Intracellular Degradation of a Mutant Serine : Pyruvate/Alanine : Glyoxylate Aminotransferase

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We recently studied a primary hyperoxaluria type 1 case who belongs to the ENZ^-/CRM^- class, and his serine : pyruvate/alanine : glyoxylate aminotransferase (SPT/AGT) gene was found to have a homogygous point mutation in the middle of the coding region (a T to C mutation resulting in a Ser to Pro substitution at residue 205). Available evidence suggested that the enzyme deficiency is due to neither a defect in transcription nor one in translation, but is due, at least in part, to instability of the mutant SPT/AGT protein in cells. The mutant SPT/AGT was also degraded faster than normal in an in vitro system involving a rabbit reticulocyte lysate, and this reaction required ATP and Mg^<2+>. A well characterized ATP-hydrolysis-dependent proteolytic machinary in reticulocyte lysates is the ubiquitin-proteasome system, but immunodepletion of proreasomes or inclusion in the reaction mixture of peptide aldehyde proteasome inhibitors had no effect on the degradation of the mutant SPT/AGT in a reticulocyte lysate. In addition, the mutant SPT/AGT was not stabilized in a yeast mutant (per1-1) with defective proteasomes. These results indicate that the inherited mutant SPT/AGT is degraded ATP-hydrolysis-dependently without involvement of proteasomes. It is thus suggested that an energy-dependent proteolytic pathway other than that involving proteasomes participates in selective elimination of abnormal proteins generated in genetic disorders.