in modulating a variety of physiologically relevant activities.
ABSTRACT: The low-conductance, highly
calcium-selective channels encoded by the Orai family of
proteins represent a major pathway for the agonist-induced entry of
calcium associated with the generation and modulation of the key intracellular
calcium signals that initiate and control a wide variety of physiologically important processes in cells. There are two distinct members of this channel family that co-exist endogenously in many cell types: the store-operated Ca(2+) release-activated
CRAC channels and the store-independent
arachidonic acid-regulated
ARC channels. Although the activities of both channels are regulated by the stromal-interacting molecule-1 (
STIM1) protein, two distinct pools of this
protein are responsible, with the major pool of STIM1 in the endoplasmic reticulum membrane regulating
CRAC channel activity, whilst the minor pool of plasma membrane STIM1 regulates
ARC channel activity. We now show that a critical feature in determining this selective activation of the two channels is the phosphorylation status of a single
threonine residue (T389) within the extensive (∼450 residue) cytosolic domain of STIM1. Specifically,
protein kinase A (PKA)-mediated phosphorylation of T389 of STIM1 is necessary for effective activation of the
ARC channels, whilst phosphorylation of the same residue actually inhibits the ability of STIM1 to activate the
CRAC channels. We further demonstrate that the PKA-mediated phosphorylation of T389 occurs at the plasma membrane via the involvement of the anchoring
protein AKAP79, which is constitutively associated with the pool of STIM1 in the plasma membrane. The novel mechanism we have described provides a means for the cell to precisely regulate the relative activities of these two channels to independently modulate the resulting intracellular
calcium signals in a physiologically relevant manner.