Spin labeling of calcium-dependent phospholipid-binding proteins.

Bovine lung annexins p32 and p34 were spin labeled with an iodoacetamidoproxyl spin label, a reagent that reportedly couples with protein methionine residues. Labeling conditions and stoichiometry were studied with the radiolabeled analogue [1-14C]iodoacetamide. As judged by this method, carboxamidomethylation of both p32 and p34 occurred up to a 0.7 mol ratio after 60 h of reaction at 37 degrees C and at pH 4. The two proteins retained Ca2(+)-dependent phospholipid-binding ability both in radiolabeled and in spin-labeled forms. Electron resonance spectra of spin-labeled p32 and p34 showed the features of a partially immobilized spin probe, with rotational correlation time values of 1.15 and 1.25 ns, respectively, which definitely indicate successful spin labeling. Quantitation of ESR spectra by computer double integration indicated 70% spin labeling of both proteins, as anticipated by radiolabeling. The use of spin-labeled p32 and p34 in the study of Ca2(+)-dependent interaction of annexins with biomembranes is proposed.