Involved in the active translocation of vitamin B12 (cyanocobalamin) across the outer membrane to the periplasmic space. It derives its energy for transport by interacting with the trans-periplasmic membrane protein TonB. Is also a receptor for bacteriophages BF23 and C1, and for A and E colicins. Calcium increases vitamin B12 binding affinity by a factor of 50-100. Interacts with TonB. Constitutively expressed. Primary control of btuB expression by cobalamin occurs at the level of translation initiation. Belongs to the TonB-dependent receptor family. BtuB (TC 1.B.14.3.1) subfamily.