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). 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. Belongs to the CAPAM family. The role of N(6),2'-O-dimethyladenosine cap (m6A(m)) on transcripts is unclear and subject to discussion. According to a report, m6A(m) promotes the translation of capped mRNAs (PubMed:30467178). However, another study did not observe a clear effect on mRNA translation, but reported an increased stability of a subset of m6A(m) transcripts (By similarity). According to a third report, m6A(m) inhibits mRNA translation without affecting mRNA stability (By similarity). Contaminating sequence. Potential poly-A sequence.