Molecular studies of ion channel function have primarily focused on the proteins themselves or on their interactions with other proteins or ions. G protein-gated K channels (KG) for example are thought to be directly activated by the betagamma subunits of GTP binding proteins (Gbetagamma subunits). Current models of KG channel activation involve G protein subunit separation (which renders them active) and interaction with the channel subunits. Our preliminary results suggest that the Gbetagamma subunit/KG channel interaction requires the presence of PIP2 in the membrane in order to manifest its effects on channel activity. This surprising result is accompanied by other effects directly attributable to PIP2, such as the MgATP-dependent sensitization of KG channels to gating by internal Na ions and possibly the MgATP-dependent rundown of G protein stimulation of KG channel activity. These results together with two recent reports on the related inwardly rectifying channel KATP and on the Na / Ca transporter (but not on Na channels or Na / K pumps) herald the potential of an unexplored area of research, crucial to the functional integrity of membrane proteins. Our proposal aims to study in detail the effects of lipids, and in particular phospholipids on KG channel function. The experiments outlined will test further the molecular basis and significance of the PIP2 effects on KG channel activity and the dependence of G protein subunit KG channel activation on the presence of PIP2. It has been proposed that the lipid effects are electrostatic in nature. We will test this hypothesis and seek to identify the basic residues in the channel sequence constituting sites of interaction with the anionic phospholipids. Phospholipids of the phosphoinositide cycle allow dynamic participation of lipids in signaling. We believe that a better appreciation of the molecular details of KG channel function afforded by this study will allow more successful manipulation of this atrial channel in the control of supraventricular arrhythmias. For example, our recent discovery of the MgATP-dependent sensitization of the atrial KG channel (KACh) to gating by internal Na ions allowed us to demonstrate that digitalis treatment causes atrial cells to activate KACh (due to the Na accumulation it causes), providing an important link to the long known effects of this drug on supraventricular rhythm.
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