Fluorescence Fluctuation Spectroscopy enables quantification of potassium channel subunit dynamics and stoichiometry

Giulia Tedeschi,Michelle A Digman,Lorenzo Scipioni,Geoffrey W Abbott,Maria Papanikolaou

doi:10.1038/s41598-021-90002-2

Giulia Tedeschi, Michelle A Digman + Show 3 more

Open Access

https://doi.org/10.1038/s41598-021-90002-2

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Abstract

Voltage-gated potassium (Kv) channels are a family of membrane proteins that facilitate K+ ion diffusion across the plasma membrane, regulating both resting and action potentials. Kv channels comprise four pore-forming α subunits, each with a voltage sensing domain, and they are regulated by interaction with β subunits such as those belonging to the KCNE family. Here we conducted a comprehensive biophysical characterization of stoichiometry and protein diffusion across the plasma membrane of the epithelial KCNQ1-KCNE2 complex, combining total internal reflection fluorescence (TIRF) microscopy and a series of complementary Fluorescence Fluctuation Spectroscopy (FFS) techniques. Using this approach, we found that KCNQ1-KCNE2 has a predominant 4:4 stoichiometry, while non-bound KCNE2 subunits are mostly present as dimers in the plasma membrane. At the same time, we identified unique spatio-temporal diffusion modalities and nano-environment organization for each channel subunit. These findings improve our understanding of KCNQ1-KCNE2 channel function and suggest strategies for elucidating the subunit stoichiometry and forces directing localization and diffusion of ion channel complexes in general.

Highlights

The KCNE subunits are single-pass transmembrane β subunits best known for modifying the functional properties of voltage-gated potassium (Kv) channel α subunits in the auditory epithelium, other epithelia, auditory neurons, and cardiac myocytes[7,8]
Focusing on the KCNQ1-KCNE2 complex that is essential for normal gastric, thyroid and choroid plexus function, we employed a toolbox of complementary imaging and biophysical analysis techniques to probe heteromeric channel stoichiometry and subunit dynamics, designed to cover the entire spatial and temporal resolution required for a full picture of dynamic processes which are responsible for the formation of different complexes
Fluorescence microscopy datasets were acquired with a total internal reflection (TIRF, Fig. 1A) microscope, which enables selective imaging of the plasma membrane of living cells, and the same dataset was analyzed with a combination of state-of-the-art fluorescence fluctuation spectroscopy (FFS) techniques

Summary

Introduction

The KCNE subunits are single-pass transmembrane β subunits best known for modifying the functional properties of voltage-gated potassium (Kv) channel α subunits in the auditory epithelium, other epithelia, auditory neurons, and cardiac myocytes[7,8]. Investigators have made important breakthroughs using high-resolution spectroscopy, cryoelectron microscopy and modeling to elucidate the structure, binding sites and mechanisms of action of KCNEs within Kv channel c omplexes[32,33,34,35] Despite these studies, some fundamental questions regarding the molecular composition and subunit dynamics of KCNE complexes remain either controversial or incompletely addressed. We applied image-derived mean square d isplacement[56,57] (iMSD, Fig. 1B), 2D pair correlation function[58] (2D-pCF, Fig. 1C) and number and brightness a nalysis[36,37] (N&B, Fig. 1D) These techniques allowed us to simultaneously obtain information about the organization of the nano-environment (iMSD), the protein directionality (2D-pCF) and oligomerization state (N&B) of the KCNQ1-KCNE2 complex. We describe a model for KCNE2 with respect to KCNQ1 dynamic interactions at nanometer scale using advanced fluorescence fluctuation analysis

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