Role of HSP90 Family Molecular Chaperone in Regulating Chloroplast Protein Homeostasis and Plant Development in Arabidopsis

Abstract

Chloroplasts carry out photosynthesis in plants and provide most food sources for living organisms. Either divided or differentiated from proplastids and etioplasts, the biogenesis of thylakoid membranes is the hallmark of chloroplast maturation and functionality, and this process requires a large set of photosynthetic proteins that must be synthesized on cytosolic ribosomes and then imported into the chloroplast stroma. Thylakoid lumen proteins travel further through the stroma and then cross the thylakoid membrane through one of the ancestral translocases. Like the cytosolic environment and the other major organelles, a cohort of molecular chaperones and proteases reside within the chloroplast to ensure proper protein homeostasis. We have identified and confirmed the interaction between the essential plastid HSP90 family heat shock protein HSP90C and a thylakoid lumen located photosystem II subunit PsbO1. To understand the biological significance of the interaction, we adopted transgenic approaches, specific HSP90 inhibitors, biochemical and in organello protein transport assays to study how the HSP90 family protein facilitates the representative lumen proteins transport across the thylakoid membrane. Specifically, we showed that the expression of the molecular chaperone is developmentally regulated, and its expression is highly correlated with some photosynthetic proteins while not with the others. Additionally, we demonstrated that HSP90C physically interacts with the thylakoid SEC translocon subunit, and that its chaperone activity is required for efficient PsbO1 transport. We also investigated the signature sequences of different HSP90 subfamily proteins and provided evidence that the chloroplast located HSP90C contains features specialized for some photosynthetic proteins interaction, and thus the chloroplast maturation and functionality.