Electron Paramagnetic Resonance Spectroscopy of Nitroxide-Labeled Calmodulin

Abstract

Calmodulin (CaM) is a highly conserved calcium-binding protein consisting of two homologous domains, each of which contains two EF-hands, that is known to bind well over 300 proteins and peptides. In most cases the (Ca(2+))(4-)form of CaM leads to the activation of a key regulatory enzyme or protein in a myriad of biological processes. Using the nitroxide spin-labeling reagent, 3-(2-iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyl oxyl, bovine brain CaM was modified at 2-3 methionines with retention of activity as judged by the activation of cyclic nucleotide phosphodiesterase. X-band electron paramagnetic resonance (EPR) spectroscopy was used to measure the spectral changes upon addition of Ca(2+) to the apo-form of spin-labeled protein. A significant loss of spectral intensity, arising primarily from reductions in the heights of the low, intermediate, and high field peaks, accompanied Ca(2+) binding. The midpoint of the Ca(2+)-mediated transition determined by EPR occurred at a higher Ca(2+) concentration than that measured with circular dichroic spectroscopy and enzyme activation. Recent data have indicated that the transition from the apo-state of CaM to the fully saturated form, [(Ca(2+))(4-)CaM], contains a compact intermediate corresponding to [(Ca(2+))(2-)CaM], and the present results suggest that the spin probes are reporting on Ca(2+) binding to the last two sites in the N-terminal domain, i.e. for the [(Ca(2+))(2)-CaM] → [(Ca(2+))(4-)CaM] transition in which the compact structure becomes more extended. EPR of CaM, spin-labeled at methionines, offers a different approach for studying Ca(2+)-mediated conformational changes and may emerge as a useful technique for monitoring interactions with target proteins.