Insights into the chloroplastic unfolded protein response

CANTOIA, ALEJO; CECCARELLI, EDUARDO A; ROSANO, GERMÁN L

Paraná

Congreso; LIV Reunión Anual de la Sociedad Argentina de Investigación en Bioquímica y Biología Molecular (SAIB); 2018

Sociedad Argentina de Investigación en Bioquímica y Biología Molecular (SAIB)

Proteolysis plays a key role in maintaining cellular proteostasis. It is very important in organelles where protein turnover and replacement arehighly elevated. Chaperones and proteases recognize and remove unnecessary, unfolded or aggregated proteins. In certain situations the qualitycontrol system is overloaded and cells trigger a response to deal with the accumulation of unwanted proteins. This process is called ―unfoldedprotein response‖ (UPR), which is a mechanism that activates signaling pathways leading to increased expression of specific chaperones andproteases, with the aim of restoring proteostasis. The UPR in endoplasmic reticulum and mitochondria has been described, yet there are noreports of a chloroplastic UPR. We propose that the accumulation of misfolded proteins in chloroplasts can trigger a chloroplastic UPR in plants.To test this, we constructed a mutant of ferredoxin NADP+ reductase (FNR) bearing a deletion in Asp289, Trp290 and Ile291 (Taq3-FNR). Thisresulted in a marked decrease in solubility. Taq3-FNR was cloned into a binary vector with a chloroplastic transit peptide. Taq3-FNR wastransiently expressed in Nicotianabenthamiana after Agrobacterium-mediated infiltration. The expression of the chloroplastic chaperone ClpBwas assessed by Western blot. A two-fold increase in ClpB expression was found in leaves infiltrated with Taq3-FNR in comparison with leavesthat express FNR wild type and mock-infiltrated samples. Our results suggest that Taq3-FNR generates a specific response in chloroplasts. AsClpB is a nuclear encoded protein, the response involves chloroplasts-to-nucleus communication, a common feature in other UPRs.