G-Protein Mediated Trafficking of Inwardly Rectifying Potassium Channels.

Understanding the mechanisms which regulate trafficking and surface expression of membrane-associated proteins is crucial to understanding their function in a physiological context. We have used a combination of electrophysiological, biochemical and imaging techniques to address G-protein mediated trafficking of inwardly rectifying potassium channels. Inwardly rectifying potassium channels play an important role in both cardiac and neuronal cells by stabilizing the resting membrane potential and shaping the action potential. Kir2 channels underlie the cardiac inwardly rectifying potassium conductance termed Ik1 and alterations in Kir2 expression have significant consequences for cardiac function, leading to potentially fatal arrhythmias. We show that surface expression of Kir2 channels is regulated by another class of proteins important for cardiac functioning, RhoGTPases. These proteins act as molecular switches for a variety of signaling cascades. We show specifically that Kir2.1, but not Kir2.2 or 2.3, subunits are regulated by Rac1. This suggests that Kir2 surface expression may be more actively regulated than previously thought, and that formation of native heteromeric channels may contribute to the diversity of Kir2 function. G-protein coupled inwardly rectifying potassium channels of the Kir3 family associate with heterotrimeric G-proteins of the Galphai/o class. In neuronal cells, signaling to Kir3 channels mediates slow post-synaptic inhibition. Evidence for signaling complexes consisting of Kir3 channels, G-proteins and G-protein coupled receptors (GPCRs) has been shown recently. We show here that Kir3 channels can be trafficked as a complex with certain GPCRs, including muscarinic m2 and GABAB receptors. Furthermore, m2 and GABAB receptors are capable of direct interactions which alter the trafficking of both the receptors and associated Kir3 channels. This represents a novel mechanism for regulation of m2 receptor trafficking, and suggests that macromolecular complex formation may have important consequences for Kir3 signaling in neuronal cells. This work provides evidence for novel mechanisms of regulating surface expression of inwardly rectifying potassium channels. We propose that trafficking of these channels is a complex process that depends upon co-localization with effector proteins and formation of macromolecular complexes. This work will aid us in understanding the role of inwardly rectifying potassium channels in both cardiac and neuronal function.