Editing ABC transporter (section)

=== Human ABCB1/MDR1 P-glycoprotein ===
{{main|P-glycoprotein}}

P-glycoprotein (3.A.1.201.1) is a well-studied protein associated with multi-drug resistance. It belongs to the human ''ABCB (MDR/TAP)'' family and is also known as ''ABCB1'' or ''MDR1 Pgp''. MDR1 consists of a functional monomer with two transmembrane domains (TMD) and two nucleotide-binding domains (NBD). This protein can transport mainly cationic or electrically neutral substrates as well as a broad spectrum of amphiphilic substrates. The structure of the full-size ABCB1 monomer was obtained in the presence and absence of nucleotide using [[electron crystallography|electron cryo crystallography]]. Without the nucleotide, the TMDs are approximately parallel and form a barrel surrounding a central pore, with the opening facing towards the extracellular side of the membrane and closed at the intracellular face. In the presence of the nonhydrolyzable ATP analog, AMP-PNP, the TMDs have a substantial reorganization with three clearly segregated domains. A central pore, which is enclosed between the TMDs, is slightly open towards the intracellular face with a gap between two domains allowing access of substrate from the lipid phase. Substantial repacking and possible rotation of the TM helices upon nucleotide binding suggests a helix rotation model for the transport mechanism.<ref name=pohl/>