Calmodulin is a 147 amino acid protein that changes its shape when calcium ions are bound. It’s quite important, quite ancient and evolutionarily stable (human calmodulin differs at only 3/147 amino acids from the fruitfly’s).
When calcium is bound, the shape change puts hydrophobic (water hating) amino acid side chains its surface. Normally such side chains are hidden in the oily interior of most proteins. The hydrophobic surface patch allows Calmodulin to bind to over 300 cellular proteins. So calmodulin functions as a calcium dependent switch which changes the function of the proteins it binds to.
Naturally the 300 proteins have amino acid sequences (motifs) whose 3 dimensional shape in the native protein is capable of recognizing (e.g. binding to) calmodulin when it contains calcium.
What is quite surprising, is how different from each other in amino acid sequence these calmodulin recognition motifs actually are. Also these sequences are ‘often’ partially or largely disordered in the absence of calmodulin.
The structure of calcium bound calmodulin is also quite dynamic (translation: it flits about between a bunch of different three dimensional structures).
The authors of a recent study [ Proc. Natl. Acad. Sci. vol. 110 pp. 20545 – 20550 ’13 ] claim that the binding of calmodulin to a target requires both to undergo mutually induced changes in shape. They call this induced fit. But it’s really a mating dance. Happy New Year and Good Luck to all.
Chemiotics II 