The chloroplast 22-Ku heat-shock protein: A lumenal protein that associates with the oxygen evolving complex and protects photosystem II during heat stress

An evolutionarily conserved small heat-shock protein localizes to the chloroplast (chlpsHsp) in all major phyla of terrestrial plants examined to date. The chlpsHsp is known to protect photosynthetic electron transport, specifically that of Photosystem II, during heat stress. Suborganellar examination indicated that there are two forms of the chlpsHsp, a 25-Ku and a 22-Ku form. Protease treatment of isolated subchloroplast fractions (thylakoid grana vs, stroma) demonstrated that the 22-Ku chlpsHsp form is found in the thylakoid lumen. Co-immunoprecipitation of the chlpsHsp using a concentration of Triton X-100 below its critical micelle concentration showed that this protein associates with proteins of Photosystem II. Coimmunoprecipitation of the chlpsHsp using a concentration of Triton X-100 significantly above its critical micelle concentration showed that this protein specifically interacts with proteins of the thermolabile Oxygen Evolving Complex of Photosystem II. The chlpsHsp does not reactivate heat-denatured Photosystem II, bur instead protects this complex from damage during heat stress.