Molecular Mechanism of ABC Exporters
ATP-binding cassette (ABC) proteins belong to one of the largest superfamilies,
which expands from prokaryotes to man. Most ABC proteins are integral membrane ATPases that transport a wide variety of substrates across
membranes. High-resolution structural information on full-length ABC proteins is very limited, and predominantly of bacterial importers
that are very different from exporters. In addition, crystal structures of membrane
proteins in detergent are static representations of proteins outside their native
bilayer environment, and biochemical and biophysical studies are required to validate
the structures and to elucidate the mechanisms of their function.
General domain structure of ABC proteins. A. The basic core structure is shown in yellow. TMD: transmembrane domain; NBD: nucleotide-binding
domain. B. Different polypeptide arrangements forming the basic core domain of ABC
proteins are illustrated. Separate polypeptides are depicted in different colors.
The various functions of ABC proteins (e.g., ion channels, lipid transporters, drug transporters) depend on the divergent transmembrane
domains, whereas nucleotide binding and hydrolysis, common to all ABC proteins, require
the conserved NBDs. ABC transporters have a central translocation pathway accessible
to only one side of the membrane at a time (alternate accessibility model), where
the accessibility is controlled by the NBDs, the motor domains of ABC proteins. In
the ATP-bound state, two ATP molecules are sandwiched between the NBDs, and it has been proposed that nucleotide-binding with the resulting NBD dimerization
constitute the power stroke associated with active transport. ATP hydrolysis is required
to dissociate the dimer by electrostatic repulsion between the products of ATP hydrolysis
(ADP and Pi). The crystal structure of the lipid transporter MsbA supports this view, but that of the S. aureus ABC exporter Sav1866 challenges it. The latter supports the notion that structural constraints limit the
magnitude of the conformational changes (i.e., the power stroke triggered by ATP binding consists of only moderate rearrangements
that originate at the interface between the NBDs and propagate to the TMDs).
Our general goal is to elucidate at the molecular level the conformational
changes in ABC proteins during the transport cycle. In particular, we are working
on defining the domain movements in P-glycoprotein and Sav1866 during the transport cycle. These studies will allow us to discriminate
between the different proposed molecular models (e.g., large vs. small NBD movements).