The ABC proteins constitute the largest family of proteins. They are present in all living species from Archaea to Homo sapiens. They make up to 4% of the full genome complement of bacteria such as Escherichia coli or Bacillus subtilis. Each eukaryote genome contains several dozens of members (over 100 in the plant Arabidopsis thaliana).
They are recognized by a consensus ATP-binding region of approximately 100 amino acids which include the two Walker A and B motifs encompassing a linker or C region. The ABC proteins catalyze a wide variety of physiological functions, most (but not all) of which being related to transport. This article describes the major physiological and biochemical functions as well as the structural properties of some of the best-known ABC transporters using examples from the yeast Saccharomyces cerevisiae and Homo sapiens.