XVI. B. 5. Extrinsic Proteins of PSII
Four nuclear-encoded proteins with molecular masses of 10, 18, 23, and 33 kDa can be released from PSII particles (for a review see Akerlund, 1993). Treatment with 1 M NaCl releases the 18- and 23-kDa proteins, 1 M CaCl2 releases the 18-and 33-kDa proteins, and 1 M NaCl/0.06% Triton X-100 releases the 18-, 23- and 10-kDa proteins as well as a large part of the 22-kDa protein. These proteins lack prosthetic groups. They appear to be located on the luminal side of the membrane, and are not likely involved in light capture or electron transfer. The 18-, 23- and 33-kDa proteins are found in equal amounts. They are synthesized in the cytoplasm with an N-terminal targeting sequence which is removed by a membrane-bound peptidase upon transfer to the thylakoid lumen.
Both the 18- and 23-kDa proteins are believed to be essential for high affinity binding of Cl- and Ca2+, two ions required for PSII activity.
The 33-kDa protein is believed to be bound to components of the PSII core such as D1 and D2. It also appears to be required for the stabilization of manganese binding, and may provide one ligand to manganese. It is also required for the binding of the 23-kDa protein. It has been suggested that the 18-, 23-, and 33-kDa proteins form a cavity or a regulatory cap around the water-splitting site.
The 10-kDA protein appears to be closely associated with the 23- and 33-kDa proteins. It is rich in hydrophobic amino acids. Beside its suggested involvement in binding the 23-kDa protein to PSII, little is known about its function.
References
- Akelund, H-E (1993). Function and organization of photosystem II. In: Pigment-Protein Complexes In Plastids: Synthesis and Assembly . C. Sundqvist and M. Ryberg, (eds.), p 419-445, Academic Press, New York.