Modulation of Kir4.2 rectification properties and pHi-sensitive run-down by association with Kir5.1

Abstract

Inwardly rectifying K+ channels (Kir) comprise seven subfamilies that can be subdivided further on the basis of cytosolic pH (pHi) sensitivity, rectification strength and kinetics, and resistance to run-down. Although distinct residues within each channel subunit define these properties, heteromeric association with other Kir subunits can modulate them. We identified such an effect in the wild-type forms of Kir4.2 and Kir5.1 and used this to further understand how the functional properties of Kir channels relate to their structures. Kir4.2 and a Kir4.2-Kir5.1 fusion protein were expressed in HEK293 cells. Inward currents from Kir4.2 were stable over 10 min and pHi-insensitive (pH 6 to 8). Conversely, currents from Kir4.2-Kir5.1 exhibited a pHi-sensitive run-down at slightly acidic pHi. At pHi 7.2, currents in response to voltage steps positive to EK were essentially time independent for Kir4.2 indicating rapid block by Mg2+. Coexpression with Kir5.1 significantly increased the blocking time constant, and increased steady-state outward current characteristic of weak rectifiers. Recovery from blockade at negative potentials was voltage dependent and 2 to 10 times slower in the homomeric channel. These results show that Kir5.1 converts Kir4.2 from a strong to a weak rectifier, rendering it sensitive to pHi, and suggesting that Kir5.1 plays a role in fine-tuning Kir4.2 activity.

Expression of inwardly rectifying potassium channel subunits in native human retinal pigment epithelium

“…Unlike Kir4.1 channels, Kir4.2 channels expressed in mammalian cells are insensitive to intracellular pH within the physiological range (Lam et al, 2006). Our results suggest that Kir4.2 channels exist as homomeric channels in the RPE, where they likely contribute to K + efflux under resting conditions.…”

Expression, localization, and functional properties of inwardly rectifying K+channels in the kidney

“…Like K ir 4.1, K ir 4.2 can form both a homomeric channel and a functional heterotetramer with K ir 5.1. The heteromerization of K ir 4.2 with K ir 5.1 converts K ir 4.2 from a strong to a weak rectifier, which renders it sensitive to intracellular pH (28,39). Although K ir 4.2 and K ir 5.1 are coexpressed in the DCT, the presence of a K ir 4.2/K ir 5.1 channel has been proposed but not validated in the kidney (31).…”

Potassium‐dependent activation of Kir4.2 K+ channels

“…Kir4.2 can combine with Kir5.1 in heterologous expression systems, producing hetermultimeric channels with altered biophysical properties (Lam et al 2006). It is not known whether this occurs in vivo .…”