Interactive Protein Tutorial

C2 Domain

C2 domains occur in a wide range of proteins that are found in eukaryotic cells. Many of these proteins are implicated in signalling mechanisms. (C2 domains take their name from the domain of protein kinase C that binds Ca²⁺).

The ball and stick structure on the right, obtained by x-ray diffraction, represents the C2 domain of protein kinase Cβ.

Click on the the buttons below to view important features of this and other C2 domain structures.

Here are some questions that you should try to answer.

How are the regions of secondary structure arranged to provide a stable fold?
How much does the 3D structure depend on sequence?

NAME

The C2 domain takes its name from the early structural description of protein kinase C, one of the first kinases to be identified. The α, β and γ isoforms, known as classical PKCs, possess similar domains, labelled C1 to C4. C3 and C4 form the catalytic domain; C1 and C2 form the regulatory domain. Other isoforms of PKC possess permutations and modifications of these basic domains. C1 is the binding site for diacylglycerol and for the tumour promoter PMA.

BACKBONE OF THE PKCβ C2 DOMAIN

Press the button to simplify this ball and stick model to show the backbone structure. There two molecules per crystal unit cell so two identical proteins are shown. Just concentrate upon one of them.

SECONDARY STRUCTURE

This is shown more clearly by a ribbon diagram. The computer calculates where regions of secondary structure occur and draws them as ribbons. The α-helical regions are now distinguishable and there are also extensive regions of β-structure.

The 7 β strands are arranged in an antiparallel “greek key” structure to form a β sandwich. The residues forming this structure are well conserved. The connecting loops however are variable.

CALCIUM BINDING

Now the Ca²⁺ ions that bind are shown in green. The cation binding occurs in the region between the loops.

Now let's simplify the picture by omitting one of the two C2 structures

CO-ORDINATING OXYGENS

Asking the program to display oxygen atoms close to the bound calciums reveals an incomplete co-ordination sphere. It is presumed that acidic phospholipids in the membrane provide the remaining elements of the cage.

Zoom in to see this more clearly

The amino acid sidechains in the loop region are mostly positively charged. However, those that contribute to the Ca²⁺ binding pocket are anionic. When calcium is bound, the negative charges are neutralised and the protein can then bind to anionic lipid. For this reason, it has also been termed an 'electrostatic switch'.

C2 domains have also been called CaLB domains ('calcium and lipid binding').

SYNAPTOTAGMIN

Here is different example. Synaptotagmin is a transmembrane protein found on synaptic and other types of secretory vesicle. The cytosolic chain of synaptotagmin has two C2 domains, C2A and C2B. In nerve cells, it is thought that the C2A domain detects the increase in cytosol Ca²⁺ concentration that is the signal that triggers exocytosis of neurotransmitter. The structure here shows both the C2A and C2B domains of synaptotagmin III. These are linked by a 7 residue chain. To obtain structure data, the protein crystal in this case did not contain Ca²⁺ ions. Instead Mg²⁺ ions (green spheres) occupy the sites normally filled by Ca²⁺.

A sulphate anion (the sulphur is yellow, the four oxygens are red) shows the location at which phospholipid binding is thought to occur.

Start with a ball and stick model (press the button under the JMol window) and use the built-in JMol menus to study more about the structural properties of this C2 domain.