What is the function of the ABC transporter superfamily?
ATP-binding cassette superfamily constitutes one of largest and most functionally diverse transporter groups present in all domains of life, with 48 members in human genome implicated in cystic fibrosis, Tangier disease and multidrug resistance. Each functional unit minimal consists of two transmembrane domains typically six helices each forming translocation pathway determining substrate specificity and two cytosolic nucleotide binding domains containing highly conserved Walker A P-loop GXXGXGKS binding phosphate, Walker B hhhhDE binding magnesium, signature motif LSGGQ unique to ABC, and Q-loop. Upon ATP binding NBDs dimerize sandwiching two ATP, inducing conformational switch from inward facing to outward facing releasing substrate, then hydrolysis resets. Transported substrates range widely: ions chloride via CFTR channel, inorganic molecules, amino acids, sugars, lipids including cholesterol and phospholipids, bile acids, peptides for antigen presentation, siderophores, and structurally unrelated chemotherapeutic drugs effluxed by P-glycoprotein ABCB1 conferring multidrug resistance in cancer. Some members regulate channels. Thus function extends far beyond ion conduction to broad organic export and import.
Ref: Dean et al., Genome Research, ABC Transporter Superfamily Substrates and Multidrug Resistance.