S24-016 Biochemical characterization of two thylakoid lumenal cyclophilins - TLP20 and TLP40 A Andersson1, S Eshaghi2, P Weber3, RG Herrmann3, AV Vener1 and B Andersson1, 2 1 Division of Cell Biology, Linköping University, SE-581 85 Linköping, Swed

Abstract

The complex cyclophilin-like protein TLP40 (Thylakoid Lumen PPIase of 40 kDa) was the first protein with peptidyl-prolyl cis-trans isomerase (PPIase) activity identified in the thylakoid lumen. This unconventional PPIase has a cyclophilin domain and an additional domain that can participate in interactions with other proteins. It has been shown that TLP40 regulates a PP2A-type phosphatase involved in dephosphorylation of the photosystem II core proteins. The phosphatase activity is up-regulated when TLP40 is released from the inner surface of the membranes into the thylakoid lumen. To obtain more insights into this regulatory process we have developed an assay where we were able to study the reversible interactions between TLP40 and inside-out vesicles from thylakoid membranes under various conditions. During purification of TLP40 from spinach thylakoids we identified an additional PPIase with a conventional size of ~20 kDa (TLP20). The PPIase activity of TLP20 in the thylakoid lumen is higher than that of TLP40. Addition of the immunosuppressive drugs Cyclosporin A (CsA) and FK506 to a highly enriched fraction of TLP20 shows inhibition of the PPIase activity with low concentrations (nM) of CsA, but no inhibition when FK506 is added. This indicates that TLP20 belongs to the cyclophilin family of PPIases. We have found a similar protein in the Arabidopsis thaliana database. This protein contains an N-terminal signal peptide, targeted to the chloroplast, followed by a cyclophilin-like PPIase-domain. In conclusion, we have characterised two distinct PPIases in the thylakoid lumen. The soluble conventionally sized TLP20 with high PPIase activity and TLP40, a multidomain protein that reversibly interacts with the inner surface of thylakoid membranes.