Structural insights into ABC transporter mechanism

Abstract

ATP-binding cassette (ABC) transporters utilize the energy from ATP hydrolysis to transport substances across the membrane. In recent years, crystal structures of several ABC transporters have become available. These structures show that both importers and exporters oscillate between two conformations: an inward-facing conformation with the substrate translocation pathway open to the cytoplasm and an outward-facing conformation with the translocation pathway facing the opposite side of the membrane. In this review, conformational differences found in the structures of homologous ABC transporters are analyzed to understand how alternating-access is achieved. It appears that rigid-body rotations of the transmembrane subunits, coinciding with the opening and closing of the nucleotide-binding subunits, couples ATP hydrolysis to substrate translocation.

Figure 1. Alternating-access in ABC importers

A. Structures of the inward-facing molybdate/tungstate transporter AfModBC-A (left) and the outward-facing maltose transporter MalFGK₂-E (right). For each, the binding protein is colored in purple, the transmembrane subunits are colored in blue and yellow, and the ATPase subunits are colored in red and green. Translocation substrates, tungstate and maltose, are shown in CPK format while ATP is shown in ball-and-stick format. B. A model for alternating-access in ABC importers, modified from [8].

Figure 2. Substrate binding sites defined in ABC importers

A. Left, a slab view of the transmembrane cavity in the maltose transporter with the maltose substrate shown in CPK format. Residues of MBP that form the substrate-binding site in the open conformation are colored cyan. Right, a close-up view of the binding site with hydrogen bonds indicated by dashed lines. Figure modified from [8]. B. Ribbon representation of the methionine transporter (MetNI) and the molybdate/tungstate transporter (ModBC) in a “trans-inhibited” state. Substrates bound to the ATPase subunits are shown in CPK format.

Figure 3. Structures of large importers revealing a flexible inner subdomain

TM subunits are colored in blue and yellow with the inner subdomains (TM3/4/5/5a) highlighted in thicker ribbons (cyan and orange). The binding protein BtuF is colored purple and the ATPase subunits are colored in red and green.

Figure 4. Structures of ABC exporters

A. Structure of the outward-facing Sav1866. One subunit is colored in blue (TMD) and green (NBD) and the other subunit in yellow (TMD) and red (NBD). The bound nucleotide, AMP-PNP is shown in ball-and-stick format. B. Structures of MsbA in open inward- (left), closed inward- (middle) and outward-facing (right) conformations. The extracellular loops (ECL) and cytoplasmic coupling helices (ICL) are indicated. C. Transitions from the open inward- to outward facing conformations. For each state TM1-6 (blue) of one TMD and TM4-5 (yellow) of the other TMD are shown. Rotations of TM4-5 (blue) and TM3-6 (blue) necessary for the transitions are indicated by black arrows.