S-adenosyl-L-methionine-dependent methyltransferase that adds a methylphosphate cap at the 5'-end of 7SK snRNA (7SK RNA), leading to stabilize it (By similarity). 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:23154982). The 7SK RNP complex also promotes snRNA gene transcription by RNA polymerase II via interaction with the little elongation complex (LEC) (By similarity). In the 7SK RNP complex, MEPCE is required to stabilize 7SK RNA and facilitate the assembly of 7SK RNP complex (By similarity). MEPCE has a non-enzymatic function in the 7SK RNP complex; it has a non-enzymatic function; 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 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). Interacts with METTL16. 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. Belongs to the methyltransferase superfamily.