Title : Dynamic O-GlcNAcylation of the small heat shock protein alpha alpha B-crystallin
Abstract :
alphaB-Crystallin , originally described as a structural lens protein , is now known to be a member of the small heat shock protein family and is expressed in a number of nonlens tissues
This highly conserved 20 kDa protein aggregates with homologous proteins , including alphaA-crystallin and the small heat shock protein HSP28 , to form large heteromeric complexes
Recently, Roquemore et al. (1992) have established that both phosphorylated and unphosphorylated forms of lens alphaB-crystallin are modified with O-linked N-acetylglucosamine, a dynamic posttranslational modification abundant on nuclear and cytoplasmic proteins
In this paper, we have identified the major site of O-GlcNAcylation on lens alphaB as Thr 170
We have further shown that this modification is not restricted to lens alphaB-crystallin but occurs on alphaB isolated from rat heart tissue and human astroglioma cells
Two-dimensional electrophoresis of rat heart alphaB-crystallin revealed two O-GlcNAcylated forms with mobilities corresponding to the unphosphorylated form (alphaB2) and an unidentified, slightly more acidic form
Phosphorylated alphaB-crystallin ( alphaB1 ) was not detected in the rat heart preparation
The major O-GlcNAcylation site on alphaB-crystallins from rat heart also appears to be at Thr 170
Metabolic pulse-chase labeling studies of U373-MG astroglioma cells indicated that turnover of the carbohydrate on alphaB-crystallin is not static but proceeds many-fold more rapidly than turnover of the protein backbone itself, consistent with a regulatory role for O-GlcNAc on this small heat shock protein