Using mammalian GIRK2 channels reconstituted in defined lipid membranes, we also determined that cholesterol directly activates GIRK2 channels. By contrast, cholesterol inhibits the activity of many different types of ion channels, including other members of the inwardly rectifying potassium channel family (Kir)24, 26, 53, 54. Like alcohol, cholesterol activation of GIRK2 depends on the presence of PIP2. Both activators appear to increase the apparent affinity of the channel for PIP2, similar to Gβγ activation34, suggesting that a tighter association of PIP2 with the channel may contribute to the increase in channel activity. In addition, we demonstrate that PIP2 is not only necessary, but is sufficient to activate GIRK2 channels with cytoplasmic Na+ in the absence of Gβγ subunits. As such, PIP2 could be considered an ‘agonist’ for GIRK channels. Moreover, molecules typically described as channel activators, i.e., Gβγ subunits and alcohol, could be mechanistically referred to as positive allosteric modulators (PAMs). PAMs are molecules that cannot activate the channel on their own but increase the efficacy or potency of an agonist, i.e., PIP2. For example, benzodiazepine is a PAM for
