S-adenosyl-L-methionine-dependent methyltransferase that adds a methylphosphate cap at the 5'-end of 7SK snRNA (7SK RNA), leading to stabilize it. Also has a non-enzymatic function as part of the 7SK RNP complex: the 7SK RNP complex sequesters the positive transcription elongation factor b (P-TEFb) in a large inactive 7SK RNP complex preventing RNA polymerase II phosphorylation and subsequent transcriptional elongation. The 7SK RNP complex also promotes snRNA gene transcription by RNA polymerase II via interaction with the little elongation complex (LEC). In the 7SK RNP complex, MEPCE is required to stabilize 7SK RNA and facilitate the assembly of 7SK RNP complex: it has a non-enzymatic function (By similarity). MEPCE has a non-enzymatic function in the 7SK RNP complex; interaction with LARP7 within the 7SK RNP complex occluding its catalytic center (By similarity). a 5'-end triphospho-guanosine-ribonucleotide-snRNA + S-adenosyl-L-methionine = a 5'-end methyltriphosphate-guanosine-ribonucleotide-snRNA + S-adenosyl-L-homocysteine Component of the 7SK snRNP complex. Transcription elongation and RNA splicing defects, as well as aberrant anterior (brain) morphologies. Belongs to the methyltransferase superfamily.