Article
- The EMBO Journal (2007) 26, 589 - 599
- doi:10.1038/sj.emboj.7601492
Published online: 4 January 2007
Subject Category:
Structure of GlnK1 with bound effectors indicates regulatory mechanism for ammonia uptake
Özkan Yildiz1,a, Christoph Kalthoff1,a, Stefan Raunser2 and Werner Kühlbrandt1
- Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt am Main, Germany
- Department of Cell Biology, Harvard Medical School, Boston, MA, USA
Correspondence to:
Werner Kühlbrandt, Department of Structural Biology, Max Planck Institute of Biophysics, Max-von-Laue-Str. 3, Frankfurt am Main 60438, Germany. Tel.: +49 69 6303 3000; Fax: +49 69 6303 3002; E-mail: werner.kuehlbrandt@mpibp-frankfurt.mpg.de
aThese authors contributed equally to this work
Received 14 August 2006; Accepted 10 November 2006
Abstract
A binary complex of the ammonia channel Amt1 from Methanococcus jannaschii and its cognate PII signalling protein GlnK1 has been produced and characterized. Complex formation is prevented specifically by the effector molecules Mg-ATP and 2-ketoglutarate. Single-particle electron microscopy of the complex shows that GlnK1 binds on the cytoplasmic side of Amt1. Three high-resolution X-ray structures of GlnK1 indicate that the functionally important T-loop has an extended, flexible conformation in the absence of Mg-ATP, but assumes a compact, tightly folded conformation upon Mg-ATP binding, which in turn creates a 2-ketoglutarate-binding site. We propose a regulatory mechanism by which nitrogen uptake is controlled by the binding of both effector molecules to GlnK1. At normal effector levels, a 2-ketoglutarate molecule binding at the apex of the compact T-loop would prevent complex formation, ensuring uninhibited ammonia uptake. At low levels of Mg-ATP, the extended loops would seal the ammonia channels in the complex. Binding of both effector molecules to PII signalling proteins may thus represent an effective feedback mechanism for regulating ammonium uptake through the membrane.
Keywords:
- electron microscopy,
- membrane transport,
- nitrogen metabolism,
- signalling mechanism,
- X-ray crystallography
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