p-gp is a 170 kda transmembrane glycoprotein, which includes 10-15 kda of n-terminal glycosylation. the n-terminal half of the molecule contains 6 transmembrane domains, followed by a large cytoplasmic domain with an atp-binding site, and then a second section with 6 transmembrane domains and an atp-binding site that shows over 65% of amino acid similarity with the first half of the polypeptide. in 2009, the first structure of a mammalian p-glycoprotein was solved (3g5u). the structure was derived from the mouse mdr3 gene product heterologously expressed in "pichia pastoris" yeast. the structure of mouse p-gp is similar to structures of the bacterial abc transporter msba (3b5w and 3b5x) that adopt an inward facing conformation that is believed to be important for binding substrate along the inner leaflet of the membrane. additional structures (3g60 and 3g61) of p-gp were also solved revealing the binding site(s) of two different cyclic peptide substrate/inhibitors. the promiscuous binding pocket of p-gp is lined with aromatic amino acid side chains.