Manganese stabilizing protein of photosystem II is a thermostable, natively unfolded polypeptide

The thermostability of manganese stabilizing protein of photosystem II was examined by biochemical and spectroscopic techniques. Samples of both nativ e and recombinant spinach manganese stabilizing protein incubated at 90 deg rees C and then cooled to 25 degrees C were capable of rebinding to, and of reactivating, the O-2-evolution activity of photosystem II membranes from which the native protein had been removed. Far-UV circular dichroism and FT -IR spectroscopies were used to analyze the structural consequences of heat ing manganese stabilizing protein. The data obtained from these techniques show that heating causes a complete loss of the protein's secondary structu re, and that this is a reversible, noncooperative phenomenon. Upon cooling, the secondary structures of the heat-treated proteins return to a state si milar to, but not identical with, that of the native, unheated controls. Re storation of a near-native tertiary structure is confirmed both by size-exc lusion chromatography and by near-UV circular dichroism. The functional and structural thermostability of manganese stabilizing protein reported here, in conjunction with additional known properties of this protein (acidic pI , high random coil and turn content, anomalous hydrodynamic behavior), iden tifies manganese stabilizing protein as a natively unfolded protein [Weinre b et al. (1996) Biochemistry 35, 13709-13715]. Although these proteins lack amino acid sequence identity, their functional solution conformations unde r physiological conditions are said to be "natively unfolded". We suggest t hat, as with other members of this family of proteins, the natively unfolde d structure of manganese stabilizing protein facilitates the highly effecti ve protein-protein interactions that are necessary for its assembly into ph otosystem II.