UDP-GLUCURONOSYLTRANSFERASE, THE ROLE OF THE AMINO-TERMINUS IN DIMERIZATION

UDP-glucuronosyltransferases (UGTs) comprise an important enzyme syste m in mammals that is involved in detoxification of a variety of small hydrophobic compounds of both endogenous and exogenous origin, Some ev idence suggests that these enzymes may function as oligomers; however, little is known about the domain of interaction or the mechanism of o ligomerization, In this work, evidence for a functional dimerization b etween UGTs is provided by studies on mutated forms of UGTSB1. When tw o inactive forms of UGT2B1 were coexpressed in cell culture, catalytic activity was restored, indicating that UGT2B1 forms functional dimers . To delineate the dimerization domain, inactive fusion proteins conta ining the amino-or carboxyl-terminal domains of UGT2B1 were generated and expressed with active UGT2B1, Expression of a fusion protein conta ining only the amino-terminal half of UGT2B1 with active UGT2B1 caused a reduction in UGT2B1 catalytic activity, This reduction in activity was not observed when UGT2B1 was co-expressed with a fusion protein co ntaining only the carboxyl terminal half of UGT2B1, strongly suggestin g that the amino-terminal domain is involved in dimerization. Truncati on of the immediate amino terminus of UGT2B1 abolished UGT2B1 activity and dimer formation. Activity was also abolished by an L4R substituti on in this region of the mature protein, which is highly conserved in the UGT family. These results indicate that UGTs can interact through their amino-terminal domains to form catalytically active dimers. Poss ible mechanisms resulting in the formation and stabilization of the UG T2B1 dimer are discussed.