Post-translational Modifications of Aquaporin 0 (AQP0) in the Normal Human Lens: Spatial and Temporal Occurrence

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• 作者：Lauren E. Ball ; Donita L. Garland ; Rosalie K. Crouch ; and Kevin L. Schey
• 刊名：Biochemistry
• 出版年：2004
• 出版时间：August 3, 2004
• 年：2004
• 卷：43
• 期：30
• 页码：9856 - 9865
• 全文大小：162K
• 年卷期：v.43,no.30(August 3, 2004)
• ISSN：1520-4995

文摘

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 withfiber cell age. To map the distribution of modified forms of AQP0 within the lens, normal human lensesranging 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 resultingC-terminal peptides of AQP0 were analyzed by HPLC tandem mass spectrometry, permitting theidentification of modifications and estimation of their abundance. Consistent with earlier reports, the majorphosphorylation site was Ser 235, and the major sites of backbone cleavage occurred at residues 246 and259. New findings suggest that cleavage at these sites may be a result of nonenzymatic truncation atasparagine residues. In addition, this approach revealed previously undetected sites of truncation at residues249, 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 modifiedAQP0 within the lens indicated an increase in truncation and racemization/isomerization with fiber cellage, whereas the level of Ser 235 phosphorylation increased from the outer to inner cortex but decreasedin the nucleus. Furthermore, the remarkably similar pattern and distribution of truncation products fromlenses from three donors suggest specific temporal mechanisms for the modification of AQP0.