Abstract

Mammalian Na+/H+ exchangers of the SLC9A family are widely expressed and involved in numerous essential physiological processes. Their primary function is to mediate the 1:1 exchange of Na+ for H+ across the membrane in which they reside, and they play central roles in regulation of body, cellular, and organellar pH. Their function is tightly regulated through mechanisms involving interactions with multiple protein and lipid-binding partners, phosphorylations, and other posttranslational modifications. Biochemical and mutational analyses indicate that the SLC9As have a short intracellular N-terminus, 12 transmembrane (TM) helices necessary and sufficient for ion transport, and a C-terminal cytoplasmic tail region with essential regulatory roles. No high-resolution structures of the SLC9As exist; however, models based on crystal structures of the bacterial NhaAs support the 12 TM organization and suggest that TMIV and XI may form a central part of the ion-translocation pathway, whereas pH sensing may involve TMII, TMIX, and several intracellular loops. Similar to most ion transporters studied, SLC9As likely exist as coupled dimers in the membrane, and this appears to be important for the well-studied cooperativity of H+ binding.

The aim of this work is to summarize and critically discuss the currently available evidence on the structural dynamics, regulation, and binding partner interactions of SLC9As, focusing in particular on the most widely studied isoform, SLC9A1/NHE1. Further, novel bioinformatic and structural analyses are provided that to some extent challenge the existing paradigm on how ions are transported by mammalian SLC9As.

Keywords

Na+/H+ exchanger

NHE1

NhaA

Acid–base transport, cellular pH regulation

Structure

Intrinsic disorder

Phosphorylation

Abbreviations

at  Amphiuma tridactylum

CaM calmodulin

CaMKII calmodulin kinase II

CaN calcineurin

CHP calcineurin homologous protein

CK casein kinase

DAG diacylglycerol

EL extracellular loop

EM electron microscopy

ER endoplasmic reticulum

ERM ezrin, radixin, and moesin

FLIM fluorescence-lifetime imaging microscopy

FRET fluorescence resonance energy transfer

ID intrinsic disorder

IDPs intrinsically disordered proteins

IDR intrinsically disordered region

IFD interfacial domain

IL intracellular loop

IP immunoprecipitation

ITC ion-translocation center

ITD ion-translocation domain

LC liquid chromatography

LID lipid interaction domain

LPA lysophosphatitic acid

MAPK mitogen-activated protein kinase

Mj  M. jannaschii

MS mass spectrometry

NHE  Na+/H+ exchanger

NIK Ste-20 like Nck-interacting kinase

NMR nuclear magnetic resonance

NOE nuclear Overhauser effect

PDZ PSD-95/discs large/ZO-1

PE phorbol esters

PI(4,5)P2 phosphatidylinositol-4,5-bisphosphate

PKA protein kinase A

PKB protein kinase B

PKC protein kinase C

POT proton-dependent oliogopeptide transporter

PP1 protein phosphatase 1

RACK1 receptor for activated C kinase-1

SAXS small-angle X-ray scattering

SGK1 serum- and glucocorticoid-inducible kinase 1

SLC9A solute carrier 9A

TM transmembrane

Y2H yeast 2 hybrid