Expression of chimeric cDNAs in cell culture defines a region of UDP glucuronosyltransferase involved in substrate selection

The cDNAs encoding two forms of UDP glucuronosyltransferase have been expressed in cultured cells to demonstrate that one form, UDPGTr-3, glucuronidates testosterone, whereas the second form, UDPGTr-4, is mainly active toward etiocholanolone (Mackenzie, P.I. (1986) J. Biol. Chem. 261, 14112-14117; Mackenzie, P.I. (1987) J. . Chem. 262, 9744-9749). In order to localize areas of the polypeptide chain involved in substrate selection, the 5' regions of UDPGTr-3 and -4 cDNAs were exchanged to form two chimeric cDNAs. A 53-kDa protein was synthesized in COS cells transfected with the chimeric UDPGTr-3.4 cDNA, which encodes the amino-terminal 298 residues of UDPGTr-3 and the carboxyl-terminal 232 residues of UDPGTr-4. This protein glucuronidated testosterone rather than etiocholanolone and had a faster electrophoretic mobility when transfected COS cells were cultured in the presence of tunicamycin, an inhibitor of N-linked glycosylation. The unglycosylated variant produced by this treatment also glucuronidated testosterone. In contrast, a 50-kDa protein that was more active toward etiocholanolone as substraste was synthesized in COS cells transfected with UDPGTr-4.3, a chimeric cDNA that encodes the amino-terminal region of UDPGTr-4 joined to the carboxyl-terminal region of UDPGTr-3. The electrophoretic mobility of this chimeric protein was unaffected by tunicamycin treatment. These results demonstrate that amino acid sequences that specify substrate specificity are localized in the amino-terminal half of the UDP glucuronosyltransferase polypeptide chain and that the presence of N-linked oligosaccharide chains on the protein does not affect the choice of substrate.