Most chemicals cannot easily cross the barrier between the outer environment and the inside of the cell. And yet every cell needs to transfer different compounds to and fro across its membrane. To do this, cells use special protein complexes, transmembrane proteins that pierce through the membrane and facilitate the transport of molecules across it.
The bacterium Pseudomonas aeruginosa can cause a number of unpleasant infections: from pneumonia, through urinary tract infections, to septic shock. Its virulence is dictated by the presence of lipopolysaccharide (a lipid with a sugar residue attached) in its outer membrane. This compound is manufactured in the bacterial cytosol and transported to the outside by a transmembrane protein complex: the ATP-binding cassette (ABC) transporter. The ABC transporter is made up of two proteins – Wzt and Wzm.
It is easy to imagine that if we can interfere with this transport, perhaps we will find a way to fight off this dangerous pathogen. This, however, depends on our understanding of the ABC transporter structure – and this is one area where until recently our knowledge was lacking.
In this recent paper in Integrative Biology, Valerică Raicu and colleagues from Universities of Wisconsin-Milwaukee and Syracuse, USA, describe how their latest improvement of a fluorescence-based method called Fluorescence Resonance Energy Transfer (FRET) in combination with optical microspectroscopy help unveil how the bacterial ABC transporter is built. The researchers use this technique to probe ABC transporter’s subunits and to establish the complex’s stoichiometry and the subunits’ relative orientation.
They find that the complex is constructed of two protein tetramers, one of Wzt and one of Wzm, with one rhombus-shaped tetramer (Wzt) embedded in the membrane and the other square-shaped tetramer (Wzm) attached to the first one on the inside of the cell. Upon binding to Wzm, the Wzt tetramer changes shape from a rhombus to a square. Although the arrangement of the ABC transporter subunits into multimers has been previously shown in other proteins of this family, this is the first study demonstrating this protein’s association into such an elaborate complex performed in living cells.
The importance of the authors’ discovery also lies in the studied protein’s cargo. While the molecules transported by many other ABC transporters are rather small, the ABC transporter of P.aeruginosa is responsible for the transport of large molecules, which means that its mechanism of action is likely also quite different – and once we work out the mechanism, we’ll be one step closer to new anti-P.aureginosa antibiotics.
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Determination of the quaternary structure of a bacterial ATP-binding cassette (ABC) transporter in living cells
Deo R. Singh, Mohammad M. Mohammad, Suparna Patowary, Michael R. Stoneman, Julie A. Oliver, Liviu Movileanu and Valerică Raicu
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Published on behalf of Rafal Marszalek, Molecular BioSystems web science writer. Rafal is an Assistant Editor of Genome Biology at BioMed Central