Kevin Schey
Last active: 3/24/2020

Post-translational modifications of aquaporin 0 (AQP0) in the normal human lens: spatial and temporal occurrence.

Ball LE, Garland DL, Crouch RK, Schey KL
Biochemistry. 2004 43 (30): 9856-65

PMID: 15274640 · DOI:10.1021/bi0496034

Because of the lack of protein turnover in fiber cells of the ocular lens, Aquaporin 0 (AQP0), the most abundant membrane protein in the lens, undergoes extensive post-translational modification with fiber cell age. To map the distribution of modified forms of AQP0 within the lens, normal human lenses ranging in age from 34 to 38 were concentrically dissected into several cortical and nuclear sections. Membrane proteins still embedded in the membranes were digested with trypsin, and the resulting C-terminal peptides of AQP0 were analyzed by HPLC tandem mass spectrometry, permitting the identification of modifications and estimation of their abundance. Consistent with earlier reports, the major phosphorylation site was Ser 235, and the major sites of backbone cleavage occurred at residues 246 and 259. New findings suggest that cleavage at these sites may be a result of nonenzymatic truncation at asparagine residues. In addition, this approach revealed previously undetected sites of truncation at residues 249, 260, 261, and 262; phosphorylation at Ser 231 and to a lower extent at Ser 229; and racemization/isomerization of l-Asp 243 to d-Asp and d-iso-Asp. The spatial distribution of C-terminally modified AQP0 within the lens indicated an increase in truncation and racemization/isomerization with fiber cell age, whereas the level of Ser 235 phosphorylation increased from the outer to inner cortex but decreased in the nucleus. Furthermore, the remarkably similar pattern and distribution of truncation products from lenses from three donors suggest specific temporal mechanisms for the modification of AQP0.

MeSH Terms (21)

Adult Amino Acid Sequence Aquaporins Asparagine Cellular Senescence Chromatography, Liquid Eye Proteins Humans Hydrolysis Lens, Crystalline Male Membrane Glycoproteins Molecular Sequence Data Peptide Fragments Phosphorylation Protein Conformation Protein Processing, Post-Translational Sequence Deletion Spectrometry, Mass, Electrospray Ionization Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization Trypsin

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