Protein engineering and NMR studies of calmodulin.

The calcium regulatory protein calmodulin (CaM) plays a role as an on-off switch in the activation of many enzymes and proteins. CaM has a dumbbell shaped structure with two folded domains, which are connected by a flexible linker in solution. The calmodulin-binding domains of the target proteins are contained in 20 residue long amino acid sequences, that share no obvious amino acid sequence homology. In this contribution, we discuss the features of CaM, which allow it to be rather promiscuous, and bind effectively to all these distinct domains. In particular, we describe the role of the methionine-rich hydrophobic surfaces of the protein in providing a malleable and sticky surface for binding many hydrophobic peptides. The enzyme activation properties of various Met --> Leu mutants of CaM are discussed. In addition, the role of the flexible linker region that connects the two domains is also analyzed. Finally, we describe various NMR and spectroscopic experiments that aid in determining the CaM-bound structures of synthetic peptides containing various CaM-binding domains. All structures analyzed to date are alpha-helical when bound to CaM, and they interact with CaM only through amino acid sidechains. This form of protein-protein interaction is rather unique, and may contribute to CaM's capacity to bind effectively to such a wide range of distinct partners.