Catalyzes the two serial methylation steps for the conversion of the 7-monomethylguanosine (m(7)G) caps of snRNAs and snoRNAs to a 2,2,7-trimethylguanosine (m(2,2,7)G) cap structure. The enzyme is specific for guanine, and N7 methylation must precede N2 methylation. Hypermethylates the m3G cap on TLC1 telomerase which affects telomere silencing and telomere length regulation. Required for pre-mRNA splicing, pre-rRNA processing and small ribosomal subunit synthesis. Involved in nucleolar structural organization. a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-methionine = a 5'-end (N(2),N(7)-dimethyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + H(+) + S-adenosyl-L-homocysteine a 5'-end (N(2),N(7)-dimethyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-methionine = a 5'-end (N(2),N(2),N(7)-trimethyl 5'-triphosphoguanosine)-(ribonucleoside) in mRNA + H(+) + S-adenosyl-L-homocysteine Substrate inhibited by S-adenosyl-L-homocysteine. Monomer (By similarity). Interacts with the spliceosomal snRNP core component SMB1 and the snoRNP components CBF5 and NOP58. Belongs to the methyltransferase superfamily. Trimethylguanosine synthase family.