Of blood, brains and bacteria, the Amt/Rh transporter family: emerging role of Amt as a unique microbial sensor.
Members of the Amt/Rh family of transporters are found almost ubiquitously in all forms of life. However, the molecular state of the substrate (NH(3) or NH(4)(+)) has been the subject of active debate. At least for bacterial Amt proteins, the model emerging from computational, X-ray crystal and mutational analysis is that NH(4)(+) is deprotonated at the exterior, conducted through the membrane as NH(3), and reprotonated at the cytoplasmic interface. A proton concomitantly is transferred from the exterior to the interior, although the mechanism is unclear. Here we discuss recent evidence indicating that an important function of at least some eukaryotic and bacterial Amts is to act as ammonium sensors and regulate cellular metabolism in response to changes in external ammonium concentrations. This is now well documented in the regulation of yeast pseudohyphal development and filamentous growth. As well, membrane sequestration of GlnK, a PII signal transduction protein, by AmtB has been shown to regulate nitrogenase in some diazotrophs, and nitrogen metabolism in some gram-positive bacteria. Formation of GlnK-AmtB membrane complexes might have other, as yet undiscovered, regulatory roles. This possibility is emphasized by the discovery in some genomes of genes for chimeric Amts with fusions to various regulatory elements.
Tremblay
Pier-Luc
PL
Département de microbiologie et immunologie, Université de Montréal, Montréal, Québec H3C 3J7, Canada.
Hallenbeck
Patrick C
PC
D001419:Bacteria / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D026901:Membrane Transport Proteins / Q000378:metabolism*; D051922:PII Nitrogen Regulatory Proteins / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism
D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; D051922:PII Nitrogen Regulatory Proteins; D000644:Quaternary Ammonium Compounds