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 chlor oplast (chlpsHsp) in all major phyla of terrestrial plants examined to date . The chlpsHsp is known to protect photosynthetic electron transport, speci fically that of Photosystem II, during heat stress. Suborganellar examinati on indicated that there are two forms of the chlpsHsp, a 25-Ku and a 22-Ku form. Protease treatment of isolated subchloroplast fractions (thylakoid gr ana vs, stroma) demonstrated that the 22-Ku chlpsHsp form is found in the t hylakoid lumen. Co-immunoprecipitation of the chlpsHsp using a concentratio n 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 i ts critical micelle concentration showed that this protein specifically int eracts with proteins of the thermolabile Oxygen Evolving Complex of Photosy stem II. The chlpsHsp does not reactivate heat-denatured Photosystem II, bu r instead protects this complex from damage during heat stress.