Cap-specific adenosine methyltransferase that catalyzes formation of N(6),2'-O-dimethyladenosine cap (m6A(m)) by methylating the adenosine at the second transcribed position of capped mRNAs (PubMed:30467178, PubMed:30487554, PubMed:31279658, PubMed:31279659, PubMed:33428944). Recruited to the early elongation complex of RNA polymerase II (RNAPII) via interaction with POLR2A and mediates formation of m6A(m) co-transcriptionally (PubMed:30467178). a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyladenosine) in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(N(6),2'-O-dimethyladenosine) in mRNA + H(+) + S-adenosyl-L-homocysteine Cap-specific adenosine methyltransferase activity is inhibited by zinc. kcat is 0.036 min(-1) for m7GpppAm capped mRNA (PubMed:31279659). kcat is 0.34 min(-1) for m7GpppAm dinucleotide (PubMed:31279659). kcat is 1.76 min(-1) for capped mRNA analog (at pH 5.4) (PubMed:33428944). kcat is 1.44 min(-1) for S-adenosyl-L-methionine (at pH 5.4) (PubMed:33428944). kcat is 0.67 min(-1) for capped mRNA analog (at pH 8.0) (PubMed:33428944). kcat is 0.63 min(-1) for S-adenosyl-L-methionine (at pH 8.0) (PubMed:33428944). kcat is 1.18 min(-1) for capped mRNA analog (at pH 9.4) (PubMed:33428944). kcat is 1.68 min(-1) for S-adenosyl-L-methionine (at pH 9.4) (PubMed:33428944). Interacts with POLR2A; interacts with the phosphorylated C-terminal domain (CTD) of POLR2A. Ubiquitous. The WW domain is sufficient for direct and specific interaction with the phosphorylated CTD of RNAPII largest subunit. Belongs to the CAPAM family. The role of N(6),2'-O-dimethyladenosine cap (m6A(m)) on transcripts is unclear and subject to discussion (PubMed:31279658, PubMed:31279659, PubMed:30467178). According to a report, m6A(m) promotes translation of capped mRNAs (PubMed:30467178). In contrast, another study did not observe a clear effect on mRNA translation, but reported an increased stability of a subset of m6A(m) transcripts (PubMed:31279658). According to a third report, m6A(m) inhibits mRNA translation without affecting mRNA stability (PubMed:31279659).