S-adenosyl-L-methionine-dependent methyltransferase that adds a methylphosphate cap at the 5'-end of 7SK snRNA (7SK RNA), leading to stabilize it (PubMed:17643375, PubMed:19906723, PubMed:30559425). 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 (PubMed:17643375). The 7SK RNP complex also promotes snRNA gene transcription by RNA polymerase II via interaction with the little elongation complex (LEC) (PubMed:28254838). In the 7SK RNP complex, MEPCE is required to stabilize 7SK RNA and facilitate the assembly of 7SK RNP complex (PubMed:19906723). MEPCE has a non-enzymatic function in the 7SK RNP complex; interaction with LARP7 within the 7SK RNP complex occluding its catalytic center (PubMed:19906723). a 5'-end triphospho-guanosine-ribonucleotide-snRNA + S-adenosyl-L-methionine = a 5'-end methyltriphosphate-guanosine-ribonucleotide-snRNA + S-adenosyl-L-homocysteine Core component of the 7SK RNP complex, at least composed of 7SK RNA, LARP7, MEPCE, HEXIM1 (or HEXIM2) and P-TEFb (composed of CDK9 and CCNT1/cyclin-T1) (PubMed:17643375, PubMed:19906723). Interacts with METTL16 (PubMed:29051200). Interacts with RBM7; upon genotoxic stress this interaction is enhanced, triggering the release of inactive P-TEFb complex from the core, yielding to P-TEFb complex activation (PubMed:30824372). Expressed in chronic myeloid leukemia cells, adrenal gland, brain, cerebellum, kidney, lung, mammary gland and testis (PubMed:12358911). Weakly or not expressed in other tissues (PubMed:12358911). Belongs to the methyltransferase superfamily.