Uvr motifs regulate the chloroplast Clp chaperone-protease system.

Chloroplast proteostasis relies on diverse proteases, including the essential Clp chaperone-protease system. Two chloroplast ClpC AAA+ chaperones and the plant-specific adaptor ClpF contain an Uvr motif with predicted coiled-coiled structures implicated in protein-protein interactions. Head-to-head contacts between Uvr motifs in middle (M)-domains regulate the oligomerization and activation of several bacterial Clp chaperones. Interestingly, in arabidopsis (Arabidopsis thaliana), this Uvr motif is found in six additional chloroplast proteins (Executer1, Executer2, and Uvr1-4). Here, we first summarize evidence that Uvr motifs regulate proteostasis in bacteria. Based on this evidence and recent results in arabidopsis, we postulate that arabidopsis Uvr motif proteins regulate chloroplast Clp proteolysis. We propose specific working hypotheses to test the function of the Uvr motif in chloroplast proteostasis.