Cap binding complex

The 5' cap of eukaryotic messenger RNA is bound at all times by various cap-binding complexes (CBCs).

Surface model of the cap-binding complex (heavy chain pink, light chain yellow, m7G and GDP as balls), after PDB 1H2T.

Nuclear cap-binding complex

In the nucleus, freshly transcribed mRNA molecules are bound on the 5' cap by the nuclear cap-binding complex of Cbc1/Cbc2 in yeast or CBP20/CBP80 in metazoans. These aid in the export of the mRNA and protect it from decapping. They also serve as a marker for the so-called pioneer round of translation when the message is examined by nonsense mediated decay.^[1]

Cytoplasmic cap-binding complex

After the first round of translation ("pioneer round"), CBC20/80 is replaced by the translation initiation factor eIF4E.^[2] The eIF4F complex (eIF4E, eIF4G and eIF4A) then regulates translation in response to the state of the cell via its phosphorylation state and again protects the message from decapping.^[3]

Decapping complex

When translationally repressed or marked for decay by various mechanisms the 5' cap is bound by the mRNA decapping enzyme DCP2. A host of proteins accompany it including UPF1, UPF2, UPF3A, Dcp1, Dhh1, XRN1, and others. The decapping enzyme removes the 5' cap leading to destruction of the message.^[4]