Calmodulin: a versatile calcium mediator protein

Abstract

The level of intracellular calcium is strictly regulated in all cells. In a resting cell, the [Ca2+] is < or = 10(-7) M and during activation it rises to approximately 10(-6) M. Calmodulin (CaM) is the secondary messenger protein that has to translate this modest rise in intracellular calcium into a physiological response in all eukaryotic cells. CaM can activate almost 30 different target systems, including smooth muscle contraction, protein kinases and phosphatases, nitric oxide synthases, and calcium-extruding pumps. It is an acidic protein of 148 amino acids with four helix-loop-helix calcium-binding domains and it has a characteristic dumbbell shape in the crystal structure. In this review I discuss which features of CaM allow it to be such a universal and versatile calcium regulator. First of all, the positive cooperative calcium binding to all four binding sites of CaM in the presence of a target protein allows the protein to act effectively during a calcium transient. Secondly, the high Met content of two hydrophobic surface patches on the two domains of CaM creates a flexible and pliable, yet sticky, interaction surface that does not place high demands on the specificity of the interaction. Consequently, calcium-CaM can bind effectively to the CaM-binding domains of all its target proteins, despite their lack of amino acid sequence homology; their only common feature is that they are hydrophobic basic peptides that have a propensity to form an alpha-helix. CaM's capacity to recognize its CaM-binding domains is further enhanced by its third crucial feature, the intrinsic flexibility of the central linker region; this allows the two domains of CaM to slide over the surface of the alpha-helical bound peptide, to find their most favourable binding orientation. In this review I have also presented selected examples of a variety of experimental techniques that have contributed to our understanding of this unique multitasking protein. These include studies with well-established techniques such as site-directed mutagenesis, chemical modification, limited proteolysis, circular dichroism, and two-dimensional nuclear magnetic resonance (NMR), as well as novel or less common approaches involving the use of unnatural amino acids, metal-ion NMR, lysine pKa determinations, and isotope-edited Fourier transform infrared spectroscopy. In combination with available structural information, these studies have provided considerable detail in our understanding of this versatile calcium regulatory protein.