NMR studies of the methionine methyl groups in calmodulin

Calmodulin (CaM) is a ubiquitous Ca(2+)-binding protein that can regulate a wide variety of cellular events. The protein contains 9 Met out of a total of 148 amino acid residues. The binding of Ca2+ to CaM induces conformational changes and exposes two Met-rich hydrophobic surfaces which provide the main protein-protein contact areas when CaM interacts with its target enzymes. Two-dimensional (1H,13C)-heteronuclear multiple quantum coherence (HMQC) NMR spectroscopy was used to study selectively 13C-isotope labelled Met methyl groups in apo-CaM, Ca(2+)-CaM and a complex of CaM with the CaM-binding domain of skeletal muscle Myosin Light Chain Kinase (MLCK). The resonance assignment of the Met methyl groups in these three functionally different states were obtained by site-directed mutagenesis (Met-->Leu). Chemical shift changes indicate that the methyl groups of the Met residues are in different environments in apo-, calcium-, and MLCK-bound-CaM. The T1 relaxation rates of the individual Met methyl carbons in the three forms of CaM indicate that those in Ca(2+)-CaM have the highest mobility. Our results also suggest that the methyl groups of the unbranched Met sidechains in general are more flexible than those of aliphatic amino acid residues such as Leu and Ile.