Vitamin B12 import proteins, molecular model. This complex is the import proteins btuC, btuD, and btuF. The first two together form BtuCD, an adenosine triphosphate (ATP)-binding cassette transporter. Its role is to translocate vitamin B12 from BtuF (a periplasmic binding protein) into the cytoplasm of the bacterium Escherichia coli. ATP-binding cassette transporters (ABC transporters) are being studied in relation to their role in a range of inherited diseases and processes, including resistance to tumours, and conditions such as cystic fibrosis.