Acts probably as a methyl group carrier between MttB and either MtbA or MtaA. One-carbon metabolism; methanogenesis from trimethylamine. Can form a complex with MttB. 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.