Measurement of amide proton exchange rates and NOEs with water in 13C/15N-enriched calcineurin B.

A rapid and sensitive 2D approach is presented for measuring amide proton exchange rates and the NOE interaction between amide protons and water. The approach is applicable to uniformly 13C/15N-enriched proteins and can measure magnetization exchange rates in the 0.02 to > 20 s-1 range. The experiments rely on selective excitation of the water resonance, coupled with purging of underlying H alpha resonances, followed by NOESY- or ROESY-type transfer to amide protons, which are dispersed by the amide 15N frequencies in an HSQC-type experiment. Two separate but interleaved experiments, with and without selective inversion of the H2O resonance, yield quantitative results. The method is demonstrated for a sample of the calcium-binding protein calcineurin B. Results indicate rapid amide exchange for the five calcineurin B residues that are analogous to the five rapidly exchanging residues in the 'central helix' of the homologous protein calmodulin.