Disease Associated Mutations in KIR Proteins Linked to Aberrant Inward Rectifier Channel Trafficking

Abstract

The ubiquitously expressed family of inward rectifier potassium (K_IR) channels, encoded by KCNJ genes, is primarily involved in cell excitability and potassium homeostasis. Channel mutations associate with a variety of severe human diseases and syndromes, affecting many organ systems including the central and peripheral neural system, heart, kidney, pancreas, and skeletal muscle. A number of mutations associate with altered ion channel expression at the plasma membrane, which might result from defective channel trafficking. Trafficking involves cellular processes that transport ion channels to and from their place of function. By alignment of all K_IR channels, and depicting the trafficking associated mutations, three mutational hotspots were identified. One localized in the transmembrane-domain 1 and immediately adjacent sequences, one was found in the G-loop and Golgi-export domain, and the third one was detected at the immunoglobulin-like domain. Surprisingly, only few mutations were observed in experimentally determined Endoplasmic Reticulum (ER)exit-, export-, or ER-retention motifs. Structural mapping of the trafficking defect causing mutations provided a 3D framework, which indicates that trafficking deficient mutations form clusters. These "mutation clusters" affect trafficking by different mechanisms, including protein stability.

Keywords: KCNJ; KIR; alignment; disease; inward rectifier channel; mutation; structure; trafficking.

Figure 1

Two opposing domains of the K_IR channel with structural common features highlighted. The membrane is indicated by dotted lines. The selectivity filter (SF) is highlighted by a dotted box. Ions inside the SF are shown as green spheres. Residues E138 and R148 (K_IR2.1) are shown as spheres.

Figure 2

Schematic representation of intracellular trafficking pathways of K_IR channels. TGN, trans-Golgi network; MVB, multivesicular body.

Figure 3

Schematic representation of inward rectifier channels (K_IR1–7) sequence alignment. Red: mutations associated with aberrant trafficking; mutation hotspots are boxed. Orange: transmembrane domain; blue: K_IR protein sequence; gray: sequence gap.

Figure 4

K_IR1–7 sequence alignment of the C-terminal mutation hotspot. Red: Mutations associated with aberrant trafficking; Green: Residues whose mutations are currently not related to impaired trafficking. Numbers at the right refer to the last amino-acid residue in the respective sequence shown. Conserved residues among all K_IR members are shaded gray. Locations of the di-acidic ER exit, G-loop and the Golgi-Export signal sequence (see text) are indicated below the alignment.

Figure 5

K_IR1–7 sequence alignment of the transmembrane domain 1 mutation hotspot. Red: mutations associated with aberrant trafficking; Green: residues whose mutations are currently not related to impaired trafficking. Numbers at the right refer to the last amino-acid residue in the respective sequence shown. Conserved residues among all K_IR members are shaded gray. Location of the transmembrane domain 1 (orange) is indicated below the alignment.

Figure 6

Structural mapping of trafficking mutants mapped on the K_IR2.2 structure. For clarity reasons, only three of the four subunits are shown in side view, with the disease associated mutations highlighted in one subunit only. Mutations of different K_IR channel family members are color-coded and shown as spheres of their respective Cα atoms. Deletions are indicated by colored regions on the secondary structure elements.

Figure 7

Structure-based IgLD hotspot (mapped on the K_IR2.2 structure), with disease associated mutations highlighted. Mutations of the different family members are color-coded and shown as spheres of their respective Cα atoms.