Conjugation of ubiquitin to proteins from green plant tissues

Conjugation of the polypeptide ubiquitin to endogenous proteins was studied in oat (Avena sativa L.) plants, and particularly in green tissues. Conjugating activity in leaf extracts was different from that in root extracts, and in both was less than in etiolated tissue. The conjugates were identified by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and their formation was both time- and ATP-dependent and had a pH optimum of about 8.2. The assay had a high affinity for ATP with a probable K_m of less than 50 micromolar. The ubiquitin conjugating system was also shown to be present in isolated chloroplasts, and ubiquitin could be conjugated to endogenous proteins of lyzed chloroplasts in which the ATP concentrations were reduced by preincubation or desalting. SDS-PAGE analysis led to the suggestion that the large and small subunits of ribulose-1,5-bisphosphate carboxylase (RuBPCase) may be able to be ubiquitinated, and we have shown that ubiquitin can stimulate the in vitro breakdown of ¹²⁵I-labeled RuBPCase. These results invite the speculation that ubiquitin may be involved in the regulation of protein turnover in green plants.