Crystal Structures of the ATPase Subunit of the Glucose ABC Transporter from Sulfolobus solfataricus: Nucleotide-free and Nucleotide-bound Conformations

Structure and function of ABC transporters: the ATP switch provides flexible control

“…Similarly, we have little idea of the conformational changes which facilitate ATP binding by the NBDs. It has been suggested from structures of both the HlyB-NBD and the GlcV NBD in their nucleotide-free forms [39,40] that unusual conformations of the Walker A loop occlude the nucleotide-binding pockets, and displacement of this loop in response to a signal transmitted from the TMDs might increase affinity for ATP. Alternatively, and perhaps more probably, ATP may always have access to, and ability to bind with low affinity to, the NBDs in the 'open dimer' configuration, but a conformational change transmitted from the TMDs is required to align the 'ABC signature motifs' from the opposing NBD to form a nucleotide-binding pocket of high affinity and enable 'closed NBD dimer' formation.…”

“…Biochemical studies of intact transport complexes suggest that the conformational changes at the NBDs are relatively small [43,44], implying a transition between an 'open NBD dimer' and 'closed NBD dimer' configuration rather than a major reorientation of the NBDs with respect to other domains. Free NBDs in solution appear relatively flexible in the absence of ATP [39,42] and cannot form a stable dimer as the buried interface would be relatively small. By comparing the structures of nucleotide-free monomers and ATP-bound dimers (only possible with mutant NBDs with key changes to prevent hydrolysis), it has become clear that high-affinity ATP binding involves 'rigid body' rotation of the α-helical subdomain with respect to the core subdomain (9, 10, 42; Fig.…”

“…More likely, ATP hydrolysis is an automatic consequence of 'closed NBD dimer' formation. The observation that stable 'closed dimers' of isolated NBDs are difficult to obtain in the presence of ATP [50,39] suggests that the NBDs may be autocatalytic, and crystals of the isolated LolD and HlyB-NBD homodimers with bound ATP could only be obtained once specific mutations preventing hydrolysis were introduced. The structure of the mutant LolD homodimer in its ATP-bound state shows a water molecule positioned with ideal geometry for hydrolytic attack on the γ-phosphate group of ATP, but which cannot be activated because the required H-bond acceptor, the catalytic base E171, has been mutated stabilising the structure ( [10]; Fig.…”

The motor domains of ABC-transporters

“…Additionally, when crystallizing GlcV, Verdon et al (2003a) obtained two different crystal forms of the nucleotide free protein which also show different structural conformations. These alternative structures are due to rotation of the helical domain with respect to the catalytic domain and due to a change in flexibility of the Q-loop.…”

“…This is coupled to the displacement of the Qloop glutamine from the position where it can interact with the γ-phosphate of the ligand. However, the crystal structures of GlcV, the NBD of the glucose importer of the hyperthermoacidophilic Sulfolobus solfataricus (Verdon et al 2003a), in various states lack most of these features. When comparing the nucleotide free state with the Mg*ADP bound state one sees that the Q-loop glutamine is flipped inwards to make a water mediated interaction with the Mg 2+ -ion, whereas in the MJ structures with Mg*ADP the equivalent glutamine is flipped away from the binding site.…”