title: Spin label method reveals barnase-barstar interaction: a temperature and viscosity dependence approach.
abstract: Spin labeling was used to study the protein-protein interaction between the enzyme barnase (Bn) and its inhibitor barstar (Bs). A mutant of barstar (C40A), which contains only one cysteine, C82, located near the Bn-Bs contact region, was selectively modified by two spin labels having different lengths and structures of the flexible tether. The formation of a strong protein complex resulted in significant restriction of spin label mobility at the C82 residue of barstar, as indicated by notable changes in the recorded EPR spectra. The dependence of the separation between broad outer peaks of the EPR spectra on viscosity at constant temperature was used to evaluate the order parameter S and the rotational correlation time tau (a temperature-viscosity dependence approach). The order parameter S, which characterizes fast reorientation of a spin label relative to the protein molecule, sharply increases and approaches unity when Bs binds to Bn. In addition, formation of a Bs-Bs complex was observed; it is also accompanied by restriction of spin label mobility. At the same time, the rotational correlation times tau of spin-labeled Bs, its complex with Bn, and the Bs dimer in solution agree well with their molecular masses. This indicates that barstar, its complex with barnase, and barstar dimer are rigid protein entities. The described approach is suitable for studying any spin-labeled macromolecular complexes.
authors.affiliation: Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, Vavilova str. 32, 119991 Moscow, Russia. tim@eimb.ru
authors.forename: V P
authors.initials: VP
authors.lastname: Timofeev
authors.forename: V V
authors.initials: VV
authors.lastname: Novikov
authors.forename: Y V
authors.initials: YV
authors.lastname: Tkachev
authors.forename: T G
authors.initials: TG
authors.lastname: Balandin
authors.forename: A A
authors.initials: AA
authors.lastname: Makarov
authors.forename: S M
authors.initials: SM
authors.lastname: Deyev
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D015394:Molecular Structure; D017434:Protein Structure, Tertiary; D012260:Ribonucleases / Q000737:chemistry* / Q000378:metabolism; D013113:Spin Labels; D013696:Temperature; D014783:Viscosity
chemical_list: D001426:Bacterial Proteins; D013113:Spin Labels; C057438:barstar protein, Bacillus amyloliquefaciens; D012260:Ribonucleases; C019760:Bacillus amyloliquefaciens ribonuclease