Study on polypeptides of cyanobacterial photosystem 2

Abstract

The main objective of this work was to characterize cyanobacterial PS2 polypeptides with particular respect to calcium binding. A number of cyanobacteria were studied by trypsin digestion of their thylakoid membranes which showed
that using the same method of preparation some cyanobacteria produce everted thylakoid vesicles whilst others produce
right sided vesicles. No direct correlation between the sidedness of the produced thylakoids and any growth or
metabolic characteristic of the organithm was found. In Synechocystis 6308 a polypeptide of 27kDa , apparently unrelated to known polypeptides of the oxygen evolving complex, was shown to disappear on trypsin treatment, a disappearance which correlated to the loss of oxygen evolving activity.
Calcium binding studies performed on Synechococcus 7942 showed the presence of a calmodulin-like calcium binding
site within both thylakoids and PS2. The protein on which this site resided was not proven but results pointed to the
involvement of the Dl protein. Synechococcus 7942 produces two forms of the Dl protein and mutants in which one form
was deleted showed differing affinities for calcium both in vivo (during growth) and in vitro (during reactivation experiments). Differences were also noted for spectra from the mutant strains with one form appearing to retain greater quantities of phycocyanin on the thylakoid membranes.
Protein kinase assays performed on the Dl mutants also showed differences between these and the wild type organism.
These differences were noted for small polypeptides of 8, 10 and 12kDa. A phosphoprotein of 14kDa was shown to be related to the psb H gene product and a phosphoprotein of approximately 32kDa, possibly Dl, was also obtained.A
further 58kDa phosphoprotein which may be related to the calmodulin site or to the protein kinase responsible for PS2
phosphorylation was noted. Both calcium and magnesium were shown to have specific effects on phosphorylation primarily
with respect to the small polypeptides mentioned above.
Studies on the possible glycosylation of P52 and thylakoid polypeptides produced evidence to suggest that either the Dl
or D2 polypeptides are glycosylated and that this may provide a mechanism by which the PS2 complex is held together. Evidence in support of glycosylation of phycobilisome components was also produced with polypeptides
of 34.5, 45 78 and 90kDa appearing to be glycosylated.
A polypeptide of 50kDa which is closely associated with PS2 was shown to be related , in terms of homology, to a
20kDa DCCD-binding protein of plants which is thought to be a LHC protein.