Practice question

A unifying structural signature distinguishes ABC ATPase pumps from other transporter classes like P-type or SLC carriers. Functionally active unit requires two hydrophobic transmembrane domains, each generally composed of six alpha-helices that create substrate-binding chamber accessible alternately to cytoplasm or extracellular milieu, and two peripherally located nucleotide-binding domains in cytoplasm that dimerize upon ATP ligation. The nucleotide-binding domains contain highly conserved Walker A, Walker B, Q-loop, D-loop, H-loop and ABC signature LSGGQ motif that contacts ATP across dimer interface. In many eukaryotic exporters such as P-glycoprotein ABCB1, CFTR ABCC7 and BSEP ABCB11, all four domains fuse into single polypeptide with two homologous halves connected by linker containing regulatory insertions. Bacterial importers often split domains into separate subunits plus extracellular solute-binding proteins delivering cargo. Intracellular coupling helices in TMDs dock into grooves of NBDs transducing ATP-driven motions to transmembrane reorientation enabling alternating access independent of ion gradient or membrane voltage, fundamental for energetic coupling.