Catalyzes the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, a cosubstrate for homocysteine remethylation to methionine. Represents a key regulatory connection between the folate and methionine cycles. (6S)-5-methyl-5,6,7,8-tetrahydrofolate + NADP(+) = (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + H(+) + NADPH (6S)-5-methyl-5,6,7,8-tetrahydrofolate + NAD(+) = (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + H(+) + NADH Allosterically regulated by S-adenosylmethionine (SAM). One-carbon metabolism; tetrahydrofolate interconversion. Homodimer. Contains a serine-rich phosphorylation region at the N-terminal and an eukaryote-only S-adenosylmethionine (SAM)-binding domain at the C-terminal. Through asymmetric homodimerization, the two regions are positioned next to each other and N-terminal phosphorylation increases sensitivity to SAM binding and inhibition. Phosphorylation of an N-terminal serine-rich phosphorylation region increases sensitivity to S-adenosylmethionine and inhibition. Belongs to the methylenetetrahydrofolate reductase family.