Metal-protein speciation using metal-sensors and metallochaperones
“To begin with, for you to be here now trillions of drifting atoms had somehow to assemble in an intricate and curiously obliging manner to create you. Why atoms take this trouble is a bit of a puzzle.”*
Among the assembled atoms are essential metals such as copper, zinc and iron. It is estimated that about a third of all gene products need one or another metal to function although there is variance in the metallo-proteomes of different cells. Remarkably, we do not understand how each protein acquires the right metal. A naïve expectation was that proteins would tightly bind the correct metal and only bind all others weakly, or not at all.
However, the ‘Irving-Williams’ order of stability defines a universal series of metal-affinities which for these atoms is copper>zinc>ferrous iron. How can a cell contain proteins that require the most competitive metals and, simultaneously, proteins that require less competitive ones? A simplistic prediction is that copper should bind them all. Part of the solution lies in the actions of metal-sensors that impose controls on the numbers of each atom in a cell. We have found that metal-selectivity in DNA-binding, metal-responsive, transcriptional-regulators is not a simple function of metal-affinity. For example, the allosteric switch that alters DNA-binding by nickel-sensing NmtR discerns weak binding of the correct metal in an octahedral geometry from tight binding of a wrong metal (zinc) in a tetrahedral geometry. We have also discovered a pathway by which copper is passed to plastocyanin via two copper transporters (CtaA and PacS) plus a metallochaperone (Atx1) and have visualized copper being handed by ligand-exchange between Atx1 and a soluble domain of PacS. Atx1 does not interact with equivalent domains of transporters for other metals such as zinc-ZiaA. In this manner copper en route to plastocyanin is kept away from sites that must be occupied by less competitive metals. This implies that specific protein interactions can dictate which metals are acquired by metallo-proteins in vivo thereby overriding inherent metal-affinities.
*taken from the opening lines of ‘A short history of nearly everything’, Bryson, 2003
Robinson, NJ (2007) A more discerning zinc exporter. Nature Chemical Biology, Vol 3, 11, 692-693.
Tottey S, Rondet SA, Borrelly GP, Robinson PJ, Rich PR, Robinson NJ (2002) A copper metallochaperone for photosynthesis and respiration reveals metal-specific targets, interaction with an importer, and alternative sites for copper acquisition. Journal of Biological Chemistry 277, 5490-5497.
Banci L, Bertini I, Ciofi-Baffoni S, Kandias NG, Robinson NJ, Spyroulias GA, Su XC, Tottey S, Vanarotti M (2006) The delivery of copper for thylakoid import observed by NMR. Proceedings of the National Academy of Sciences, USA 103, 8320-8325.
Cavet JS, Meng W, Pennella MA, Appelhoff RJ, Giedroc DP, Robinson NJ (2002) A nickel-cobalt-sensing ArsR-SmtB family repressor. Contributions of cytosol and effector binding sites to metal selectivity. Journal of Biological Chemistry 277, 38441-38448.
