C2 domain

A C2 domain is a protein structural domain involved in targeting proteins to cell membranes. It is a beta-sandwich composed of 8 β-strands that co-ordinates two or three calcium ions, which bind in a cavity formed by the first and final loops of the domain, on the membrane binding face.

Coupling with other domains
C2 domains are frequently found coupled to enzymatic domains; for example, the C2 domain in PTEN, brings the phosphatase domain into contact with the membrane where it can dephosphorylate its substrate, phosphatidylinositol (3,4,5)-trisphosphate (PIP3), without removing it from the membrane - which would be energetically very costly. In addition to this, phosphatidylinositol 3-kinase (PI3-kinase), an enzyme that phosphorylates phosphoinositides on the 3-hydroxyl group of the inositol ring, also uses a C2 domain to bind to the membrane (e.g. 1e8w PDB entry).

C2 domains are also found in clostridial alpha toxins, where they are used to bring the catalytic phospholipase domain into contact with the plasma membrane, conferring the toxic activity on the protein. These are the only known examples of C2 domains in prokaryotes.

Lipid selectivity
C2 domains are unique among membrane targeting domains in that they show wide range of lipid selectivity for the major components of cell membranes, including phosphatidylserine and phosphatidylcholine. This C2 domain is about 116 amino-acid residues and is located between the two copies of the C1 domain in Protein Kinase C (that bind phorbol esters and diacylglycerol) (see PDOC00379) and the protein kinase catalytic domain (see PDOC00100). Regions with significant homology to the C2-domain have been found in many proteins. The C2 domain is thought to be involved in calcium-dependent phospholipid binding and in membrane targeting processes such as subcellular localisation.

3D structure
3D structure of C2 domains has been reported, the domain forms an eight-stranded beta sandwich constructed around a conserved 4-stranded motif, designated a C2 key. Calcium binds in a cup-shaped depression formed by the N- and C-terminal loops of the C2-key motif. Structural analyses of several C2 domains have shown them to consist of similar ternary structures in which three Ca2+-binding loops are located at the end of an 8 stranded antiparallel beta sandwich.

Human proteins containing C2 domain
ABR;      BAIAP3;    BCR;       C2CD2;     C2CD3;     C8orfK23;  CADPS;     CADPS2; CAPN5;    CAPN6;     CC2D1A;    CC2D1B;    CPNE1;     CPNE2;     CPNE3;     CPNE4; CPNE5;    CPNE6;     CPNE7;     CPNE8;     CPNE9;     DAB2IP;    DOC2A;     DOC2B; DYSF;     ESYT1;     ESYT3;     FAM62A;    FAM62B;    FAM62C;    FER1L3;    FER1L5; HECW1;    HECW2;     ITCH;      ITSN1;     ITSN2;     KIAA0528;  KIAA1228;  KIAA1957; LOC392742; MCTP1;    MCTP2;     MTAC2D1;   NEDD4;     NEDD4L;    NEDL1;     OTOF; PCLO;     PIK3C2A;   PIK3C2B;   PIK3C2G;   PLA2G4A;   PLA2G4B;   PLA2G4D;   PLA2G4E; PLA2G4F;  PLCB1;     PLCB2;     PLCB3;     PLCB4;     PLCD1;     PLCD3;     PLCD4; PLCE1;    PLCG1;     PLCG2;     PLCH1;     PLCH2;     PLCL1;     PLCL2;     PLCZ1; PRF1;     PRKCA;     PRKCB1;    PRKCE;     PRKCG;     PRKCH;     RAB11FIP1; RAB11FIP2; RAB11FIP5; RASA1;    RASA2;     RASA3;     RASA4;     RASAL1;    RASAL2;    RGS3; RIMS1;    RIMS2;     RIMS3;     RIMS4;     RPGRIP1;   RPGRIP1L;  RPH3A;     SGA72M; SMURF1;   SMURF2;    SYNGAP1;   SYT1;      SYT10;     SYT11;     SYT12;     SYT13; SYT14;    SYT14L;    SYT15;     SYT16;     SYT17;     SYT2;      SYT3;      SYT4; SYT5;     SYT6;      SYT7;      SYT8;      SYT9;      SYTL1;     SYTL2;     SYTL3; SYTL4;    SYTL5;     TOLLIP;    UNC13A;    UNC13B;    UNC13C;    UNC13D;    WWC2; WWP1;     WWP2;      sytdep;