a, Aligned core of near-intact MFS transporter structures from 17 structures, including PfHT1, mammalian (GLUT1, GLUT3 and GLUT5) and prokaryotic (XylE) systems. The ‘core’ structural elements conserved among the structures are shown in different colours (Methods). b, Motions along the first principal component (PC1) derived from the core ensemble, tracking the rocker-switch motion. c, Residue fluctuations computed from PC1, with the core helix fragments shown as shadowed areas at the base. d, Schematic of the structural basis of the sugar-porter alternating-access mechanism. To summarize, in the outward and outward-occluded conformations (PDB 4YBQ and 4ZW9) the substrate-gating helix TM7b (magenta and transparent) is mobile and samples either state, as seen in molecular dynamics simulation of human GLUT3; spontaneous gate closure is further consistent with the fact that—even in the presence of maltose—GLUT3 crystallizes in both outward-open and outward-occluded conformations4. Substrate binding conformationally stabilizes the outward-occluded state, thus increasing the likelihood for TM7b to break in the middle, completely close the substrate pocket and form contacts with TM1. In the occluded state, the salt-bridge interactions between ICH5 in the C-terminal bundle and ICH1, ICH2 ICH3 and ICH4 are lost, which indirectly destabilizes the highly conserved intrabundle salt-bridge network. Breakage of the intrabundle salt-bridge network catalyses global rocker-switch rearrangements of the N- and C-terminal bundles. In the inward-occluded conformation (PDB 4JA3), the intracellular gating helix TM10b (cyan)—which is related by inverted symmetry to TM7b—spontaneously moves outward to the inward-open conformation (PDB 4YB9). After sugar release, the sugar porter spontaneously resets itself to the outward-facing conformation through an ‘empty’ occluded state22. Spontaneous resetting means that the energetic barriers separating opposite-facing states must be low enough that the occluded state can form in the absence of sugar binding. Nevertheless, consistent with a conformational-selection-driven rocker-switch mechanism, substrate binding catalyses transport as rates are substantially faster through ‘substrate-bound’ versus ‘empty’ occluded-state transitions1.