Control of subcellular localization and function of inwardly rectifying K channels by PDZ-anchoring proteins

Abstract

The control of subcellular localization is an essential mechanism for ion channels to play specific functional roles in various polarized cells, such as neurons and epithelial cells. We found that anchoring proteins that contain PDZ domains such as PSD-95/SAP90 and SAP97 play critical roles in the control of subcellular localization and function of some of inwardly rectifying K (Kir) channels. Kir4.1/KAB-2, cloned by our group, was shown to be predominantly expressed in glial cells in the brain and retina1.Takumi T. Ishii T. Horio Y. et al.A novel ATP-dependent inward rectifier potassium channel expressed predominantly in glial cells.J Biol Chem. 1995; 270: 16339-16346Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 2.Horio Y. Hibino H. Inanobe A. et al.Clustering and enhanced activity of an inwardly rectifying potassium channel, Kir4.1, by an anchoring protein, PSD-95/SAP 90 family.J Biol Chem. 1997; 272: 12885-12888Crossref PubMed Scopus (121) Google Scholar, 3.Ishii M. Horio Y. Tada Y. et al.Expression and clustered distribution of an inwardly rectifying potassium channel, KAB-2/Kir4.1, on mammalian retinal Müller cell membrane: their regulation by insulin and laminin signals.J Neurosci. 1997; 17: 7725-7735Crossref PubMed Google Scholar, 4.Nagelhus E.A. Horio Y. Inanobe A. et al.Subcellular coexpression of the inwardly rectifying K+ channel Kir4.1 and water aquaporin-4 in retinal Müller cells suggests coupling between K+ siphoning and water transport.Glia. 1999; 26: 47-54Crossref PubMed Scopus (388) Google Scholar. We found that (1) Kir4.1 is distributed in a clustered manner on the retinal Mueller cell membrane; (2) the clustering distribution is probably controlled by the interaction between Kir4.1 and SAP97, a PSD anchoring protein; (3) the C-terminal amino acids, SNV, in Kir4.1 may be critical for the protein-protein interaction; (4) laminin and insulin signals are essential for maintaining the clustered distribution of Kir4.1, which involves tyrosine kinase activity; (5) aquaporin 4 (AQP4), whose amino acids at the C-terminal end are LSSV, is co-localized with Kir4.1 on retinal Mueller cells, which seems to be controlled by associated proteins other than SAP97; and (6) SAP97 enhances the channel activity of Kir4.1 in a heterologous expression system. G protein-gated K (KG) channels generate slow inhibitory postsynaptic potentials in the brain5.Yamada M. Inanobe A. Kurachi Y. G protein regulation of potassium ion channels.Pharmacol Rev. 1998; 50: 723-757PubMed Google Scholar. Current opinion suggests that neuronal KG channels are heterotetramers of Kir3.1 and Kir3.2. Kir3.2 has at least four splicing variants named Kir3.2a-Kir3.2d. We found that the KG channels in substantia nigra (SN) are an assembly of Kir3.2a and Kir3.2c6.Inanobe A. Yoshimoto Y. Horio Y. et al.Characterization of G protein-gated K+ channels composed of Kir3.2 subunits in substantia nigra.J Neuroscience. 1999; 19: 1006-1017PubMed Google Scholar. Kir3.2c is 11 amino acids longer than Kir3.2a, which is the only divergent part between the two clones. The C-terminal amino acids of Kir3.2c are ESKV. The channels were localized specifically at the postsynaptic membrane on the dendrites of dopaminergic neurons. Kir3.2c, but not Kir3.2a, could bind a PDZ domain-containing protein, PSD-95. The heterologously expressed KG channels composed of Kir3.2a and Kir3.2c or Kir3.2a alone were activated by G-protein stimulation, but expression of Kir3.2c alone was not. However, the anchoring proteins confer the G-protein sensitivity to the homomeric Kir3.2c channel through their GK domain7.Hibino H. Inanobe A. Tanemoto M. et al.Anchoring proteins confer G protein sensitivity to an inwardly rectifying K+ channel through the GK domain.EMBO J. 2000; 19: 78-83Crossref PubMed Scopus (47) Google Scholar. Therefore, not only subcellular localization, but also the pore function of some Kir channels are critically controlled by PDZ domain-containing anchoring proteins in vivo.