<p>The SWEET family belongs to a class of transporters in
plants that undergoes large conformational changes to facilitate transport of
sugar molecules across the cell membrane. However, the structures of their
functionally relevant conformational states in the transport cycle have not
been reported. In this study, we have characterized the conformational dynamics
and complete transport cycle of glucose in OsSWEET2b transporter using
extensive molecular dynamics simulations. Using Markov state models, we
estimated the free energy barrier associated with different states as well as 1
for the glucose the transport mechanism. SWEETs undergoes structural transition
to outward-facing (OF), Occluded (OC) and inward-facing (IF) and strongly
support alternate access transport mechanism. The glucose diffuses freely from
outside to inside the cell without causing major conformational changes which
means that the conformations of glucose unbound and bound snapshots are exactly
same for OF, OC and IF states. We identified a network of hydrophobic core
residues at the center of the transporter that restricts the glucose entry to
the cytoplasmic side and act as an intracellular hydrophobic gate. The
mechanistic predictions from molecular dynamics simulations are validated using
site-directed mutagenesis experiments. Our simulation also revealed hourglass
like intermediate states making the pore radius narrower at the center. This
work provides new fundamental insights into how substrate-transporter
interactions actively change the free energy landscape of the transport cycle
to facilitate enhanced transport activity.</p>
Molecular dynamics simulations reveal distinct differences in conformational dynamics and thermodynamics between the unliganded and CD4-bound states of HIV-1 gp120
