TerC – an essential protein for the integration of CP43 into the thylakoid membrane and its assembly into photosystem II

I Summary During evolution, most genes of the cyanobacterial ancestor of plastids were transferred to the nuclear genome of the host cell (endosymbiosis). Consequently, to maintain the physiological properties of plastids, new regulatory elements and protein import machineries have evolved. For proper assembly of the photosystem II complex, whose core proteins are still plastid encoded, several nuclear encoded assembly factors like LPA1, LPA2, LPA3, PAM68 and ALB3 are required. In Arabidopsis thaliana an albinotic and hence seedling lethal mutant was described in previous publications. Thylakoids of terc-1 were devoid of subunits of photosystem II and therefore a crucial role for TerC in the biogenesis of thylakoids was suggested. During this thesis, the function of TerC during the assembly of photosystem II was characterised in-depth. Downregulation of AtTerC transcripts induced by artificial micro RNA (amiR-TerC plants) leads predominantly to a reduction of the photosynthetic performance of photosystem II and to a decreased accumulation of respective proteins. Protein-protein interaction and co-localisation experiments reveal that TerC specifically interacts with subunits of photosystem II, as well as with photosystem II assembly factors. During the stepwise assembly of photosystem II, the insertion of CP43 (Chlorophyll binding protein of 43 kDa) into the CP43 free complex controls the PSII core monomerisation. The function of TerC is attributed to this step based on two dimensional gel analysis and in vivo labelling experiments of plastidic proteins. In both experiments a reduced amount of unassembled CP43 protein and a block before photosystem II core monomerisation is observed. Taken together, a function of TerC in both, the integration of CP43 into the thylakoid membrane and the assembly of CP43 into the photosystem II, is proposed in a two step model: During the first step, TerC, LPA2 and LPA3 together with ALB3 integrate the unfolded form of the CP43 protein into the thylakoid membrane. In a second step, this complex consisting of TerC, LPA2, LPA3 and CP43 joins the CP43-free photosystem II complex and, mediated by interaction of TerC with photosystem II subunits, LPA1 and PAM68 the assembly of monomeric photosystem II is completed.Komplexes ist abgeschlossen.