S-adenosyl-L-methionine-dependent methyltransferase that mediates mRNA cap1 2'-O-ribose methylation to the 5'-cap structure of mRNAs (PubMed:32504809). Methylates the ribose of the first nucleotide of a m(7)GpppG-capped mRNA to produce m(7)GpppNmp (cap1) (PubMed:32504809). Positively regulates the Ago2-dependent small RNA pathway, with roles in both siRNA biogenesis and RISC assembly (PubMed:32504809). Involved in facilitating conversion of pre-RISC into holo-RISC, possibly by promoting the unwinding of Ago2-bound siRNA duplexes and thus the retention of the guide strand in holo-RISC (PubMed:32504809). a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyl-ribonucleoside) in mRNA + H(+) + S-adenosyl-L-homocysteine Interacts (via C-terminus) with r2d2 (via C-terminus).