Unlocking the swinging door

Novel insights into the transport mechanism
of the Type II importer MolB2C2-A

Figure from Rice et al article: TM5a residues
Fig. 1 from the article shows the CW-EPR spectroscopy of TM5a residues showing nucleotide-dependent mobility changes at the periplasmic gate. Click on the image to see a larger version of it.

ATP-binding cassette transporters, also known as ABC transporters, are classified as either importers or exporters and use ATP binding and hydrolysis to control the transport of substrates from the periplasm into the cell. The prokaryotic ABC importers have three conserved components: periplasmic or substrate binding protein, two transmembrane binding domains, and two cytoplasmic nucleotide binding domains.
ABC importers can be subcategorized further as type I or II. Much of our understanding of transport mechanisms of type II importers is based on studies conducted with the vitamin B12 transporter, BtuCD-F. In the ATP free state of BtuCD, the outward facing periplasmic gates are open wide to accept substrate from the periplasmic binding protein.
ATP binding results in the closure of the cytoplasmic and periplasmic gates, trapping substrate in the translocation pathway. Upon ATP hydrolysis, the cytoplasmic gates open, releasing vitamin B12 into the cell. Very little is known regarding the acceptance of small substrates, such as molybdate, from type II importers. The molybdate transporter, MolB2C2-A, is homologous to BtuCD-F.
In a “Paper of the Week” in The Journal of Biological Chemistry, Heather Pinkett at Northwestern University and her collaborators last year proposed the transport mechanism of the importer MolB2C2-A. Through electron paramagnetic resonance spectroscopy and disulfide cross-linking, the authors found that there are distinct differences between the transport of small and large substrates. In the ATP free state, the perplasmic gate is closed while the cytoplasmic gates are opened.
Once ATP is bound, a subtle conformational change occurs in which the perplasmic gate is unlocked and the cytoplasmic gates closed, allowing molybdate into the translocation pathway. ATP hydrolysis opens the cytoplasmic gate, allowing molybdate into the cell.
The authors state that the mechanism at the periplasmic gate is “akin to unlocking a swinging door: allowing just enough space for molybdate to slip into the cell.” The authors propose that the conformational changes at the periplasmic gate regulate the transport of small substrates. Future studies will determine if this is a conserved mechanism for the transport of small substrates.

Natasha BrooksNatasha C. Brooks (nbrooks@shrinenet.org) is the medical staff research services coordinator at Shriners Hospitals for Children in Galveston, Texas.