Function of clpF in the clp protease complex

The plant plastid Clp machinery comprises a hetero-oligomeric ClpPRT proteolytic core, ATP-dependent ClpC,D chaperones and an adaptor protein, ClpS1. ClpS1 directs a subset of substrates to the Clp protease-chaperone complex for degradation, but substrate recognition and delivery mechanisms are unknown. Here we describe ClpF, a novel chloroplast adaptor. ClpF is only found in photosynthetic eukaryotes and contains uvr/C and YccV domains and an N-terminal domain conserved across ClpF homologs. We demonstrate that ClpF acts together with ClpS1 in substrate delivery to plastid ClpC chaperones. The molecular domain interactions of ClpF to ClpS1 and the ClpC chaperones were mapped, and in vivo interactions between ClpF and the Clp substrate glutamate t-RNA reductase (GluTR1) are demonstrated. Quantitative proteomics identified subtle molecular phenotype in a ClpF null mutant and we show that GluTR1 degradation is delayed in ClpF and ClpS1 Arabidopsis null mutants.