Nuclear proteasomes carry a constitutive posttranslational modification which derails SDS-PAGE (but not CTAB-PAGE).
Pitcher DS., de Mattos-Shipley K., Wang Z., Tzortzis K., Goudevenou K., Flynn H., Bohn G., Rahemtulla A., Roberts I., Snijders AP., Karadimitris A., Kleijnen MF.
We report that subunits of human nuclear proteasomes carry a previously unrecognised, constitutive posttranslational modification. Subunits with this modification are not visualised by SDS-PAGE, which is used in almost all denaturing protein gel electrophoresis. In contrast, CTAB-PAGE readily visualises such modified subunits. Thus, under most experimental conditions, with identical samples, SDS-PAGE yielded gel electrophoresis patterns for subunits of nuclear proteasomes which were misleading and strikingly different from those obtained with CTAB-PAGE. Initial analysis indicates a novel modification of a high negative charge with some similarity to polyADP-ribose, possibly explaining compatibility with (positively-charged) CTAB-PAGE but not (negatively-charged) SDS-PAGE and providing a mechanism for how nuclear proteasomes may interact with chromatin, DNA and other nuclear components.