title: How to change the oligomeric state of a circular protein assembly: switch from 11-subunit to 12-subunit TRAP suggests a general mechanism.
abstract: BACKGROUND
  Many critical cellular functions are performed by multisubunit circular protein oligomers whose internal geometry has evolved to meet functional requirements. The subunit number is arguably the most critical parameter of a circular protein assembly, affecting the internal and external diameters of the assembly and often impacting on the protein's function. Although accurate structural information has been obtained for several circular proteins, a lack of accurate information on alternative oligomeric states has prevented engineering such transitions. In this study we used the bacterial transcription regulator TRAP as a model system to investigate the features that define the oligomeric state of a circular protein and to question how the subunit number could be manipulated.
  
  
  RESULTS
  We find that while Bacillus subtilis and Bacillus stearothermophilus TRAP form 11-subunit oligomers, the Bacillus halodurans TRAP exclusively forms 12-subunit assemblies. Significantly, the two states of TRAP are related by a simple rigid body rotation of individual subunits around inter-subunit axes. We tested if such a rotation could be induced by insertion or deletion mutations at the subunit interface. Using wild type 11-subunit TRAP, we demonstrate that removal of five C-terminal residues at the outer side of the inter-subunit axis or extension of an amino acid side chain at the opposite, inner side, increased the subunit number from 11 to 12. Our findings are supported by crystal structures of TRAP oligomers and by native mass spectrometry data.
  
  
  CONCLUSIONS
  The subunit number of the TRAP oligomer can be manipulated by introducing deletion or addition mutations at the subunit interface. An analysis of available and emerging structural data on alternative oligomeric states indicates that the same principles may also apply to the subunit number of other circular assemblies suggesting that the deletion/addition approach could be used generally to engineer transitions between different oligomeric states.
authors: York Structural Biology Laboratory, Department of Chemistry, University of York, York, United Kingdom.
authors: Chao-Sheng
authors: CS
authors: Chen
authors: Callum
authors: C
authors: Smits
authors: Guy G
authors: GG
authors: Dodson
authors: Mikhail B
authors: MB
authors: Shevtsov
authors: Natalie
authors: N
authors: Merlino
authors: Paul
authors: P
authors: Gollnick
authors: Alfred A
authors: AA
authors: Antson
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000378:metabolism; D001412:Bacillus subtilis / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D001411:Geobacillus stearothermophilus / Q000378:metabolism; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D021122:Protein Subunits / Q000737:chemistry* / Q000378:metabolism*; D012329:RNA, Bacterial / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D012399:Rotation; D014157:Transcription Factors / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D001426:Bacterial Proteins; C079092:MtrB protein, Bacteria; D021122:Protein Subunits; D012329:RNA, Bacterial; D016601:RNA-Binding Proteins; D014157:Transcription Factors