HEAT-INDUCED ALTERATIONS IN THYLAKOID MEMBRANE-PROTEIN COMPOSITION INBARLEY

Biochemical and spectroscopic studies on the effects of high temperatu res (45-47 degrees C) over a 1 h period on the protein composition, fl uorescence and photochemical activities of the barley thylakoid membra ne were made. Photosystem II (PS II) activity decreased as expected, a nd photosystem I (PS I) activity also unexpectedly decreased. Our data support previous conclusions that the decrease in PS I activity is la rgely due to inactivation (or loss) of a component between the two pho tosystems. A two-dimensional electrophoretic system permitted first th e separation of the thylakoid pigment-protein complexes of unstressed and stressed plants, followed by a determination of their subunit comp osition. The changes in the protein composition of each pigment-protei n complex in response to elevated temperatures were monitored. Heat ch anged the quaternary structure of PS II and resulted in removal of the oxygen-evolving enhancer proteins from the thylakoid, but did essenti ally no damage to the PS I complex. The PS II core complex dissociated from a dimeric form to a monomeric one, and the major LHC II componen t (LHC IIb) changed from a trimeric to a monomeric form. The pigments that are lost from thylakoids during heat stress are mainly removed fr om the PS II pigment-proteins.