RNA methyltransferases involved in 5' cap biosynthesis

Byszewska, Magdalena; Smietanski, Miroslaw; Purta, Elzbieta; Bujnicki, Janusz M

Studies of cap composition of human mRNAs conducted in mid-70s revealed that when the first nucleotide of the transcript is an adenosine, this base can be methylated to m 6 A. 8, 71 The enzyme that catalyzes the conversion of m 7 GpppAm ends of mRNA to m 7 Gpppm 6 Am has been isolated from a cytoplasmic fraction of HeLa cells. The isolated enzyme showed no activity toward internal adenosines. 72 Recently, Schwartz and coworkers studied the m 6 A mRNA methylome following depletion of multiprotein methyltransferase complex components METTL3, METTL14, KIAA1429, and WTAP, and implicated the involvement of the METTL3, METTL14, and KIAA1429 proteins in m 6 A formation at the internal sites but not at the 5 0 sites. 73 The full characterization of the cap-specific m 6 A methyltransferase activity requires further studies in vitro.