STRUCTURE AND STABILITY OF HISTONE HMF FROM THE HYPERTHERMOPHILIC ARCHAEON METHANOTHERMUS-FERVIDUS

The secondary and quaternary structures and stabilities of recombinant (r) forms of the HMfA and HMfB histones from Methanothermus fervidus have been investigated by CD spectroscopy and formaldehyde-mediated protein-protein cross-linking. Both proteins were shown to be dimers in solutions containing 5-1300 mM KCl, at pH 6-10 and 25-83 degrees C, and specifically in 1 M KCl, at pH 7.5 and 83 degrees C, conditions which approximate those in vivo in M. fervidus cells. Heat treatment of a mixture of rHMfA and rHMfB homodimers resulted in the formation of rHMfA . HMfB heterodimers, as demonstrated by two-dimensional PAGE. Heterodimer formation did not result in a CD-detectable conformational change from the homodimer states, indicating that homogeneous (rHMfA)(2) and (rHMfB)(2) preparations may be considered as structural models of heterodimers. At pH 2, both rHMfA and rHMfB were denatured under low-salt (<0.2 M KCl) conditions, and their conformations were stabilized in a cooperative manner by increasing KCl concentration, with cooperativity constants for KCI uptake of 2.7 and 3.1, respectively. The alpha-helical conformations of rHMfA and rHMfB were salt-dependent, at both pH 2 and pH 7.5, with maximal helicities in 1 M KCl of 84% and 63% at pH 2, and 72% and 65% at pH 7.5, respectively. The data obtained indicate that the structures of HMfA and HMfB, in 100-200 mM KCl at pH 7.5 and 25 degrees C, are likely to be very similar to their in vivo structures, even though these conditions are far removed from those found in vivo. Correlation of engineered primary sequence changes with their effects on the structures and stabilities of rHMfA and rHMfB established here should therefore be relevant to the in vivo situation.