An Alternatively Spliced Isoform of PSD-93/Chapsyn 110 Binds to the Inwardly Rectifying Potassium Channel, Kir2.1

Abstract

Inwardly rectifying potassium (Kir) channels are prime determinants of resting membrane potential in neurons. Their subcellular distribution and surface density thus help shape neuronal excitability, yet mechanisms governing the membrane targeting and localization of Kir channels are poorly understood. Here we report a direct interaction between the strong inward rectifier, Kir2.1, and a recently identified splice variant of postsynaptic density-93 (PSD-93), a protein involved the subcellular targeting of ion channels and glutamate receptors at excitatory synapses. Yeast two-hybrid screening of a human brain cDNA library using the carboxyl terminus of Kir2.1 as bait yielded cDNA encoding the first two PDZ domains of PSD-93, but with an extended N-terminal region that diverged from other PSD-93 isoforms. This clone represented the human homologue of the mouse PSD-93 splice variant, PSD-93␦. Reverse transcription-polymerase chain reaction analysis showed diffuse low level PSD-93␦ expression throughout the brain, with significantly higher levels in spinal cord. In vitro binding studies revealed that a type I PDZ recognition motif at the extreme C terminus of the Kir2.1 mediates interaction with all three PDZ domains of PSD-93␦, and association between Kir2 channels and PSD-93␦ was confirmed further by the ability of antiKir2.1 antibodies to coimmunoprecipitate PSD-93␦ from rat spinal cord lysates. Functionally, coexpression of Kir2.1 and PSD-93␦ had no discernible effect upon channel kinetics but resulted in cell surface Kir2.1 clustering and suppression of channel internalization. We conclude that PSD-93␦ is potentially an important regulator of the spatial and temporal distribution of Kir2 channels within neuronal membranes of the central nervous system.Inwardly rectifying potassium channels comprise a family of integral membrane proteins whose diverse cellular functions include maintenance of the resting membrane potential, control of neuronal excitability and heart rate, and potassium homeostasis (1-3). Underlying these functions is the property of inward rectification, the ability to allow potassium to move easily into the cell at membrane potentials negative to the potassium equilibrium potential (E K ) but to restrict potassium outflow at potentials positive to E K . This asymmetry in the current-voltage relation results from the channel's susceptibility to voltage-dependent block by intracellular polyamines and magnesium ions (4, 5) and ensures that, while being extremely active around E K , they pass little or no current at membrane potentials positive to Ϫ40 mV. Thus, Kir 1 channels maintain a tight control on the resting membrane potential but close in the face of significant membrane depolarization (such as that generated by the cardiac or neuronal action potential) to protect the cell from excessive K ϩ loss. This pivotal role in the regulation of the membrane potential makes Kir channels attractive candidates for modulation by neurotransmitters and hormones, since even small fluctuations in Kir channe...

Initiation of human myoblast differentiation via dephosphorylation of Kir2.1 K+ channels at tyrosine 242

“…Signaling pathways like the Ras-MAPK pathway also appear to act on Kir2.1 channels trafficking (Giovannardi et al, 2002). In addition, binding sites for anchoring proteins such as the filamin A (Sampson et al, 2003), PSD93␦ (Leyland and Dart, 2004) or SAP97 (Leonoudakis et al, 2004) are thought to stabilize Kir2.1 channels at the plasma membrane.…”

Kir2.6 Regulates the Surface Expression of Kir2.x Inward Rectifier Potassium Channels

“…Two additional regions within Kir2.1, a positively charged motif in the N terminus and a conserved YXX⌽ sequence on the membrane-proximal region of the C terminus, have been associated with export from the Golgi to the plasma membrane (25,26). The Kir2.1, Kir2.2, and Kir2.3 C termini also contain a PDZ-binding motif that permits binding to scaffolding and clustering MAGUK proteins that may direct channel localization and abundance (7,10,11,(27)(28)(29)(30)(31)(32).…”

MUPP1 complexes renal K+channels to alter cell surface expression and whole cell currents

“…Our finding that overexpression of MUPP1 in HEK-293 cells and Xenopus oocytes reduces Kir4.2 cell surface expression and Kir4.2 currents suggests that the expressed MUPP1 is binding the expressed Kir4.2 and preventing it from reaching the cell surface. This situation is different from that of other PDZ proteins including hLin7b, CIPP, PSD-93␦, and PSD-95 and their partner membrane proteins (15,24,30,34). TIP-1, essentially a single PDZ domain, appears to function as a true scaffold antagonist (2).…”