Calmodulin

Calmodulin (CaM) (an abbreviation for CALcium-MODULated proteIN) is a calcium-binding messenger protein expressed in all eukaryotic cells. CaM is a multifunctional intermediate messenger protein that transduces calcium signals by binding calcium ions and then modifying its interactions with various target proteins.:
http://en.wikipedia.org/wiki/Calmodulin

Model of mammalian Calmodulin based on the Protein Data Bank entry 3CLN:
http://www.rcsb.org/pdb/explore/explore.do?structureId=3cln

STL file created using VMD:
http://www.ks.uiuc.edu/Research/vmd/

The protein backbone is represented in "cartoon" form. Calcium ions are present as spheres with calcium-binding residues and water molecules.