“Small, ubiquitous, divalent ion with high versatility seeks partner for transient or long-term relationship”. Two web sites where you might find Ca2+'s ideal partner are The EF-Hand Calcium-Binding Proteins Data Library and The Calmodulin Target Database.

Welcoming you to the homepage of Vanderbilt's data library is an EF-hand-protein structure; a more comprehensive list of EF-hand proteins, along with their functions, is just a click away ('Summary of proteins included'). For the structural biologists among you, the 'Structural information' option provides information on aspects such as interhelical angles and distance difference matrices, and 'Picture gallery' provides an aesthetic touch, too. There's also sequence information (alignments, two-dimensional sequence maps), and — unfortunately rather hidden — what might be called 'a beginner's guide to EF hands' ('Home pages for residues'). Well worth a click-through is the 'Other web resources' option, which details reseachers and societies.

The Calmodulin Target Database, pioneered by the Ikura laboratory at the Ontario Cancer Institute, offers an introduction to calmodulin (CaM). On binding Ca2+, this EF-hand-containing Ca2+ receptor changes shape, enabling it to bind to various targets. The 'Guided tour' facility leads you through the available options, providing information on some of the known CaM-binding proteins and motifs, and lets you search for putative CaM-binding sites in your protein. If it does bind, you can do a binding-site analysis ... and find out more, through the 'Resources' option, in the up-to-date list of CaM-target reviews.