UDP-glucuronosyltransferase form 1A1 (UGT1A1) is the only bilirubin-glucuronidating isoform of this protein, and genetic deficiencies of UGT1A1 cause Crigler-Najjar syndrome, a disorder resulting from nonhemolytic unconjugated hyperbilirubinemia. Here we have focused on the instability of a translocation-deficient UGT1A1 protein, which has been found in patients with Crigler-Najjar type II, to elucidate the molecular basis underlying the deficiency in glucuronidation of bilirubin. A substitution of leucine to arginine at position 15 (L15R/1A1) is predicted to disrupt the hydrophobic core of the signal peptide of UGT1AL L15R/1A1 was synthesized in similar amounts to wild-type UGT1A1 protein (WT/1A1) in transfected COS cells. However, L15R/1A1 did not translocate across the endoplasmic reticulum membrane and was degraded rapidly with a half-life of about 50 min, in contrast to the much longer half-life of about 12.8h for WT/1A1. Our findings demonstrate that L15R/1A1 was rapidly degraded by the proteasome owing to its mislocalization in the cell. (C) 2003 Elsevier Inc. All rights reserved.