9 Capping Enzyme

Abstract

Eukaryotic mRNAs contain a modified 5′ terminal “cap” structure consisting of 7-methylguanosine linked to the penultimate nucleoside of the RNA via a 5′-5′ triphosphate bridge. The presence of this cap structure or variants on almost all eukaryotic cellular and viral mRNAs has stimulated a great deal of research on the mechanism of cap synthesis and on the role played by the cap in mRNA function. This chapter focuses on the properties of the enzymes that modify the RNA terminus to generate the cap structure m 7 GpppX, commonly known as cap zero. It describes RNA guanylyltransferase (capping enzyme), the enzyme that catalyzes the guanylylation step (the formation of GpppX) in the capping pathway. The capping and methylating enzymes of vaccinia virus have been solubilized and obtained in pure form. The vaccinia capping and methylating system consists of two separable components. The first is a multifunctional capping enzyme complex that contains RNA triphosphatase, RNA guanylyltransferase and RNA (guanine-7)-methyltransferase activities. The second component is an mRNA (nucleoside-2)-methyltransferase. Cellular enzymes that catalyze the capping and methylation reactions have been purified to varying extents and the capping and methylating activities have been separated from each other. The synthesis of the modified RNA cap structure in vivo is of great importance as the RNA cap itself plays a significant role in cellular metabolism.