Structure and activity of JAC1 J-domain implicate the involvement of the cochaperone activity with HSC70 in chloroplast photorelocation movement

Novel Role of JAC1 in Influencing Photosynthesis, Stomatal Conductance, and Photooxidative Stress Signalling Pathway in Arabidopsis thaliana

Regulation of light absorption under variable light conditions is essential to optimize photosynthetic and acclimatory processes in plants. Light energy absorbed in excess has a damaging effect on chloroplasts and can lead to cell death. Therefore, plants have evolved protective mechanisms against excess excitation energy that include chloroplast accumulation and avoidance responses. One of the proteins involved in facilitating chloroplast movements in Arabidopsis thaliana is the J domain-containing protein required for chloroplast accumulation response 1 (JAC1). The function of JAC1 relates to the chloroplast actin filaments appearance and disappearance. So far, the role of JAC1 was studied mainly in terms of chloroplasts photorelocation. Here, we demonstrate that the function of JAC1 is more complex, since it influences the composition of photosynthetic pigments, the efficiency of photosynthesis, and the CO₂ uptake rate. JAC1 has positive effect on water use efficiency (WUE) by reducing stomatal aperture and water vapor conductance. Importantly, we show that the stomatal aperture regulation is genetically coupled with JAC1 activity. In addition, our data demonstrate that JAC1 is involved in the fine-tuning of H₂O₂ foliar levels, antioxidant enzymes activities and cell death after UV-C photooxidative stress. This work uncovers a novel function for JAC1 in affecting photosynthesis, CO₂ uptake, and photooxidative stress responses.

Functional relevance of J-protein family of rice (Oryza sativa)

Protein folding and disaggregation are crucial processes for survival of cells under unfavorable conditions. A network of molecular chaperones supports these processes. Collaborative action of Hsp70 and Hsp100 proteins is an important component of this network. J-proteins/DnaJ members as co-chaperones assist Hsp70. As against 22 DnaJ sequences noted in yeast, rice genome contains 104 J-genes. Rice J-genes were systematically classified into type A (12 sequences), type B (9 sequences), and type C (83 sequences) classes and a scheme of nomenclature of these proteins is proposed. Transcript expression profiles revealed that J-proteins are possibly involved in basal cellular activities, developmental programs, and in stress. Ydj1 is the most abundant J-protein in yeast. Ydj1 deleted yeast cells are nonviable at 37 °C. Two rice ortholog proteins of yeast Ydj1 protein namely OsDjA4 and OsDjA5 successfully rescued the growth defect in mutant yeast. As Hsp70 and J-proteins work in conjunction, it emerges that rice J-proteins can partner with yeast Hsp70 proteins in functioning. It is thus shown that J-protein machine is highly conserved.