title: The Role of the Carboxyl Terminus Helix C-D Linker in Regulating KCNQ3 K+ Current Amplitudes by Controlling Channel Trafficking.
abstract: In the central and peripheral nervous system, the assembly of KCNQ3 with KCNQ2 as mostly heteromers, but also homomers, underlies "M-type" currents, a slowly-activating voltage-gated K+ current that plays a dominant role in neuronal excitability. KCNQ3 homomers yield much smaller currents compared to KCNQ2 or KCNQ4 homomers and KCNQ2/3 heteromers. This smaller current has been suggested to result either from divergent channel surface expression or from a pore that is more unstable in KCNQ3. Channel surface expression has been shown to be governed by the distal part of the C-terminus in which helices C and D are critical for channel trafficking and assembly. A sequence alignment of this region in KCNQ channels shows that KCNQ3 possesses a longer linker between helix C and D compared to the other KCNQ subunits. Here, we investigate the role of the extra residues of this linker on KCNQ channel expression. Deletion of these residues increased KCNQ3 current amplitudes. Total internal reflection fluorescence imaging and plasma membrane protein assays suggest that the increase in current is due to a higher surface expression of the channels. Conversely, introduction of the extra residues into the linker between helices C and D of KCNQ4 reduced current amplitudes by decreasing the number of KCNQ4 channels at the plasma membrane. Confocal imaging suggests a higher fraction of channels, which possess the extra residues of helix C-D linker, were retained within the endoplasmic reticulum. Such retention does not appear to lead to protein accumulation and activation of the unfolded protein response that regulates protein folding and maintains endoplasmic reticulum homeostasis. Taken together, we conclude that extra helix C-D linker residues play a role in KCNQ3 current amplitudes by controlling the exit of the channel from the endoplasmic reticulum.
authors.affiliation: Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas, United States of America.
authors.forename: Frank S
authors.initials: FS
authors.lastname: Choveau
authors.affiliation: Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas, United States of America.
authors.forename: Jie
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas, United States of America.
authors.forename: Sonya M
authors.initials: SM
authors.lastname: Bierbower
authors.affiliation: Department of Cellular and Structural Biology, University of Texas Health Science Center at San Antonio, San Antonio, Texas, United States of America.
authors.forename: Ramaswamy
authors.initials: R
authors.lastname: Sharma
authors.affiliation: Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas, United States of America.
authors.forename: Mark S
authors.initials: MS
authors.lastname: Shapiro
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D003412:Cricetulus; D004721:Endoplasmic Reticulum / Q000378:metabolism; D051656:KCNQ Potassium Channels / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D051658:KCNQ2 Potassium Channel / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D051659:KCNQ3 Potassium Channel / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013329:Structure-Activity Relationship; D056811:Unfolded Protein Response
chemical_list: D051656:KCNQ Potassium Channels; D051658:KCNQ2 Potassium Channel; C496914:KCNQ2 protein, human; D051659:KCNQ3 Potassium Channel; C496916:KCNQ3 protein, human; C117367:KCNQ4 protein, human; D011994:Recombinant Proteins