THE STABILITY OF TROPOMYOSIN AT ACID PH - EFFECTS OF ANION-BINDING

The alpha-helical coiled-coil tropomyosin homodimer; alpha alpha Tm, u nfolds cooperatively with T-1/2 = 47 degrees C, at neutral pH, 0.5 M N aCl. At pH 2, where each chain contains 55 positive charges, no cooper ative unfolding occurs to at least 80 degrees C. The NaCl and K2SO4 de pendence of thermal unfolding of alpha alpha Tm and a less stable prot ein, beta beta Tm, were studied with circular dichroism. For alpha alp ha Tm, 10 mM NaCl, or 0.5 mM K2SO4, was sufficient to increase the unf olding temperature by 80 degrees C. For beta beta Tm, similar concentr ations of NaCl and K2SO4 as for alpha alpha Tm increased the unfolding temperature of the most stable domain, Titrations indicated that two to three anions bind preferentially to the beta beta Tm intermediate. Thus anions bind to Tm at acid pH values to greatly stabilize the heli x. But even in the absence of added salt, Tm is more stable at pH 2 th an pH 7, suggesting destabilization by negatively charged amino acids. (C) 1998 Academic Press.