Cobalt ion interaction with TMEM16A calcium-activated chloride channel: Inhibition and potentiation

TMEM16A, a Ca(2+)-sensitive Cl(-) channel, plays key roles in many physiological functions related to Cl(-) transport across lipid membranes. Activation of this channel is mediated via binding intracellular Ca(2+) to the channel with a relatively high apparent affinity, roughly in the sub-μM to low μM concentration range. Recently available high-resolution structures of TMEM16 molecules reveal that the high-affinity Ca(2+) activation sites are formed by several acidic amino acids, using their negatively charged sidechain carboxylates to coordinate the bound Ca(2+). In this study, we examine the interaction of TMEM16A with a divalent cation, Co(2+), which by itself cannot activate current in TMEM16A. This divalent cation, however, has two effects when applied intracellularly. It inhibits the Ca(2+)-induced TMEM16A current by competing with Ca(2+) for the aforementioned high-affinity activation sites. In addition, Co(2+) also potentiates the Ca(2+)-induced current with a low affinity. This potentiation effect requires high concentration (mM) of Co(2+), similar to our previous findings that high concentrations (mM) of intracellular Ca(2+) ([Ca(2+)](i)) can induce more TMEM16A current after the Ca(2+)-activation sites are saturated by tens of μM [Ca(2+)](i). The degrees of potentiation by Co(2+) and Ca(2+) also roughly correlate with each other. Interestingly, mutating a pore residue of TMEM16A, Y589, alters the degree of potentiation in that the smaller the sidechain of the replaced residue, the larger the potentiation induced by divalent cations. We suggest that the Co(2+) potentiation and the Ca(2+) potentiation share a similar mechanism by increasing Cl(-) flux through the channel pore, perhaps due to an increase of positive pore potential after the binding of divalent cations to phospholipids in the pore. A smaller sidechain of a pore residue may allow the pore to accommodate more phospholipids, thus enhancing the current potentiation caused by high concentrations of divalent cations.