Four individual polypeptide chains including two TMD and two NBD subunits, may combine to form a full transporter such as in the E. coli BtuCD importer involved in the uptake of vitamin B12.
Četiri individualna polipeptidna lanca uključujući dve TMD i dve NBD podjedinice se mogu kombinovati da formiraju pun transporter, kao u E. coli BtuCD importeru koji učestvuje u preuzimanju vitamina B12.