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Membrane targeting mechanism of voltage-gated calcium channel β subunits

Membrane targeting mechanism of voltage-gated calcium channel β subunits
Alternative Title
전압 의존성 칼슘 채널에서 β-subunit 의 세포막 결합 기작 규명
Kim, Dong Il
DGIST Authors
Kim, Dong IlSuh, Byung Chang
Suh, Byung Chang
Jang, Deok Jin
Issued Date
Awarded Date
2016. 2
β2e subunitelectrostatic interactionphosphoinositidesDanio rerio voltage-sensing phosphatasecalmodulin전압 의존성 칼슘 채널β2e 소단위체전압 의존성 탈인산효소정전기적 상호작용
High voltage-gated Ca2+ (CaV) channels are protein complexes containing α1, β, and α2δ subunits. Although α1 subunit determines primarily gating property of CaV channels and their pharmacological response, as one of auxiliary subunits, β subunit is key regulator for CaV channel gating and receptor modulation. In particular, their subcellular localization is important for regulation of current inactivation and lipid-sensitivity of CaV channels. Most of the β subunit isoforms are cytosolic proteins, whereas two isoforms β2a and β2e subunits are localized in the plasma membrane. β2a is expressed in the plasma membrane via palmitoylation on two cystein residues of N-terminal region whereas membrane targeting mechanism of β2e remains unclear. Here we investigated how β2e is associated with the plasma membrane and what mechanism triggers reversible translcoation of β2e. Furthermore, we explored how such features have an influence on properties of CaV channels in terms of pore gating and lipid modulation. Through mutagenesis and liposome binding assays, we determined that the N-terminus, which contains seven basic residues and a hydrophobic residue, is responsible for this membrane tethering via electrostatic interaction with the plasma membrane. Particularly, we found that Lys2 (K2) and Trp5 (W5) proximal to N-terminus play a pivotal role in membrane association of β2e.
Using rapamycin-inducible dimerization system, β2e exhibited cytosolic distribution when membrane phosphatidylinositol 4-phosphate (PI(4)P) and phosphatidylinositol 4,5-bisphosphate (PIP2) were simultaneously depleted by membrane recruitment of PJ, which is consisting of 4-phosphatase (sac) and 5-phosphatase (INPP5E). Additionally, in M1 muscarinic receptor (M1R) expressing cells, muscarinic stimulation induced translocation of β2e. In consistent with dynamic location change of β2e, in current recording, CaV channels displayed accelerated inactivation when β2e was localized in the cytosol. In experiments with Danio rerio voltage-sensitive phosphatase (Dr-VSP) for PIP2 depletion, out data show that current inhibition with K2A or W5A was 19% by PIP2 depletion. In addition, a double mutation in the N-terminus of β2e (K2A/W5A) increased the PIP2 sensitivity of CaV2.2 and CaV1.3 channels by ~2-fold. The results suggest that membrane targeting of the β2e subunit is mediated by nonspecific electrostatic insertion and dynamically regulated by receptor stimulation. Together, the phospholipid-protein interaction observed here provides structural insight into general principles for membrane-protein association and regulatory roles of phospholipids in ion channels.
Next, we investigated the involvement of second messengers, such as protein kinases and calmodulin (CaM), in translocation of β2e to the cytosol since GPCR activation triggers various signaling molecules such as calmodulin and protein kinases. Using diverse approaches, our findings indicates that rise in cytosolic Ca2+ induces translocation of the β2e, and such shuttling is directly regulated by Ca2+ rather than other second messengers such protein kinases and CaM due to Ca2+ mediated screening effect. In addition, membrane dissociation of the β2e by rise in cytosolic Ca2+ accelerated inactivation of CaV channel even in the presence of Ca2+-insensitive CaM and augmented PIP2 sensitivity to CaV channel. Thus, we suggest that Ca2+ is a determinant for dynamic translocation of the β2e and profoundly affects gating property of CaV channels via dynamics regulation of the β2e. These results reveal a role of Ca2+ on ionic protein-lipid interaction and provide a novel regulatory mechanism of CaV channels. Collectively, we show for the first time that Ca2+ ions directly regulate the CaV channel gating through the control of membrane tethering of β2e subunit and the dynamic interplay between β2e subunit and cytosolic Ca2+ constitutes a novel feedback mechanism for the regulation of CaV channels. ⓒ 2016 DGIST
Table Of Contents
Ⅲ. RESULTS 14 --
The β2e subunit is localized on the plasma membrane and is widely expressed in brain tissues 14 --
Membrane targeting of β2e subunit is independent from lipid modification 17 --
The N-terminal 23 amino acids determine the plasma membrane localization of the β2e subunit 21 --
Basic and hydrophobic residues of the N-terminus affect subcellular distribution of the β2e subunit 25 --
Wild-type and mutants of the β2e subunit show differential effects on inhibition of CaV current by PIP2 depletion 29 --
Phospholipids determine the binding affinity of peptides to liposomes 32 --
Depletion of poly-PIs induces cytosolic translocation of β2e subunits and fast channel inactivation 36 --
Both β2e and MARCKS are tethered to the plasma membrane through electrostatic interaction 42 --
M1 receptor activation results in fast inactivation of CaV current by eliciting transient and reversible translocation of β2e subunit from membrane to cytosol 44 --
Activation of endogenous Gq type GPCRs gives rise to translocation of β2e subunit without PIP2 depletion 49 --
Rise of Ca2+ causes translocation of β2e 54 --
β2e translocation is independent of CaM and protein kinases 57 --
Divalent ions diminish FRET signaling between peptides and liposome 60 --
Ca2+ induces CDI of CaV channels with β2e regardless of CaM 63 --
Current inhibition by PIP2 depletion is predominant in the presence of Ca2+ in the medium 70 --
Brain and Cognitive Sciences
Related Researcher
  • 서병창 Suh, Byung-Chang 뇌과학과
  • Research Interests Molecular mechanisms of epilepsy and sensory pain transmission; Signaling mechanism of ion channel regulation and membrane excitability; 분자전기생리; 간질 및 통증의 분자적 기전 연구
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Department of Brain Sciences Theses Ph.D.


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