Báo cáo khoa học: The ATPase activities of sulfonylurea receptor 2A and sulfonylurea receptor 2B are

The ATPase activities of sulfonylurea receptor 2A and

sulfonylurea receptor 2B are influenced by the C-terminal

42 amino acids

Heidi de Wet, Constantina Fotinou, Nawaz Amad, Matthias Dreger and Frances M. Ashcroft

Department of Physiology, Anatomy and Genetics, University of Oxford, UK

Introduction

ATP-sensitive potassium channels (KATP channels) link

the metabolic state of the cell to its electrical excitabil￾ity [1]. They are involved in the response to cardiac

stress, ischemic preconditioning, vascular smooth mus￾cle tone, skeletal muscle glucose uptake, neuronal

excitability, transmitter release, and insulin secretion

from pancreatic b-cells [2].

The pore of the KATP channel consists of four Kir6.2

subunits, each of which is associated with a regulatory

sulfonylurea receptor (SUR) subunit. There are several

types of the latter: SUR1 in b-cells and neurons, SUR2A

in cardiac and skeletal muscle, and SUR2B in smooth

muscle and some neurons [1]. SUR2A and SUR2B are

encoded by splice variants of a single gene, ABCC9, and

differ only in their C-terminal 42 amino acids.

ATP blocks KATP channel activity by binding to

Kir6.2, whereas the SUR subunit endows the channel

with sensitivity to inhibition by sulfonylurea drugs and

to the stimulatory actions of MgADP and the KATP

channel openers [1,3]. SUR has multiple transmembrane

Keywords

ATP-binding cassette transporter; KATP

channel; sulfonylurea receptor; SUR2A;

SUR2B

Correspondence

F. M. Ashcroft, Department of Physiology,

Anatomy and Genetics, Parks Road, Oxford,

OX1 3PT, UK

Fax: +44 1865 285812

Tel: +44 1865 285810

E-mail: [email protected]

(Received 23 January 2010, revised 26

March 2010, accepted 8 April 2010)

doi:10.1111/j.1742-4658.2010.07675.x

Unusually among ATP-binding cassette proteins, the sulfonylurea receptor

(SUR) acts as a channel regulator. ATP-sensitive potassium channels are

octameric complexes composed of four pore-forming Kir6.2 subunits and

four regulatory SUR subunits. Two different genes encode SUR1 (ABCC8)

and SUR2 (ABCC9), with the latter being differentially spliced to give

SUR2A and SUR2B, which differ only in their C-terminal 42 amino acids.

ATP-sensitive potassium channels containing these different SUR2 iso￾forms are differentially modulated by MgATP, with Kir6.2 ⁄ SUR2B being

activated more than Kir6.2 ⁄ SUR2A. We show here that purified SUR2B

has a lower ATPase activity and a 10-fold lower Km for MgATP than

SUR2A. Similarly, the isolated nucleotide-binding domain (NBD) 2 of

SUR2B was less active than that of SUR2A. We further found that the

NBDs of SUR2B interact, and that the activity of full-length SUR cannot

be predicted from that of either the isolated NBDs or NBD mixtures.

Notably, deletion of the last 42 amino acids from NBD2 of SUR2 resulted

in ATPase activity resembling that of NBD2 of SUR2A rather than that of

NBD2 of SUR2B: this might indicate that these amino acids are responsi￾ble for the lower ATPase activity of SUR2B and the isolated NBD2 of

SUR2B. We suggest that the lower ATPase activity of SUR2B may result

in enhanced duration of the MgADP-bound state, leading to channel

activation.

Abbreviations

ABC, ATP-binding cassette; AMP-PCP, Adenylyl(b,c-methylene)diphosphonate; DDM, dodecylmaltoside; KATP channel, ATP-sensitive

potassium channel; MBP, maltose-binding protein; MRP1, multidrug resistance protein 1; NBD, nucleotide-binding domain; SUR,

sulfonylurea receptor, TMD, transmembrane domain.

2654 FEBS Journal 277 (2010) 2654–2662 ª 2010 The Authors Journal compilation ª 2010 FEBS