title: Isoaspartyl protein damage and repair in mouse retina.
abstract: OBJECTIVE
  To determine the propensity of retinal proteins for spontaneous damage via formation of isoaspartyl sites, a common type of protein damage that could contribute to retinal disease.
  
  
  METHODS
  Tissue extracts were obtained from retinas and brains of control mice and from mice in which the gene for protein L-isoaspartate O-methyltransferase (PIMT; an enzyme that repairs isoaspartyl protein damage) was knocked out. PIMT expression in these extracts was measured by Western blot, and its specific activity was assayed by monitoring the rate of [(3)H]methyl transfer from S-adenosyl-[methyl-(3)H]L-methionine to γ-globulin. Isoaspartate levels in extracts were measured by their capacity to accept [(3)H]methyl groups via the PIMT-catalyzed methylation reaction. To compare molecular weight distributions of isoaspartyl-rich proteins in retina versus brain, proteins from PIMT knockout (KO) and control mice were separated by SDS-PAGE and transferred to polyvinylidene difluoride (PVDF). Isoaspartyl proteins were (3)H-labeled on-blot using a PIMT overlay and imaged by autoradiography.
  
  
  RESULTS
  When normalized to the β-actin content of each tissue, retina was found to be nearly identical to brain with regard to expression and activity of PIMT and its propensity to accumulate isoaspartyl sites when PIMT is absent. The two tissues show distinct differences in the molecular weight distribution of isoaspartyl proteins.
  
  
  CONCLUSIONS
  The retina is rich in PIMT activity and contains a wide range of proteins that are highly susceptible to this type of protein damage. Recoverin may be one such protein. Isoaspartate formation, along with oxidation, should be considered as a potential source of protein dysfunction and autoimmunity in retinal disease.
authors.affiliation: Department of Molecular Biology and Biochemistry, University of California, Irvine, California.
authors.forename: Zhenxia
authors.initials: Z
authors.lastname: Qin
authors.forename: Jing
authors.initials: J
authors.lastname: Yang
authors.forename: Henry J
authors.initials: HJ
authors.lastname: Klassen
authors.forename: Dana W
authors.initials: DW
authors.lastname: Aswad
mesh_terms: D000818:Animals; D004195:Disease Models, Animal; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D026581:Isoaspartic Acid / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D015394:Molecular Structure; D040901:Proteomics / Q000379:methods*; D012160:Retina / Q000378:metabolism* / Q000473:pathology; D012164:Retinal Diseases / Q000209:etiology / Q000378:metabolism*
chemical_list: D026581:Isoaspartic Acid
keywords: brain; isoaspartate; protein damage; protein repair; retina