Purification and partial characterization of ubiquitin‐activating enzyme from Saccharomyces cerevisiae

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Abstract
Ubiquitin‐activating enzyme was purified from the yeast Saccharomyces cerevisiae by covalent affinity chromatography on ubiquitin‐Sepharose followed by HPLC anion‐exchange chromatography. Enzyme activity was monitored by the ubiquitin‐dependent ATP: 32PP1 exchange assay. The purified enzyme has a specific activity of 1.5μmol 32PP1 incorporated into ATP·min−1·mg−1 at 37°C and pH 7.0 under standard conditions for substrate concentrations as described by Ciechanover et al. (1982) J. Biol. Chem. 257, 2537–2542. The catalytic activity showed a maximum at pH 7.0. Its molecular weight both in non‐denaturing and in SDS‐gel electrophoresis was estimated to be 115 kDa, suggesting a monomeric form. The isoelectric point determined by gel electrofocusing was approximately 4.7. Two protein bands differing slightly in electrophoretic mobility could be distinguished when SDS gels were loaded with very small amounts of purified E1 and immunoblotted, the one with higher molecular weight being clearly predominant. The same two bands were also found in anti‐E1 immunoblots of crude yeast lysates prepared under broad protease inhibition.