LOCALIZATION OF THE IODOMYCIN BINDING-SITE IN HAMSTER P-GLYCOPROTEIN

P-glycoprotein, the overexpression of which is a major cause for the f ailure of cancer chemotherapy in man, recognizes and transports a broa d range of structurally unrelated amphiphilic compounds, This study re ports on the localization of the binding site of P-glycoprotein for io domycin, the Bolton-Hunter derivative of the anthracycline daunomycin. Plasma membrane vesicles isolated from multidrug-resistant Chinese ha mster ovary B30 cells were photolabeled with [I-125]iodomycin. After c hemical cleavage behind the tryptophan residues, I-125-labeled peptide s were separated by electrophoresis and high performance liquid chroma tography. Edman sequencing revealed that [I-125]iodomycin had been pre dominantly incorporated into the fragment 230-312 of isoform I of hams ter P-glycoprotein. According to models based on hydropathy plots, the amino acid sequence 230-312 forms the distal part of transmembrane se gment 4, the second cytoplasmic loop, and the proximal part of transme mbrane segment 5 in the N-terminal half of P-glycoprotein. The binding site for iodomycin is recognized with high affinity by vinblastine an d cyclosporin A.