Characterization of human torsinA and its dystonia-associated mutant form

Biochem J. 2003 Aug 15;374(Pt 1):117-22. doi: 10.1042/BJ20030258.

Abstract

Deletion of a single glutamate in torsinA correlates with early-onset dystonia, the most severe form of a neurological disorder characterized by uncontrollable muscle contractions. TorsinA is targeted to the ER (endoplasmic reticulum) in eukaryotic cells. We investigated the processing and membrane association of torsinA and the dystonia-associated Glu-deletion mutant (torsinAdeltaE). We found that the signal sequence of torsinA (residues 1-20 from the 40 amino-acid long N-terminal hydrophobic region) is cleaved in Drosophila S2 cells, as shown by the N-terminal sequencing after partial protein purification. TorsinA is not secreted from S2 cells. Consistently, sodium carbonate extraction and Triton X-114 treatment showed that torsinA is associated with the ER membrane in CHO (Chinese-hamster ovary) cells. In contrast, a variant of torsinA that contains the native signal sequence without the hydrophobic region Ile24-Pro40 does not associate with the membranes in CHO cells, and a truncated torsinA without the 40 N-terminal amino acids is secreted in the S2 culture. Thus the 20-amino-acid-long hydrophobic segment in torsinA, which remains at the N-terminus after signal-peptide cleavage, is responsible for the membrane anchoring of torsinA. TorsinAdeltaE showed similar cleavage of the 20 N-terminal amino acids and membrane association properties similar to wild-type torsinA but, unlike the wild-type, torsinAdeltaE was not secreted in the S2 culture even after deletion of the membrane-anchoring segment. This indicates that the dystonia-associated mutation produces a structurally distinct, possibly misfolded, form of torsinA, which cannot be properly processed in the secretory pathway of eukaryotic cells.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • CHO Cells
  • Carrier Proteins / chemistry
  • Carrier Proteins / genetics*
  • Cell Line
  • Cricetinae
  • Drosophila melanogaster
  • Dystonia Musculorum Deformans / genetics*
  • Dystonia Musculorum Deformans / metabolism
  • Genetic Variation
  • Humans
  • Molecular Chaperones*
  • Molecular Sequence Data
  • Mutation*
  • Peptide Fragments / chemistry
  • Protein Folding
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / metabolism
  • Sequence Deletion*
  • Transfection

Substances

  • Carrier Proteins
  • Molecular Chaperones
  • Peptide Fragments
  • Recombinant Proteins
  • TOR1A protein, human