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Transmembrane 16A (TMEM16A, anoctamin1), 1 of 10 TMEM16 family proteins, is a Cl(−) channel activated by intracellular Ca(2+) and membrane voltage. This channel is also regulated by the membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)]. We find that two splice variants of TMEM16A show different sensitivity to endogenous PI(4,5)P(2) degradation, where TMEM16A(ac) displays higher channel activity and more current inhibition by PI(4,5)P(2) depletion than TMEM16A(a). These two channel isoforms differ in the alternative splicing of the c-segment (exon 13). The current amplitude and PI(4,5)P(2) sensitivity of both TMEM16A(ac) and (a) are significantly strengthened by decreased free cytosolic ATP and by conditions that decrease phosphorylation by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Noise analysis suggests that the augmentation of currents is due to a rise of single-channel current (i), but not of channel number (N) or open probability (P(O)). Mutagenesis points to arginine 486 in the first intracellular loop as a putative binding site for PI(4,5)P(2), and to serine 673 in the third intracellular loop as a site for regulatory channel phosphorylation that modulates the action of PI(4,5)P(2). In silico simulation suggests how phosphorylation of S673 allosterically and differently changes the structure of the distant PI(4,5)P(2)-binding site between channel splice variants with and without the c-segment exon. In sum, our study reveals the following: differential regulation of alternatively spliced TMEM16A(ac) and (a) by plasma membrane PI(4,5)P(2), modification of these effects by channel phosphorylation, identification of the molecular sites, and mechanistic explanation by in silico simulation.

β subunits of high voltage-gated Ca(2+) (Ca(V)) channels promote cell-surface expression of pore-forming α1 subunits and regulate channel gating through binding to the α-interaction domain (AID) in the first intracellular loop. We addressed the stability of Ca(V) α1B–β interactions by rapamycin-translocatable Ca(V) β subunits that allow drug-induced sequestration and uncoupling of the β subunit from Ca(V)2.2 channel complexes in intact cells. Without Ca(V) α1B/α2δ1, all modified β subunits, except membrane-tethered β2a and β2e, are in the cytosol and rapidly translocate upon rapamycin addition to anchors on target organelles: plasma membrane, mitochondria, or endoplasmic reticulum. In cells coexpressing Ca(V) α1B/α2δ1 subunits, the translocatable β subunits colocalize at the plasma membrane with α1B and stay there after rapamycin application, indicating that interactions between α1B and bound β subunits are very stable. However, the interaction becomes dynamic when other competing β isoforms are coexpressed. Addition of rapamycin, then, switches channel gating and regulation by phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] lipid. Thus, expression of free β isoforms around the channel reveals a dynamic aspect to the α1B–β interaction. On the other hand, translocatable β subunits with AID-binding site mutations are easily dissociated from Ca(V) α1B on the addition of rapamycin, decreasing current amplitude and PI(4,5)P(2) sensitivity. Furthermore, the mutations slow Ca(V)2.2 current inactivation and shift the voltage dependence of activation to more positive potentials. Mutated translocatable β subunits work similarly in Ca(V)2.3 channels. In sum, the strong interaction of Ca(V) α1B–β subunits can be overcome by other free β isoforms, permitting dynamic changes in channel properties in intact cells.

The PI(4,5)P2 level in the plasma membrane is dynamically regulated by cytoplasmic ATP production and receptor-mediated transmembrane signaling cascades. In this issue of Cell Chemical Biology, Xie et al. (2016) use optogenetics to micro-manipulate membrane PI(4,5)P2 and reveal how acute PI(4,5)P2 changes can alter intracellular Ca(2+) dynamics and insulin secretion in pancreatic β cells.