Bevensee Exchangers

1. Auflage 2014
ISBN: 978-0-12-800291-9
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band Volume 73, 438 Seiten

Reihe: Current Topics in Membranes

Current Topics in Membranes is targeted toward scientists and researchers in biochemistry and molecular and cellular biology, providing the necessary membrane research to assist them in understanding the current state and future prospects of a particular field. This volume on exchangers, in conjunction with a previous volume on cotransporters (volume 70), represents an up-to-date, systematic, and comprehensive review of all the major secondary active carrier proteins responsible for the absorption, secretion, and general transport of ions and solutes in mammalian organ systems and additional species. Each chapter is devoted to a specific transporter or a grouping of related transporters based on the well-recognized nomenclature of the SoLute Carrier (SLC) gene family. This book provides readers with the latest mechanistic information on the function and structure of specific transporters, as well as their history and physiological significance. - Comprehensive review of all the major exchangers - Emphasis on protein mechanism with the most recent findings from functional and structural work - Authoritative work from experts in the field

Autoren/Hrsg.

Bevensee, Mark

Leseproben

Chapter Two

Structural Dynamics and Regulation of the Mammalian SLC9A Family of Na+/H+ Exchangers

Ruth Hendus-Altenburger*,†; Birthe B. Kragelund*; Stine Falsig Pedersen†,1    * Section for Biomolecular Sciences, Department of Biology, University of Copenhagen, Copenhagen, Denmark
† Section for Cell and Developmental Biology, Department of Biology, University of Copenhagen, Copenhagen, Denmark
1 Corresponding author: [e-mail]

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

1 Introduction

The solute carrier 9A (SLC9A) family of mammalian Na+/H+ exchangers (NHEs) play essential roles in cellular, organellar, and systemic pH regulation, in cell volume regulation, and in vectorial ion transport, and consequently, in a wide array of physiological events depending on these processes (see Alexander & Grinstein, 2006; Boedtkjer, Bunch, & Pedersen, 2012; Burckhardt, Di Sole, & Helmle-Kolb, 2002; Donowitz, Ming, & Fuster, 2013; Orlowski & Grinstein, 2004, 2011). NHE dysregulation is associated with a rapidly growing list of pathophysiological conditions, of which some of the best studied are ischemia/reperfusion damage, cancer, diabetes, hypertension, epilepsy, and several gastrointestinal disorders (see Donowitz et al., 2013; Orlowski & Grinstein, 2011; Pedersen, O'Donnell, Anderson, & Cala, 2006). All the NHEs exhibit a membrane topology of a short intracellular N-terminus, 12 predicted transmembrane (TM) helices mediating ion translocation, and a C-terminal cytoplasmic tail region with essential roles in NHE posttranslational modification and interactions with binding partners (Boedtkjer et al., 2012; Donowitz et al., 2013; Orlowski & Grinstein, 2011; Shrode, Gan, D'Souza, Orlowski, & Grinstein, 1998; Wakabayashi, Pang, Su, & Shigekawa, 2000), Fig. 2.1.

Figure 2.1 Overall structure and topology of the SLC9A family. (A) Presumed overall topology of NHE1 as representative for the SL9A family. The model shows the TM domain consisting of 12 TM a-helices and loops of varying length, as well as the short N-terminal and long C-terminal tails. Known structural elements of the tail as well as predicted and confirmed intrinsic disorder in the extracellular loop 1 (EL1) and tail are indicated. (B) Phylogram for the human SLC9A family showing the relative phylogenic distance between members. (C) Multiple sequence alignment of the SLC9A members. Horizontal bars represent aligning sequences and gaps in the alignment are depicted as horizontal lines. The positions of the TM a-helices from the Nygaard model of NHE1 are indicated. Note the long extracellular loops EL1 and extracellular loop 5 (EL5), as well as major gaps in the C-terminal tail leading to different lengths of the individual members. The multiple sequence alignment and the phylogram were made with the online software ClustalW2 (Goujon et al., 2010). Human sequences were retrieved from NCBI, and the accession numbers are as follows: NHE1 NP_003038.2, NHE2 Q9UBY0.1, NHE3 NP_004165.2, NHE4 NP_001011552.2, NHE5 AAC98696.1, NHE6 Q92581.2, NHE7 Q96T83.1, NHE8 AAI12214.1, and NHE9 NP_775924.1.

In the solute carrier (SLC) nomenclature (http://slc.bioparadigms.org/), the mammalian NHEs were recently divided into SLC9A1–9 (NHE1–9), SLC9B1–B2 (NHA1–2), and SLC9C1 (Sperm-NHE), reflecting major sequence divergence between these three groups (see also Donowitz et al., 2013). While the physiology of many of the mammalian NHEs is relatively well understood, a comprehensive understanding of their structural and biophysical properties is lacking. Here, we summarize and discuss the current insight into the structural dynamics, regulation, and binding partner interactions of the mammalian NHEs, with particular focus on SLC9A1/NHE1 (which will be referred to as NHE1 throughout this work). Furthermore, we provide novel bioinformatic and structural analyses that to some extent challenge the existing paradigm on the mechanism of ion transport by mammalian NHEs, and point to new open questions in the understanding of their regulation.

2 Basic Functional Properties: Substrates, Driving Forces and Kinetics of SLC9As

SLC9A activity is...