Coupling mechanical forces to electrical signaling: molecular motors and the intracellular transport of ion channels

Abstract

Proper localization of various ion channels is fundamental to neuronal functions, including postsynaptic potential plasticity, dendritic integration, action potential initiation and propagation, and neurotransmitter release. Microtubule-based forward transport mediated by kinesin motors plays a key role in placing ion channel proteins to correct subcellular compartments. PDZ- and coiled-coil-domain proteins function as adaptor proteins linking ionotropic glutamate and GABA receptors to various kinesin motors, respectively. Recent studies show that several voltage-gated ion channel/transporter proteins directly bind to kinesins during forward transport. Three major regulatory mechanisms underlying intracellular transport of ion channels are also revealed. These studies contribute to understanding how mechanical forces are coupled to electrical signaling and illuminating pathogenic mechanisms in neurodegenerative diseases.

Figure 1

Major binding proteins in the intracellular transport of AMPA receptors. (A) The GluR2/3 subunits of AMPA receptors interact with glutamate receptor interacting protein-1 (GRIP-1), which binds to either KIF5 or KIF1. The diagram of a tetrameric AMPA receptor/channel is on the upper right. The diagram of a single GluR subunit is on the upper left, which contains four TM segments (gray bars), extracellular N-terminal domain, and intracellular C-terminal domain. The GluR2/3 C-termini bind to the PDZ5 domain (aa. 441–658) of GRIP-1. The residue number and the last five residues (-ESVKI*) are from GluR2. The cyan boxes numbered from 1 to 7 indicate seven PDZ domains. The region between PDZ6 and PDZ7 of GRIP-1 (aa. 753–987) interacts with the KIF5 C-terminal cargo binding region (aa. 807–934 from KIF5B). The PDZ6 domain (aa. 658–753) of GRIP-1 interacts with the C-terminal region of Liprin-α, which contains sterile α motifs (red boxes). The C-terminal half of the coiled-coil domain (the dark blue box) in the Liprin-α N-terminus (aa. 351–673) binds to the C-terminal region of the KIF1A motor (aa. 657–1105). Blue balls: N-terminal motor domains of kinesins. “N,” the N-terminus of the protein; “C,” the C-terminus of the protein; the number near the C-terminus; the total residue number. The residue numbers of protein binding sites are given whenever they are available. (B) The GluR1 subunit of AMPA receptors interacts with KIF17 through mLin-10. The PDZ1 domain of mLin-10 (aa. 650–840) can interact with both the GluR1 C-terminus and the KIF17 C-terminal region (aa. 957–1038). Yellow box: phosphotyrosine binding domain.

Figure 2

Adaptor and motor proteins in the intracellular transport of NMDA receptors. During the intracellular transport of NMDA receptors, KIF17 interacts with the NR2B subunit through three PDZ-domain proteins. The NR2B C-terminus binds to the only PDZ domain of mLin-7 (aa. 127–197). The last residue number (1484) and the last four residues (-ESDV*) of NR2B are indicated. A small region right before the PDZ domain of LIN-7 (aa. 109–126) interacts with a region in the middle of LIN-2 (aa. 428–524). An N-terminal region of LIN-2 (aa. 1–247) interacts with the N-terminal region of LIN-10 (aa. 1–581). The PDZ1 of mLin-10 (aa. 650-840) binds to the C-terminal tail of KIF17 (aa. 957–1038). In the PDZ-domain proteins, cyan boxes, PDZ domains; yellow box in LIN-10/mLin-10, phosphotyrosine binding domain; orange box in LIN-2, N-terminal CaM kinase domain; green box in LIN-2, SH3 domain; red box in LIN-2, guanylate kinase domain.

Figure 3

Different adaptor proteins mediate the interaction between GABA_A receptors and KIF5 motors. Pentameric structural diagram of the GABA_A receptor is shown on the top. The first coiled-coil domain of GRIF-1 (aa. 124–283) can bind to either the second intracellular loop of GABA_A β2 (aa. 301–426) or the C-terminal tail region of KIF5B (aa. 827–957). The coiled-coil domains of huntingtin-associated protein-1 (HAP-1; aa. 153-320) interact with C-terminal of KIF5B (aa. 814–963). Whether and how HAP-1 interacts with GABA_A β3 directly remains unknown at this time. Blue boxes: coiled-coil domains.

Figure 4

Direct binding between ion channel/transport and kinesins. (A) Structural diagrams of Kv3.1b subunit (left) and tetrameric channel complex of Kv3 (right). T1 domain of Kv3.1 (aa. 1–110) interacts with the C-terminal region of KIF5B (aa. 865–934). Gray bars, transmembrane domains; black ovals, T1 domains. Importantly, only properly tetramerized T1 domains, but not T1 monomers, bind to KIF5 tail domains. In the diagram for a channel tetramer complex (upper right), for clarification, only the C-terminus of one subunit is shown in green, containing the ATM (axonal targeting motif). (B) Diagrams of CLC-5 subunit structure and KIF3. CLC-5 C-terminal domain interacts with the non-motor region (coiled-coil and cargo binding domains) of KIF3. Gray bars, transmembrane domains in CLC-5; purple bars, extra-/intracellular domains; green ovals, CBS (cystathionine beta synthase) domain. KAP3 = kinesin-associated protein 3.

Figure 5

Regulation of ion channel protein loading to kinesin motors. (A) Multiple binding proteins of GRIP-1. Cyan boxes, PDZ domains. (B) Phosphorylation of KIF17 at aa. Ser1029 by CaM kinase II causes disassociation from mLin-10. (C) Kv3.1 tetramerization is required for the interaction with KIF5 motor [Source. Adapted from Xu and others (2010)].