The mdr1 P-glycoprotein may regulate the sensitivity of murine T lymphoma cells to glucocorticoids. P-glycoproteins are ATP-dependent transporters that remove hydrophobic drugs from cells and thus cause multidrug resistance. Some steroids such as progesterone inhibit transport of drugs by P-glycoproteins. There is a high degree of selectivity in the ability of P-glycoproteins to transport corticosteroids, and specific structural features of steroids are recognition determinants for binding to and transport by P-glycoproteins. This application proposes a structure-function analysis of the murine mdr1 protein concerning its ability to bind to and transport corticosteroids. A method has been developed to select for variants of the WEHI-7 murine T lymphoma line that contain mdr1 mutations affecting the protein's ability to transport dexamethasone. Other steroids that normally inhibit mdr1 have a reduced capacity to do so in such variants. The selections are designed to ensure that mdr1 retains most of its capacity to transport other drugs. It is proposed that the putative mdr1 mutations affect the recognition of specific steroid structural features that are determinants for binding to mdr1. Assays using a variety of steroidal mdr1 inhibitors will determine which of these steroid structural determinants (initially, the 3- and 20-keto groups) are no longer recognized by the mutated mdr1. In vitro mutagenesis and expression of mutant mdr1 proteins will confirm the functional effects of each mutation.