RNA methyltransferases involved in 5' cap biosynthesis

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

In the human capping system, the cap0 methyltransferase (RNMT) consists of a catalytic subunit related to Abd1 and an obligate activating subunit, RAM (RNMT-activating miniprotein). 26 The C-terminal catalytic domain of RNMT has essentially the same structure as Abd1. RNMT also has an N-terminal domain that is conserved in mammals, but not required for catalytic activity. However, it contains 2 nuclear localization signal motifs and the nuclear localization of RNMT is essential for cell viability. The cap0 methyltransferases, members of the abovementioned family, were also identified and chartacterized in other eukaryotes, including TbCmt1 in Trypanosoma brucei, for example. 27 As mentioned above, the viral cap0 methyltransferases possess a catalytic domain that is closely related to the eukaryotic cap0 methyltransferases, but it often functions in the context of other domains. For instance, the vaccinia virus possesses an enzyme that is composed of D1 and D12 polypeptides that execute all 3 steps in cap0 biosynthesis. The D1 subunit contains triphosphatase and guanylyltransferase activities in the N-terminal domain, and a cap0 methyltransferase domain that forms a heterodimer with the D12 subunit. 28, 29 The methyltransferase active site is located entirely in the D1 subunit and has a weak cap0 modification activity that is stimulated allosterically by D12. [30] [31] [32] Interestingly, the D12 structure resembles a degenerate cap 2 0 -O-ribose methyltransferase domain (see below), but it lacks a proper SAM binding site and does not show any methyltransferase activity on its own. 33 In the SARS-coronavirus, a nonstructural protein 14 (nsp14) was initially identified as an exoribonuclease (and termed ExoN). Later, it was shown that it also exhibits cap0 methyltransferase activity. Analysis of protein variants with substitutions of conserved residues in the ExoN (N-terminal) and methyltransferase (C-terminal) domains revealed that both active sites are functionally distinct; however, the integrity of the ExoN domain turned out to be essential for the function of the cap0 methyltransferase domain. 34 Nsp14 shows little sequence similarity to known methyltransferases; however, its structure has not been determined experimentally, hence its phylogenetic relationships to other enzymes remain unclear.