Calmodulin's Flexibility Allows for Promiscuity in Its Interactions with Target Proteins and Peptides

Abstract

The small bilobal calcium regulatory protein calmodulin (CaM) activates numerous target enzymes in response to transient changes in intracellular calcium concentrations. Binding of calcium to the two helix-loop-helix calcium-binding motifs in each of the globular domains induces conformational changes that expose a methionine-rich hydrophobic patch on the surface of each domain of the protein, which it uses to bind to peptide sequences in its target enzymes. Although these CaM-binding domains typically have little sequence identity, the positions of several bulky hydrophobic residues are often conserved, allowing for classification of CaM-binding domains into recognition motifs, such as the 1-14 and 1-10 motifs. For calcium-independent binding of CaM, a third motif known as the IQ motif is also common. Many CaM-peptide complexes have globular conformations, where CaM's central linker connecting the two domains unwinds, allowing the protein to wrap around a single predominantly alpha-helical target peptide sequence. However, novel structures have recently been reported where the conformation of CaM is highly dissimilar to these globular complexes, in some instances with less than a full compliment of bound calcium ions, as well as novel stoichiometries. Furthermore, many divergent CaM isoforms from yeast and plant species have been discovered with unique calcium-binding and enzymatic activation characteristics compared to the single CaM isoform found in mammals.

Binding of trifluoperazine to apocalmodulin revealed by a combination of small-angle X-ray scattering and nuclear magnetic resonance

“…An IQ or IQ-like domain often mediates Ca 2+ -independent CaM binding (reviewed in Jurado et al, 1999;Bä hler & Rhoads, 2002). Ca 2+ -dependent target proteins primarily contain domains that can be classified into motifs based upon variations on the basic amphiphilic -helix domain involving conserved hydrophobic residues at positions 1-10, 1-14 or 1-16 (reviewed in Rhoads & Friedberg, 1997;Ikura et al, 2002;Yamniuk & Vogel, 2004). SAXS measurements of Ca 2+ CaM with an amphiphilic peptide such as mastoparan or mellitin and a peptide from myosin light chain kinase (MLCK) showed large conformational changes of Ca 2+ CaM (Matsushima et al, 1989;Heidorn et al, 1989;Kataoka et al, 1989).…”

Calmodulin has the Potential to Function as a Ca2+-Dependent Adaptor Protein

“…2A). 3 Numerous structures of these Ca 2+ -CaM-CaMBD complexes have been reported, which reveal a characteristic "wrap-around" binding mode (Fig. 1C).…”

“…Intracellular Ca 2+ transients and oscillations (Ca 2+ signals) are decoded by a large superfamily of calcium-binding proteins, the most important of which is calmodulin (CaM). [1][2][3] The prototypical CaM protein consists of four tandem helix-loop-helix "EF-hand" Ca 2+ -binding motifs that are divided into distinct N-and C-terminal globular lobes connected by a flexible linker. CaM proteins from all species including the single mammalian CaM and the many different plant CaM isoforms each undergo similar Ca 2+ -induced conformational changes involving a rearrangement of the position of its a-helices that opens distinct hydrophobic target protein-binding patches on the surface of each lobe; known as the "open" conformation (Fig.…”

“…1A) can also interact with another subset of proteins, to target CaM to certain cellular locations or facilitate Ca 2+ -independent regulatory events. [1][2][3] The CaM-dependent regulation of target proteins can occur through numerous different mechanisms. For example, Ca 2+ -CaM can relieve autoinhibition by binding to a short (20-25 residue) calmodulin-binding domain (CaMBD) sequence that is adjacent to or within an autoinhibitory region of the enzyme ( Fig.…”

“…Typically the CaM C-lobe binds with high affinity to a Trp residue within the N-terminal part of the target sequence, and the flexible central linker allows the N-lobe to pivot and bind to a second bulky hydrophobic "anchor" residue within the C-terminal part of the target sequence. 3 Truncation of this second anchor residue can lead to binding of only one CaM domain and an extended CaM conformation (Fig. 1D).…”