Experimental Data Snapshot

  • Resolution: 2.00 Å
  • R-Value Free: 0.262 
  • R-Value Work: 0.210 
  • R-Value Observed: 0.210 

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GlnK, a PII-homologue: structure reveals ATP binding site and indicates how the T-loops may be involved in molecular recognition.

Xu, Y.Cheah, E.Carr, P.D.van Heeswijk, W.C.Westerhoff, H.V.Vasudevan, S.G.Ollis, D.L.

(1998) J Mol Biol 282: 149-165

  • DOI: https://doi.org/10.1006/jmbi.1998.1979
  • Primary Citation of Related Structures:  
    1GNK, 2GNK

  • PubMed Abstract: 

    GlnK is a recently discovered homologue of the PII signal protein, an indicator of the nitrogen status of bacteria. PII occupies a central position in the dual cascade that regulates the activity of glutamine synthetase and the transcription of its gene. The complete role of Escherichia coli GlnK is yet to be determined, but already it is known that GlnK behaves like PII and can substitute for PII under some circumstances thereby adding to the subtleties of nitrogen regulation. There are also indications that the roles of the two proteins differ; the expression of PII is constitutive while that of GlnK is linked to the level of nitrogen in the cell. The discovery of GlnK begs the question of why E. coli has both GlnK and PII. Clearly, the structural similarities and differences of GlnK and PII will lead to a better understanding of how PII-like proteins function in E. coli and other organisms. We have crystallised and solved the X-ray structure of GlnK at 2.0 A resolution. The asymmetric unit has two independent copies of the GlnK subunit and both pack around 3-fold axes to form trimers. The trimers have a barrel-like core with recognition loops (the T-loops) that protrude from the top of the molecule. The two GlnK molecules have similar core structures to PII but differ significantly at the C terminus and the loops. The T-loops of the two GlnK molecules also differ from each other; one is disordered while the conformation of the other is stabilised by lattice contacts. The conformation of the ordered T-loop of GlnK differs from that observed in the PII structure despite the fact that their sequences are very similar. The structures suggest that the T-loops do not have a rigid structure and that they may be flexible in solution. The presence of a turn of 310 helix in the middle of the T-loop suggests that secondary structure could form when it interacts with soluble receptor enzymes.Co-crystals of GlnK and ATP were used to determine the structure of the complex. In these crystals, GlnK occupies a position of 3-fold symmetry. ATP binds in a cleft on the side of the molecule. The cleft is suitably positioned for ATP to influence the flexible T-loops. It is found at the junction of two beta sheets and is formed by two peptides one of which contains a variant of the "Gly-loop" found in other mononucleotide binding proteins. This sequence, Thr-Gly-X-X-Gly-Asp-Gly-Lys-Ile-Phe, forms part of the B-loop and is conserved in a wide variety of organisms that include bacteria, algae and archeabacteria. This sequence is more highly conserved than the functional T-loop, suggesting that ATP has an important role in PII-like proteins.

  • Organizational Affiliation

    Research School of Chemistry, Australian National University, GPO 414, Canberra, ACT, 2601, Australia.

Find similar proteins by:  (by identity cutoff)  |  3D Structure
Entity ID: 1
MoleculeChains Sequence LengthOrganismDetailsImage
PROTEIN (NITROGEN REGULATORY PROTEIN)112Escherichia coliMutation(s): 0 
Find proteins for P0AC55 (Escherichia coli (strain K12))
Explore P0AC55 
Go to UniProtKB:  P0AC55
Entity Groups  
Sequence Clusters30% Identity50% Identity70% Identity90% Identity95% Identity100% Identity
UniProt GroupP0AC55
Sequence Annotations
  • Reference Sequence
Small Molecules
Ligands 1 Unique
IDChains Name / Formula / InChI Key2D Diagram3D Interactions
Query on ATP

Download Ideal Coordinates CCD File 
C10 H16 N5 O13 P3
Experimental Data & Validation

Experimental Data

  • Resolution: 2.00 Å
  • R-Value Free: 0.262 
  • R-Value Work: 0.210 
  • R-Value Observed: 0.210 
  • Space Group: P 63
Unit Cell:
Length ( Å )Angle ( ˚ )
a = 57.45α = 90
b = 57.45β = 90
c = 54.79γ = 120
Software Package:
Software NamePurpose
DENZOdata reduction
SCALEPACKdata scaling

Structure Validation

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Entry History 

Deposition Data

Revision History  (Full details and data files)

  • Version 1.0: 1999-07-23
    Type: Initial release
  • Version 1.1: 2008-04-27
    Changes: Version format compliance
  • Version 1.2: 2011-07-13
    Changes: Derived calculations, Version format compliance
  • Version 1.3: 2023-12-27
    Changes: Data collection, Database references, Derived calculations