Conformational effects of amino acid substitutions in the P-glycoprotein of the mdr 1 gene

The P-glycoprotein of the mdr 1 gene is responsible for the phenomenon of multidrug resistance in human cells. The presumed drug-binding site of the wild-type P-glycoprotein contains a glycine at position 185. A mutant P-glycoprotein which contains valine at this position causes cells to retain resistance to colchichine, but to lose cross-resistance to other drugs such as the chemotherapeutic agents vinblastine and Adriamycin. This has been hypothesized to be due to a conformational change in the protein induced by the amino acid substitution. Using conformational energy analysis, we have determined the allowed three-dimensional structures for the wild-type and mutant proteins in the region of position 185. The results indicate that the wild-type protein adopts a unique left-handed conformation at position 185 which is energetically unfavorable for the protein with l-amino acids (including valine) at this position. This conformational change induced by amino acid substitutions for Gly 185 could explain the differences in binding to the P-glycoprotein of various drugs and, hence, the differences in drug resistance exhibited by various cell lines expressing these proteins.