Functional analysis of chimeric genes obtained by exchanging homologous domains of the mouse mdr1 and mdr2 genes.

A full-length cDNA clone for the mouse mdr1 gene can confer multidrug resistance when introduced by transfection into otherwise drug-sensitive cells. In the same assay, a full-length cDNA clone for a closely related member of the mouse mdr gene family, mdr2, fails to confer multidrug resistance. To identify the domains of mdr1 which are essential for multidrug resistance and which may be functionally distinct in mdr2, we have constructed chimeric cDNA molecules in which discrete domains of mdr2 have been introduced into the homologous region of mdr1 and analyzed these chimeras for their capacity to transfer drug resistance. The two predicted ATP-binding domains of mdr2 were found to be functional, as either could complement the biological activity of mdr1. Likewise, a chimeric molecule in which the highly sequence divergent linker domain of mdr2 had been introduced in mdr1 could also confer drug resistance. However, the replacement of either the amino- or carboxy-terminus transmembrane (TM) domain regions of mdr1 by the homologous segments of mdr2 resulted in inactive chimeras. The replacement of as few as two TM domains from either the amino (TM5-6) or the carboxy (TM7-8) half of mdr1 by the homologous mdr2 regions was sufficient to destroy the activity of mdr1. These results suggest that the functional differences detected between mdr1 and mdr2 in our transfection assay reside within the predicted TM domains.