Table of Contents
Advances in Biology
Volume 2014, Article ID 523591, 20 pages
http://dx.doi.org/10.1155/2014/523591
Review Article

Function, Structure, and Evolution of the Major Facilitator Superfamily: The LacY Manifesto

Department of Physiology, University of California, Los Angeles, CA 90095, USA

Received 5 June 2014; Accepted 7 August 2014; Published 18 September 2014

Academic Editor: Luis Loura

Copyright © 2014 M. Gregor Madej. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The major facilitator superfamily (MFS) is a diverse group of secondary transporters with members found in all kingdoms of life. A paradigm for MFS is the lactose permease (LacY) of Escherichia coli, which couples the stoichiometric translocation of a galactopyranoside and an across the cytoplasmic membrane. LacY has been the test bed for the development of many methods applied for the analysis of transport proteins. X-ray structures of an inward-facing conformation and the most recent structure of an almost occluded conformation confirm many conclusions from previous studies. Although structure models are critical, they are insufficient to explain the catalysis of transport. The clues to understanding transport are based on the principles of enzyme kinetics. Secondary transport is a dynamic process—static snapshots of X-ray crystallography describe it only partially. However, without structural information, the underlying chemistry is virtually impossible to conclude. A large body of biochemical/biophysical data derived from systematic studies of site-directed mutants in LacY suggests residues critically involved in the catalysis, and a working model for the symport mechanism that involves alternating access of the binding site is presented. The general concepts derived from the bacterial LacY are examined for their relevance to other MFS transporters.