Discovery of an Auto-Regulation Mechanism for the Maltose ABC Transporter MalFGK2

Huan Bao,Franck Duong,Bostjan Kobe

doi:10.1371/journal.pone.0034836

Abstract

The maltose transporter MalFGK2, together with the substrate-binding protein MalE, is one of the best-characterized ABC transporters. In the conventional model, MalE captures maltose in the periplasm and delivers the sugar to the transporter. Here, using nanodiscs and proteoliposomes, we instead find that MalE is bound with high-affinity to MalFGK2 to facilitate the acquisition of the sugar. When the maltose concentration exceeds the transport capacity, MalE captures maltose and dissociates from the transporter. This mechanism explains why the transport rate is high when MalE has low affinity for maltose, and low when MalE has high affinity for maltose. Transporter-bound MalE facilitates the acquisition of the sugar at low concentrations, but also captures and dissociates from the transporter past a threshold maltose concentration. In vivo, this maltose-forced dissociation limits the rate of transport. Given the conservation of the substrate-binding proteins, this mode of allosteric regulation may be universal to ABC importers.

Highlights

ATP binding cassette (ABC) transporters utilize ATP to transport a wide range of substrates across cellular membranes [1]
ABC transporters are typically made of two nucleotidebinding domains and two transmembrane domains that alternate into two distinct conformations: Inward facing (P-closed) and outward-facing (P-open)
We report that maltose-free MalE binds the P-open transporter with high affinity (Kd,79 nM), whereas at saturating maltose concentration, MalE captures the sugar and dissociates from the transporter

Summary

Introduction

ATP binding cassette (ABC) transporters utilize ATP to transport a wide range of substrates across cellular membranes [1]. ABC transporters are typically made of two nucleotidebinding domains and two transmembrane domains that alternate into two distinct conformations: Inward facing (P-closed) and outward-facing (P-open). This ATP-driven alternate conformational change allows the capture of the substrate on one side of the membrane and its release to the other side; the so-called alternate access model [2]. The protein usually consists of two symmetrical lobes that rotate toward each other to capture the substrate with high-affinity; so-called closed-liganded conformation [5]. The function of MalE is essential to increase the affinity of the transporter for the substrate, and the efficiency of transport

Methods

Results

Conclusion