Practice question

ATP-binding cassette superfamily includes hundreds of members across all domains of life encoded by 49 human genes divided into ABCA to ABCG subfamilies. Core architecture comprises two transmembrane domains forming substrate path and specificity filter, and two cytosolic nucleotide-binding domains containing Walker A P-loop GXXGXGKS/T for phosphate binding, Walker B hhhhDE coordinating Mg2+ and catalytic glutamate, and signature LSGGQ hallmark. ATP binding drives NBD dimerization sandwiching two ATPs, converting TMDs to outward-facing conformation; hydrolysis and release of ADP and Pi resets inward-facing state enabling vectorial translocation against gradient. In bacteria they primarily serve as high-affinity importers for sugars, amino acids, phosphate, vitamins and siderophores using periplasmic binding proteins. In eukaryotes they act as exporters moving phospholipids, sterols, bile acids, retinoids, heme, xenobiotics, and immunologically TAP1/TAP2 transport peptides for MHC I presentation. CFTR ABCC7 even evolved into chloride channel gated by ATP binding, illustrating diversification of same ATP-switch engine for varied functions.