Catalyzes the transfer of the methyl group from monomethylamine to the corrinoid cofactor of MtmC. Co(I)-[methylamine-specific corrinoid protein] + H(+) + methylamine = methyl-Co(III)-[methylamine-specific corrinoid protein] + NH4(+) One-carbon metabolism; methanogenesis from methylamine. Can form a complex with MtmC. Belongs to the monomethylamine methyltransferase family. Truncated C-terminus. Truncated C-terminus. Truncated C-terminus. Truncated C-terminus.