Characterisation and functional analysis of a lumenal proline isomerase from photosynthetic membranes of higher plants and cyanobacteria

TLP40 is the first complex immunophilin which was described from
plant chloroplasts (Fulgosi et al., 1998). For this protein a role
in regulation of PSII protein phosphorylation has been suggested
(Fulgosi et al., 1998; Vener et al., 1999; Rokka et al., 2000).
Homologous proteins were found in Arabidopsis thaliana and in
Synechocystis. In Synechocystis, different from higher plants, the
relevant PSII proteins are not phosphorylated. The main topic of
this work was therefore to characterise the homologous protein of
TLP40 in Synechocystis (named cTLP40, cyanobacterial TLP40). The
investigation shows: • cTLP40 contains the major structural domains
of TLP40 both at the N- and at the C-termini. A higher homology is
found in the immunophilin domain located at the C-terminal end. •
As in chloroplasts, cTLP40 is also located in the thylakoid lumen
where it is present in free form or associated to the membrane. • A
Synechocystis strain lacking cTLP40 ( ∆sll0408) could grow
photoautotrophycally under normal grown conditions in a way
comparable to wild-type. However, after adaptation to strong light
the mutant strain showed higher photosensibility. Under these
conditions, there was a decrease in oxygen evolution. • The total
amount of PSII dimer was reduced in the mutant under high light.
The lower amount of PSII can be attributed to a slower assembly
rate of the complex and/or to higher degradation rate. Protein
synthesis was not impaired under any of the tested conditions. •
The PPIAse activity of cTLP40 was tested in vitro on synthetic
prolinecontaining peptides of PSII proteins which are exposed to
the lumenal face in thylakoids. The in vitro assays showed that
cTLP40 possesses PPIAse activity on control peptides but can not
efficiently isomerise the specific synthetic peptides.