D12, Vaccinia Virus

The D12 subunit forms a stable complex with the methyltransferase domain of D1. D12 has no known enzymatic activity but serves to stimulate the otherwise relatively weak intrinsic methyltransferase activity of D1.The D1/D12 heterodimer is a required cofactor for viral early gene transcription termination. Of the three enzymatic activities required for mRNA capping, the guanylyltransferase and methyltransferase activities are compromised while the triphosphatase activity and the D12 subunit interaction are unaffected.