Catalyzes the 2 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. Hypermethylation of the m7G cap of U snRNAs leads to their concentration in nuclear foci, their colocalization with coilin and the formation of canonical Cajal bodies (CBs). Plays a role in transcriptional regulation. 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 Optimum pH is 8.5-9.5. May form homooligomers. Interacts with CREBBP/CBP, EED/WAIT1, EP300/P300, NCOA6/PRIP, PPARBP/PBP and SMN. A 90 kDa isoform is found in the nucleus while a 55 kDa isoform is found in the cytoplasm and colocalizes with the tubulin network. Ubiquitously expressed. High expression in heart, skeletal muscle, kidney, liver and placenta. Belongs to the methyltransferase superfamily. Trimethylguanosine synthase family. Truncated N-terminus. Truncated N-terminus.