CHARACTERISTICS OF PROTEIN-TURNOVER IN THE DEVELOPING FIRST LEAF OF OATS (AVENA-SATIVA L)

The primary leaves of 7- and 15-day-old oat plants were labelled with S-35-methionine and the in viva degradation of the synthesized protein s was followed by two-dimensional polyacrylamide gel electrophoresis o ver a period of 2 weeks. During both leaf development and senescence p rotein breakdown comprised a rather quick turnover of a large number o f proteins and a slow decline of a smaller number of persisting spots, many of which appeared to be chloroplast-associated. This pattern:of breakdown was compared with that in vitro in soluble-protein extracts at pHs 5.5 and 7.5, the optimal pH values of the acidic and neutral pr oteases, respectively Although 75% of the proteins were degraded simil arly in vivo and in vitro, substantial differences in the disappearanc e of thr other proteins and their relative rates of degradation occurr ed under the different conditions. Notably, ribulosebisphosphate carbo xylase (Rubisco) was degraded very slowly in vivo, but rather quickly and yielding different degradation products in vitro at pH 5.5. In viv a protein degradation more closely resembled in vitro breakdown pH 7.5 . These results suggest that the vacuole has only a Limited role in en dogenous protein turnover and that several mechanisms are simultaneous ly active in protein degradation in oat leaves.