Temperature-Induced Protein Conformational Changes in Barley Root Plasma Membrane-Enriched Microsomes: III. Effect of Temperature and Cations on Protein Sulfhydryl Reactivity.

Temperature and cations modified the reaction of barley (Hordeum vulgare L. cv Conguest) root plasma membrane protein sulfhydryl groups with N-4-(7-diethylamino-4-methylcoumarin-3-yl)-phenyl maleimide (CPM). The pseudo-first-order rate constants for the formation of fluorescent CPM-protein adducts increased as the temperature was raised above 30 degrees C, suggesting changes in protein conformation. Monovalent [K(I), Na(I), L(I)] and certain divalent cations [Ba(II), Mg(II)] increased the reaction rates. Other divalent cations [Ca(II), Mn(II), Ni(II), Co(II), Sr(II), Cd(II), Hg(II)] decreased the rate of fluorescent adduct formation. Na(I) promoted and Ca(II) delayed the onset of the temperature-dependent increases in reaction rates. The results are discussed in terms of lipid-mediated, temperature-dependent changes in membrane protein conformation and ion-protein interactions.