Acts as a methyl group carrier between MtbB1 and MtbA. Binds 1 corrinoid cofactor per protein, is subsequently demethylated by MtbA. One-carbon metabolism; methanogenesis from dimethylamine. Copurifies with MtbA. Induced by growth on trimethylamine but not methanol or monomethylamine. Part of the mtbC-mttB-mttC and mtbC-mttB-mttC-mttP-mtbB1 operons. Belongs to the methylamine corrinoid protein family.