Temperature-Induced Protein Conformational Changes in Barley Root Plasma Membrane-Enriched Microsomes

The membrane-bound proteins of barley (Hordeum vulgare L. cv Conquest) root plasma membrane-enriched microsomes displayed fluorescence typical of protein-associated trytophan residues. The protein fluorescence intensity was sensitive to variations in sample temperature. The temperature-induced decline in protein fluorescence intensity was nonlinear with slope discontinuities at about 12 and 32 degrees C. Detergents at levels above their critical micelle concentration enhanced protein fluorescence. Glutaraldehyde reduced protein fluorescence. Protein fluorescence polarization increased at temperatures above 30 degrees C. Both the rate of tryptophan photoionization and the fluorescence intensity of the photoionization products suggested alterations in membrane protein conformation between 12 and 32 degrees C. The quenching of the intrinsic protein fluorescence by acrylamide and potassium iodide indicated changes in accessibility of the extrinsic agents to the protein tryptophan residues beginning at about 14 degrees C. The results indicate thermally induced changes in the dynamics of the membrane proteins over the temperature range of 12 to 32 degrees C which could account for the complex temperature dependence of the barley root plasma membrane ATPase.