Rapid proteasomal degradation of translocation-deficient UDP-glucuronosyltransferase 1A1 proteins in patients with Crigler-Najjar type II.
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 UGT1A1. 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 50min, 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.
Ohnishi
Aki
A
Department of Life Science, Faculty of Science, Himeji Institute of Technology, Harima Science Park City, Hyogo, Japan.
Emi
Yoshikazu
Y
D000818:Animals; D001120:Arginine / Q000737:chemistry; D019556:COS Cells; D002462:Cell Membrane / Q000378:metabolism; D003414:Crigler-Najjar Syndrome / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005260:Female; D014453:Glucuronosyltransferase / Q000737:chemistry / Q000378:metabolism*; D006026:Glycoside Hydrolases / Q000378:metabolism; D006801:Humans; D007930:Leucine / Q000737:chemistry; D008875:Middle Aged; D009097:Multienzyme Complexes / Q000378:metabolism*; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D011233:Precipitin Tests; D011480:Protease Inhibitors / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex; D021382:Protein Sorting Signals; D021381:Protein Transport; D013997:Time Factors; D014162:Transfection; D014357:Trypsin / Q000494:pharmacology; D000099148:UGT1A1 Enzyme
D001120:Arginine; D003546:Cysteine Endopeptidases; D014453:Glucuronosyltransferase; D006026:Glycoside Hydrolases; D007930:Leucine; D009097:Multienzyme Complexes; D011480:Protease Inhibitors; D046988:Proteasome Endopeptidase Complex; D021382:Protein Sorting Signals; D014357:Trypsin; D000099148:UGT1A1 Enzyme