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TR Shearer, H Ma, M Azuma; Phosphorylation of Lens-Specific Calpain Lp82: Possible Regulatory Mechamism? . Invest. Ophthalmol. Vis. Sci. 2002;43(13):455.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Lp82 is a lens-specific member of the calpain family (EC 34.22.17) of non-lysosomal cysteine proteases. Lp82 is of particular interest because the enzyme is stable and probably more active than ubiquitous m-calpain in young rodent models of cataract. Lp82 is also less sensitive to endogenous calpain inhibitor calpastatin than m-calpain. The half-maximal calcium-activation requirement for Lp82 at 30 µM is more than six times lower than m-calpain. However, this level of calcium is still much higher than normal physiological levels of calcium in lens fibers, and the activation mechanism for Lp82 in normal lens remains unknown. Recent data suggest that phosphorylation activates m-calpain (D. Goll, unpublished), but no similar data exist for of Lp82. Thus, the purpose of present experiment was to compare phosphorylation in Lp82 to m-calpain. Methods: Lp82 and m-calpain were partially purified from 12 day old rat lens by DEAE-HPLC. Immunoblotting was performed with antibodies against p-Ser and p-Thr (Zymed Laboratories) and with chemiluminescent immunodetection (WesternBreeze, Invitrogen). Results: p-Thr and p-Ser immunostaining were detected on partially purified Lp82 from the lens cortex and nucleus. In the nucleus, phosphorylation levels on Lp82 appeared qualitatively at least as high or higher than the positive control m-calpain. Conclusions: Phosphorylation of Lp82 may be a regulatory mechanism for control of Lp82 in lens. Future studies will be directed towards determining if phosphorylation lowers the calcium-activation requirement and if Lp82 phosphorylation levels change with lens maturation and cataract formation.
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