An N-Terminal ER Export Signal Facilitates the Plasma Membrane Targeting of HCN1 Channels in Photoreceptors
| Autor: | Joseph G. Laird, Yuan Pan, David M. Yamaguchi, Sheila A. Baker |
|---|---|
| Jazyk: | angličtina |
| Rok vydání: | 2015 |
| Předmět: |
Membrane potential
Endoplasmic reticulum Cell Membrane ER retention Biology Endoplasmic Reticulum Immunohistochemistry Cell biology Cell membrane Animals Genetically Modified Xenopus laevis medicine.anatomical_structure Membrane protein Retinal Cell Biology Cyclic nucleotide-binding domain Models Animal Synapses medicine Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels Animals Endomembrane system sense organs Signal transduction Photoreceptor Cells Vertebrate Signal Transduction |
| Popis: | Hyperpolarization-activated cyclic nucleotide-gated channels (HCN1, HCN2, HCN3, and HCN4) are differentially expressed in the retina and brain. They are important for multiple neuronal functions including shaping resting membrane potential, modulating synaptic output, and dendritic integration.1 HCN1, which is most abundant in the inner segment (IS) of rods and cones, carries a feedback current that shapes vision at mesopic and photopic conditions.2–4 Trafficking is one of the key regulatory mechanisms for HCN1 function, exemplified by the altered HCN1 trafficking observed in CA1 pyramidal neurons after temporal lobe epilepsy.5 HCN1 trafficking involves a series of signals that are beginning to be unraveled. The large intracellular C-terminal domain of HCN1 is a hot-spot for trafficking signals. This region has two binding sites for the tetratricopeptide repeat-containing Rab8b interacting protein (TRIP8b), an accessory subunit of HCN channels.1 TRIP8b maintains the concentration of HCN1 at the distal dendrites in hippocampal CA1 and neocortical layer V neurons.6–10 However, neither HCN1 compartmentalization nor surface expression in the retina is TRIP8b-dependent.11 The C-terminus of HCN1 also contains the cyclic nucleotide binding domain (CNBD) common to all HCN channels. The CNBD was shown to regulate the surface expression of HCN2 and may participate in the trafficking of other HCN channels.12 Following the CNBD, there is a binding site to filamin-A, which is required for clustering HCN1 at the cell surface and promotes HCN1 internalization in hippocampal neurons.13,14 We have previously identified a di-arginine endoplasmic reticulum (ER) retention signal at the C-terminus of HCN1 using X. laevis rod photoreceptors.15 This ER retention signal negatively regulates the surface expression of HCN1. However, the process of overcoming the retention signal under physiological conditions remains unclear. An ER retention signal is usually counteracted by a forward trafficking signal that promotes the movement of the protein from the endomembrane system to the surface (i.e., plasma membrane). However, the overall effect of fusing the HCN1 C-terminus on a reporter membrane protein is to retain the reporter in the ER. This suggests that the forward trafficking signal is present in other regions of HCN1. One candidate for mediating the forward trafficking signal is protocadherin 15, which interacts with the N-terminus (NT) of HCN1 in inner ear cells.16 However, HCN1 and protocadherin 15 do not colocalize in photoreceptors17,18; raising the question as to which other HCN1 trafficking signals or regulators may promote its trafficking in this cell. The goal of this study was to identify novel forward trafficking signals using the established transgenic X. laevis approach for probing membrane protein trafficking pathways.19 We found that in rods, the intracellular NT of HCN1 is necessary for the protein to target the IS plasma membrane (ISPM). Through investigating a series of truncation mutants, we identified a leucine-based ER export signal that can override the di-arginine ER retention signal. This finding of combinatorial trafficking signals controlling HCN1 localization provides insight into how the amount of HCN1 functioning at the cell surface is regulated under normal and disease conditions. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|