Regions of EF-Hand CaBPs

EF-hands almost always occur in pairs in proteins. The only protein which is known from structural studies to have an odd number of EF-hands is calpain domain VI (the calcium-binding domain of the small subunit of calpain). This domain has five EF-hands. However, isolated calpain domain VI forms homodimers both in the crystal structure and in solution (InfoCard). In vivo, domain VI interacts with the homologous EF-hand calcium-binding domain of the large subunit, domain IV. The homodimerization of domain VI is thought to be a good model for the domain VI-domain IV interaction.

The First EF-Hand

There are two main types of EF-hands common as the first EF-hand in a pair: the canonical EF-hand such as the one seen in calmodulin, and the pseudo EF-hand seen in the S100 proteins. The primary difference between these two types of EF-hands is in the binding loop: the canonical EF-hand has a twelve residue loop that chelates calcium mostly via sidechain carboxylates, while the pseudo EF-hand has a fourteen residue binding loop that chelates calcium mostly via backbone carbonyls. In both types of binding loops, and in fact in all high affinity binding loops studied to date, the last residue provides bidentate chelation via sidechain carboxylates. This residue is almost always a glutamate, but is occasionally an aspartate.

The Second EF-Hand

The second EF-hand in a pair is of the canonical type in most proteins, including the S100 proteins.

The Linker Loop

The loop linking the two EF-hands in a pair, termed the linker loop, is one of the most variable regions of EF-hand calcium-binding proteins. It varies both in composition and in length. In proteins with more than one pair of EF-hands, the connections between the pairs of EF-hands are also highly variable.