Consequences of the loss of catalytic triads in chloroplast CLPPR protease core complexes in vivo.

The essential chloroplast CLP protease system consists of a tetradecameric proteolytic core with catalytic P (P1, 3-6) and non-catalytic R (R1-4) subunits, CLP chaperones and adaptors. The chloroplast CLP complex has a total of ten catalytic sites,but it is not known how many of these catalytic sites can be inactivated before plants lose viability. Here we show that CLPP3 and the catalytically inactive variant CLPP3S164A fully complement the developmental arrest of the null mutant, even under environmental stress. In contrast, whereas the inactive variant CLPP5S193A assembled into the CLP core, it cannot rescue the embryo lethal phenotype of the null mutant. This shows that CLPP3 makes a unique structural contribution but its catalytic site is dispensable, whereas the catalytic activity of CLPP5 is essential. Mass spectrometry of affinity-purified CLP cores of the complemented lines showed highly enriched CLP cores. Other chloroplast proteins were co-purified with the CLP cores and are candidate substrates. A strong overlap of co-purified proteins between the CLP core complexes with active and inactive subunits indicates that CLP cores with reduced number of catalytic sites do not over-accumulate substrates, suggesting that the bottle-neck for degradation is likely substrate recognition and unfolding by CLP adaptors and chaperones, upstream of the CLP core.