Plant protein quality control

Adverse environmental conditions including high temperatures, cold, drought, and salinity cause protein stress and aggregation in plants as well as in other model organisms. Recycling of damaged proteins is achieved by the action of molecular chaperones but, when recycling is not possible, toxic aggregated proteins have to be degraded by the action of proteases to avoid cellular damage. Chaperones and proteases act coordinately and constitute protein quality control (PQC) systems that are required for organismal survival. In chloroplasts, similar mechanisms ruled by the chaperone HSP70 survey for maintaining protein homeostasis (proteostasis).

In our group, we have unveiled a novel synergistic collaboration between the chaperone HSP70 and the protease CLP in preventing the accumulation of protein aggregates. In these synergistic interaction DNAJ proteins are key factors that provide specificity to the system, adaptors that recognise unfolded substrates and transfer them to the HSP70 for refolding, often in cooperation with CLPB3. However, when refolding is not possible, the DNAJ/HSP70 system transfer substrates to CLPC1, a subunit of the CLP protease for degradation. The accumulation of protein aggregates unleashes a chloroplast-to-nucleus retrograde signalling pathway that specifically activates the transcription of chaperones at the nucleus, the chloroplast unfolded protein response (cpUPR).

In our goal to design new strategies to engineering plant stress tolerance, we are currently engaged in the study of three interconnected objectives::

– To identify novel substrates of DNAJ proteins in chloroplasts.

– To untangle the specificity of the different CLPC adaptors.

– To unveil components of the cpUPR signalling pathway.