  ATP-binding cassette (ABC) transporters are integral membrane proteins that
mediate the exchange of diverse substrates across membranes powered by ATP
hydrolysis. We report results of coarse-grained dynamical simulations performed
for the bacterial heme transporter HmuUV. Based on the nucleotide-free
structure, we have constructed a ligand-elastic-network description for this
protein and investigated ATP-induced conformational motions in structurally
resolved computer experiments. As we found, interactions with nucleotides
resulted in generic motions which are functional and robust. Upon binding of
ATP-mimicking ligands the structure changed from a conformation in which the
nucleotide-binding domains formed an open shape, to a conformation in which
they were found in tight contact and the transmembrane domains were rotated.
The heme channel was broadened in the ligand-bound complex and the gate to the
cytoplasm, which was closed in the nucleotide-free conformation, was rendered
open by a mechanism that involved tilting motions of essential transmembrane
helices. Based on our findings we propose that the HmuUV transporter behaves
like a `simple' mechanical device in which, induced by binding of ATP ligands,
linear motions of the nucleotide-binding domains are translated into rotational
motions and internal tilting dynamics of the transmembrane domains that control
gating inside the heme pathway.
