SiaP complex

Experimental Data Snapshot

  • Resolution: 1.05 Å
  • R-Value Free: 0.150 
  • R-Value Work: 0.125 
  • R-Value Observed: 0.126 

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This is version 1.7 of the entry. See complete history


Tripartite ATP-Independent Periplasmic (Trap) Transporters Use an Arginine-Mediated Selectivity Filter for High Affinity Substrate Binding.

Fischer, M.Hopkins, A.P.Severi, E.Hawkhead, J.Bawdon, D.Watts, A.G.Hubbard, R.E.Thomas, G.H.

(2015) J Biol Chem 290: 27113

  • DOI: https://doi.org/10.1074/jbc.M115.656603
  • Primary Citation of Related Structures:  
    2XWI, 2XWK, 2XWO, 2XWV

  • PubMed Abstract: 

    Tripartite ATP-independent periplasmic (TRAP) transporters are secondary transporters that have evolved an obligate dependence on a substrate-binding protein (SBP) to confer unidirectional transport. Different members of the DctP family of TRAP SBPs have binding sites that recognize a diverse range of organic acid ligands but appear to only share a common electrostatic interaction between a conserved arginine and a carboxylate group in the ligand. We investigated the significance of this interaction using the sialic acid-specific SBP, SiaP, from the Haemophilus influenzae virulence-related SiaPQM TRAP transporter. Using in vitro, in vivo, and structural methods applied to SiaP, we demonstrate that the coordination of the acidic ligand moiety of sialic acid by the conserved arginine (Arg-147) is essential for the function of the transporter as a high affinity scavenging system. However, at high substrate concentrations, the transporter can function in the absence of Arg-147 suggesting that this bi-molecular interaction is not involved in further stages of the transport cycle. As well as being required for high affinity binding, we also demonstrate that the Arg-147 is a strong selectivity filter for carboxylate-containing substrates in TRAP transporters by engineering the SBP to recognize a non-carboxylate-containing substrate, sialylamide, through water-mediated interactions. Together, these data provide biochemical and structural support that TRAP transporters function predominantly as high affinity transporters for carboxylate-containing substrates.

  • Organizational Affiliation

    York Structural Biology Laboratory, Departments of Chemistry, University of York, P. O. Box 373, York YO10 5YW.

Find similar proteins by:  (by identity cutoff)  |  3D Structure
Entity ID: 1
MoleculeChains Sequence LengthOrganismDetailsImage
SIALIC ACID-BINDING PERIPLASMIC PROTEIN SIAP312Haemophilus influenzaeMutation(s): 0 
Find proteins for P44542 (Haemophilus influenzae (strain ATCC 51907 / DSM 11121 / KW20 / Rd))
Explore P44542 
Go to UniProtKB:  P44542
Entity Groups  
Sequence Clusters30% Identity50% Identity70% Identity90% Identity95% Identity100% Identity
UniProt GroupP44542
Sequence Annotations
  • Reference Sequence
Small Molecules
Ligands 1 Unique
IDChains Name / Formula / InChI Key2D Diagram3D Interactions
Query on SLB

Download Ideal Coordinates CCD File 
B [auth A]N-acetyl-beta-neuraminic acid
C11 H19 N O9
Experimental Data & Validation

Experimental Data

  • Resolution: 1.05 Å
  • R-Value Free: 0.150 
  • R-Value Work: 0.125 
  • R-Value Observed: 0.126 
  • Space Group: P 21 21 21
Unit Cell:
Length ( Å )Angle ( ˚ )
a = 47.723α = 90
b = 74.51β = 90
c = 86.93γ = 90
Software Package:
Software NamePurpose
MOSFLMdata reduction
SCALEPACKdata scaling

Structure Validation

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Ligand Structure Quality Assessment 

Entry History 

Deposition Data

Revision History  (Full details and data files)

  • Version 1.0: 2011-11-16
    Type: Initial release
  • Version 1.1: 2011-11-23
    Changes: Structure summary
  • Version 1.2: 2015-09-16
    Changes: Database references
  • Version 1.3: 2015-09-23
    Changes: Database references
  • Version 1.4: 2015-11-18
    Changes: Database references
  • Version 1.5: 2019-03-06
    Changes: Data collection, Experimental preparation, Other
  • Version 1.6: 2020-07-29
    Type: Remediation
    Reason: Carbohydrate remediation
    Changes: Data collection, Derived calculations, Other, Structure summary
  • Version 1.7: 2024-05-08
    Changes: Data collection, Database references, Structure summary