Structures and General Transport Mechanisms by the Major Facilitator Superfamily (MFS)

Abstract

The major facilitator superfamily (MFS) is the largest known superfamily of secondary active transporters. MFS transporters are responsible for transporting a broad spectrum of substrates, either down their concentration gradient or uphill using the energy stored in the electrochemical gradients. Over the last 10 years, more than a hundred different MFS transporter structures covering close to 40 members have provided an atomic framework for piecing together the molecular basis of their transport cycles. Here, we summarize the remarkable promiscuity of MFS members in terms of substrate recognition and proton coupling as well as the intricate gating mechanisms undergone in achieving substrate translocation. We outline studies that show how residues far from the substrate binding site can be just as important for fine-tuning substrate recognition and specificity as those residues directly coordinating the substrate, and how a number of MFS transporters have evolved to form unique complexes with chaperone and signaling functions. Through a deeper mechanistic description of glucose (GLUT) transporters and multidrug resistance (MDR) antiporters, we outline novel refinements to the rocker-switch alternating-access model, such as a latch mechanism for proton-coupled monosaccharide transport. We emphasize that a full understanding of transport requires an elucidation of MFS transporter dynamics, energy landscapes, and the determination of how rate transitions are modulated by lipids.

Insights into the Role of the Discontinuous TM7 Helix of Human Ferroportin through the Prism of the Asp325 Residue

“…In some cases, the substrate gating helices are easy to ascertain, as they are broken and contain unwound regions that connect to half-helices. A well-studied example is that of Glucose Transporters (GLUTs), which have a broken TM7 helix, with a short TM7b segment playing a key role in extracellular gating, even being directly involved in substrate preference through specific changes in gating dynamics [10,25]. A similar topology is observed for TM7 in the C-lobe of ferroportin [18][19][20][21], although here the exact coupling mechanism between iron binding and extracellular gating is unclear.…”

“…As stated in the Introduction, the "rocker-switch" model is not sufficient to account for the different states occurring during conformational transitions in MFS transporters. Rather than symmetrical rocking of two structurally similar N-and C-terminal lobes and consequently only three expected conformations (outward-facing, occluded, and inwardfacing), many MFS transporters have at least five distinct conformations: outward-facing, outward-occluded, occluded, inward-occluded, and inward facing [9,10]. The partially occluded states are induced by local changes in TMs referred to as "gating helices", which block the substrate from exiting, but the MFS transporter is yet to undergo the global rockerswitch transition to its opposite-facing conformation [25].…”

“…FPN1 belongs to the Major Facilitator Superfamily (MFS), a large family of secondary active transporters that control the flow of a wide range of substrates (inorganic ions, metabolites, neurotransmitters, and drugs) across biological membranes [7][8][9][10]. Despite low sequence identities, MFS members share a common architecture comprising two bundles, each including 6 transmembrane (TM) helices (N-lobe: TM1-TM6; C-lobe: TM7-TM12), related by twofold pseudosymmetry.…”

“…This "alternating access" model has been refined in recent years to better reflect the specific characteristics of MFS proteins within the lipid bilayer [9,10,12]. The OF conformation can be converted to IF, and vice versa (IF to OF), by concerted conformational changes in the two lobes over a rotation axis that crosses a central-binding site and involves occluded intermediates.…”

“…In the more recent "clamp-and-switch" model [9], it is suggested that occlusion of the binding site from the cytoplasmic side is mainly achieved through bending the cytoplasmic ends of helices TM4 and TM10, and occasionally the flanking helices TM5 and TM11, whereas occlusion from the extracellular side is mainly achieved through bending the extracellular ends of helices TM1 and TM7 and flanking TM2 and TM8 helices. The conformational transitions are orchestrated by the formation and disruption of non-covalent inter-and intra-domain interactions, among which salt-bridges are particularly important [9,10,14].…”