Apo-calmodulin

Zhang M, Tanaka T, and Ikura M [ 1995] Calcium-induced conformational transition revealed by the solution structure of apo calmodulin. Nat Struct Biol 2 758-767... 

Figure 6.22 X-ray crystallographic structures of (A) Ca -saturated calmodulin, PDB lEXR and (B) apo-calmodulin,PDB 1QX5. Visualized using CambridgeSoft Chem3D Ultra 10.0 with notations in ChemDraw Ultra 10.0. (Printed with permission of CambridgeSoft Corporation.) (See color plate)...

More recently, workers in Japan published the solution structure of yeast (Saccharomyces cerevisiae) apo-calmodulin (PDB ILKJ). Yeast calmodulin is 60% identical in its amino acid sequence with vertebrate CaMs. The ILKJ N-terminal domain with its two helix-loop-helix calcium-binding domains looks quite similar to those of IDMO and ICFD (see Figure 6.23). 

To validate the method, we applied PLIMSTEX to determine the binding constants (Xa), stoichiometry (1 protein to n ligands), and the protection against H/D exchange in various interactions. We chose as tests the binding of Mg to gua-nosine diphosphate (GDP)-bound human ras protein, of Ca + to apo calmodulin... 

Ca2+-activated K+ channels controls assembly with apo-calmodulin. J. Biol. Chem. 277 4558-4564. 

Andre, I., Linse, S., Mulder, F.A. Residue-specific pKa determination of lysine and arginine side chains by indirect 15N and 13C NMR spectroscopy Application to apo calmodulin. J. Am. Chem. Soc. 2007,129,15805-13. 

We pulled this protein from its two ends and obtained F-E curves for two types of calmodulin, one with (holo) and the other without (apo) Ca++ ions [27]. The F-E curves shown in Fig. 9 clearly show differences in the mechanical properties between holo and apo calmodulin, the former having a sharp force peak after extension of about 70% of the chain length. 

The E-E curve of apo calmodulin showed a lower stiffness compared with that of holo-calmodulin, but it was still clearly higher than that of the WLC model up to approximately 80% extension. It was confirmed by CD spectroscopy that the modified calmodulin had the same level of helicity as the native protein but not longer oligomers. Dimer of calmodulin gave similar E-E curves with a force peak... 

Fig. 9 F-E curves of holo and apo calmodulin (Reproduced from [26] with permission)...

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