Authors
Creative Biogene
Summary
ABC (ATP‑binding cassette) proteins form one of the largest and most diverse protein superfamilies, whose members can be found in all eukaryotic and prokaryotic organisms studied to date. The most of ABC proteins are transporters that translocate allocrites across biological membranes. Almost half of the 48 human ABC transporter proteins are thought to facilitate the ATP-dependent translocation of lipids or lipid-related compounds. Such substrates include cholesterol, plant sterols, phospholipids, bile acids and sphingolipids. Mutations in a substantial number of the 48 human ABC transporters have been related to human disease.
Introduction
The ABCs of lipid transport
ABC transporters are transmembrane proteins that utilize the hydrolysis of ATP to facilitate the movement of a wide variety of substrates across membrane bilayers. There are more than 250 members in the ABC superfamily that can be thought of as importers or exporters. Importers are typically associated with the uptake of hydrophilic nutrients, such as peptides, sugars and ions, into the cell, and are common in bacteria and absent from eukaryotes. Both eukaryotes and prokaryotes possess ABC transporters that function to export substrates out of the cytosol, either into an organelle or across the plasma membrane and out of the cell.
Proteins are classified as ABC transporters based on the organization of their ATP-binding cassette, a region that spans 180 amino acids, and contains three highly conserved motifs: the Walker A/P-loop (12 amino acids), the Walker B motif (5 amino acids) and a signature motif/C-loop (5 amino acids). Functional ABC transporters contain two transmembrane domains (TMD), each commonly thought to consist of six transmembrane a-helices and two ABCs (Figure 1). ABC transporters can be further divided into either ‘half’ or ‘full’ transporters. Half-transporters, in which the polypeptide chain encodes only one TMD and one ABC, are thought to homo- or hetero-dimerize to form a functional transporter. In full transporters, the two TMD and two ABCs are encoded by a single polypeptide, where the two ABCs interact and hydrolyze ATP, thereby generating energy for substrate transport.
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