14-3-3 proteins control proteolysis of nitrate reductase in spinach leaves.

To test a possible role of 14-3-3 proteins in the degradation of nitrate reductase (NR) in leaves, we monitored 14-3-3s bound to NR in leaf extracts. The amount of 14-3-3s that coimmunoprecipitated with serine 543 phospho-NR (p-NR) increased upon a light/dark transition. This was accompanied by a similar increase in the protein turnover rate of NR in leaves. Purified NR was degraded in extracts from darkened but not from illuminated leaves. Removal of 14-3-3s from such extracts prevented NR degradation. We conclude that the availability of 14-3-3s for p-NR regulates the stability of NR.