Outer capsid protein involved in mRNA capping. Catalyzes the last 3 enzymatic activities for formation of the 5' cap structure on the viral plus-strand transcripts, namely the RNA guanylyltransferase, RNA-7N- and RNA-2'O-methyltransferase activities (By similarity). a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) = a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + diphosphate a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-homocysteine Belongs to the orthoreovirus lambda-2 protein family.