title: Identification of protein side chains near the membrane-aqueous interface: a site-directed spin labeling study of KcsA.
abstract: This study presents an approach to identifying surface residues on membrane proteins that are exposed toward the membrane-aqueous interface. The method employs a lipid Ni(II) chelate that localizes the metal ion to a region near the membrane-aqueous interface. Lateral diffusion of the lipid chelate results in Heisenberg exchange (HE) with nitroxide side chains in the protein only if direct contact occurs between the paramagnetic species during a collision. Thus, HE serves as a signature for residues facing the bilayer in the neighborhood of the membrane-aqueous interface. To evaluate the method, 13 surface residues on the extracellular half of KcsA, a prokaryotic potassium channel of known structure, were examined for HE with the Ni(II) chelate. The HE rate between the two species is found to depend strongly on the vertical position of the nitroxide with respect to the membrane-aqueous interface. Nitroxides introduced near the interface experience relatively high HE rates, whereas nitroxides that are immersed in the bilayer interior or sterically sheltered from collision experience low or undetectable rates. The results indicate that residues near the interface can be identified on the basis of their high rates of collision with the headgroup region of the bilayer.
authors.affiliation: Jules Stein Eye Institute and Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA. a-gross@northwestern.edu
authors.forename: Adrian
authors.initials: A
authors.lastname: Gross
authors.forename: Wayne L
authors.initials: WL
authors.lastname: Hubbell
mesh_terms: D019943:Amino Acid Substitution; D001426:Bacterial Proteins; D003545:Cysteine / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D008958:Models, Molecular; D015221:Potassium Channels / Q000737:chemistry*; D011487:Protein Conformation; D013113:Spin Labels
chemical_list: D001426:Bacterial Proteins; D015221:Potassium Channels; D013113:Spin Labels; C107790:prokaryotic potassium channel; D003545:Cysteine