The alternating-access mechanism thought to underlie the transport of most MFS transport is classically described as the "rocker-switch" mechanism.
In this model, the transporter opens to either the extracellular space or cytoplasm and simultaneously seals the opposing face of the transporter, preventing a continuous pathway across the membrane.
For example, in the best studied MFS transporter, LacY, lactose and protons typically bind from the periplasm to specific sites within the aqueous cleft.
This drives closure of the extracellular face, and opening of the cytoplasmic side, allowing substrate into the cell.
Upon substrate release, the transporter recycles to the periplasmic facing orientation.

Exporters and antiporters of the MFS family follow a similar reaction cycle, though exporters bind substrate in the cytoplasm and extrude it to the extracellular or periplasmic space, while antiporters bind substrate in both states to drive each conformational change.
 While most MFS structures suggest large, rigid body structural changes with substrate binding, the movements may be small in the cases of small substrates, such as the nitrate transporter NarK.
The alternating-access mechanism thought to underlie the transport of most MFS transport is classically described as the "rocker-switch" mechanism. In this model, the transporter opens to either the extracellular space or cytoplasm and simultaneously seals the opposing face of the transporter, preventing a continuous pathway across the membrane. For example, in the best studied MFS transporter, LacY, lactose and protons typically bind from the periplasm to specific sites within the aqueous cleft. This drives closure of the extracellular face, and opening of the cytoplasmic side, allowing substrate into the cell. Upon substrate release, the transporter recycles to the periplasmic facing orientation.
Exporters and antiporters of the MFS family follow a similar reaction cycle, though exporters bind substrate in the cytoplasm and extrude it to the extracellular or periplasmic space, while antiporters bind substrate in both states to drive each conformational change.  While most MFS structures suggest large, rigid body structural changes with substrate binding, the movements may be small in the cases of small substrates, such as the nitrate transporter NarK.
Major facilitator superfamily