Catalyzes the reversible reduction of methylene-H(4)MPT to methyl-H(4)MPT. 5-methyl-5,6,7,8-tetrahydromethanopterin + H(+) + oxidized coenzyme F420-(gamma-L-Glu)(n) = 5,10-methylenetetrahydromethanopterin + reduced coenzyme F420-(gamma-L-Glu)(n) Requires the presence of relatively high concentrations of either sulfate or phosphate for maximal activity. kcat is 275 sec(-1). One-carbon metabolism; methanogenesis from CO(2); methyl-coenzyme M from 5,10-methylene-5,6,7,8-tetrahydromethanopterin: step 1/2. Homotetramer composed of two loosely associated dimers. Belongs to the mer family.