II. Functional investigation of the chloroplast HSP70/HSP90 chaperones.

Rationale

Chaperones are a specialized class of proteins, whose most well-known function is to help other proteins to assume and maintain the native folding state. In addition, chaperones participate in numerous other activities like the translocation of proteins across biomembranes, protein complex assembly/disassembly, maturation of signal transduction components, tagging of unfoldable proteins for degradation, etc. While chaperones have been extensively studied in bacteria, mitochondria, the ER, and the eukaryotic cytosol, comparably little is known about their activities in chloroplasts. This is surprising, since chloroplasts harbour a unique compartment which is the ultimate energy source for almost all life on earth - the thylakoid membranes. Moreover, in light of the growing importance of the chloroplast as the compartment of choice for the expression of recombinant proteins an understanding of its protein folding machinery is indispensable.

Aim

To biochemically characterize the chloroplast chaperone networks and to elucidate their roles in chloroplast biogenesis and maintenance of chloroplast functions.

Current State and Focus

We have identified and biochemically characterized many components of the chloroplast HSP70 and HSP90 chaperone systems in Chlamydomonas (HSP70B, HSP90C, CGE1, HEP2, CDJ1-5). Moreover, we have gained insights into the regulation of HSP70B’s chaperone activity and identified the vesicle inducing protein in plastids 1 (VIPP1) as a main substrate of the chloroplast HSP70/90 systems. We have revealed an important function of VIPP1 and the chaperones in the biogenesis of thylakoid membrane protein complexes and have a strong focus on elucidating the underlying mechanisms.