S-adenosyl-L-methionine-dependent methyltransferase that acts as a component of the wybutosine biosynthesis pathway. Wybutosine is a hyper modified guanosine with a tricyclic base found at the 3'-position adjacent to the anticodon of eukaryotic phenylalanine tRNA. Catalyzes the final 2 independent reactions, methylation of the alpha-carboxy group of wybutosine-72 to form wybutosine-58, and methoxycarbonylation of alpha-amino group of wybutosine-58 through the fixation of CO(2) to complete wybutosine. 7-[(3S)-3-amino-3-carboxypropyl]wyosine(37) in tRNA(Phe) + S-adenosyl-L-methionine = 7-[(3S)-(3-amino-3-methoxycarbonyl)propyl]wyosine(37) in tRNA(Phe) + S-adenosyl-L-homocysteine 7-[(3S)-(3-amino-3-methoxycarbonyl)propyl]wyosine(37) in tRNA(Phe) + CO2 + S-adenosyl-L-methionine = 2 H(+) + S-adenosyl-L-homocysteine + wybutosine(37) in tRNA(Phe) tRNA modification; wybutosine-tRNA(Phe) biosynthesis. Belongs to the methyltransferase superfamily. LCMT family.