The activity of the 20S proteasome is maintained in detached wheat leaves during senescence in darkness
2002
Roberts, I. ((Universidad de Buenos Aires (Argentine). IBYF-CONICET, Facultad de Agronomia)) | Fernandez Murray, P. | Passeron, S. | Barneix, A.
In the present paper, we studied the participation of the 20S proteasome, the proteolytic component of the ubiquitin-proteasome pathway, in the remobilization of bulk proteins in senescing wheat leaves. The detached leaves of 15-d-old plants were incubated in darkness for several days, and various proteolytic activities were analysed in soluble extracts prepared at 0, 48 and 96 h after detachment. The endoproteolytic activity, measured at pH 7.5 and 5.4, increased more than 10-fold and the total peptidasic activity increased up to 5-fold after 96 h of incubation in the dark, when expressed as specific activity. In the same period, the leaf-protein content decreased to less than 50/ of that present at the initial time. The 20S proteasome chymotrypsin-like activity remained constant when it was expressed as activity per leaf fresh weight and resulted 2-fold higher in terms of specific activity. The western blot analysis showed that the amount of 20S proteasome protein and ubiquitin-protein conjugates also remained constant until 4 d of incubation in darkness. These results indicate that the ubiquitin-proteasome pathway remains functional until the late phases of senescence suggesting that it may participate in the regulatory aspects of the process rather than in the massive protein breakdown
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