SPECIFIC INTERACTION BETWEEN H1-HISTONE AND HIGH-MOBILITY PROTEIN HMG1

High mobility group proteins HMG1 and -2 and histone H1 are structural components of chromatin. Previously, we reported that HMG1 interacts with H1 histone in a way that modulates the ability of H1 to condense DNA in vitro, suggesting that these proteins may act together in vivo to regulate locally the condensation state of chromatin, possibly affe cting replication and/or transcription. Here we show that reduced (nat ive) HMG1 binds to H1 cooperatively at pH 6.0 as a tetramer with a dis sociation constant of 3.4 x 10(-8) M, and at pH 7.5 as a monomer with a dissociation constant less than 10(-9) M. Denaturation through oxida tion of sulfhydryl groups has a strong effect on the interaction of HM G1 with Hl histone, suggesting that the reduced state of HMG1 is criti cal to its function. Oxidized HMG1 failed to bind H1 at pH 7.5, and it s binding al pH 6 was biphasic; the first three (or two) molecules of H1 were bound with a dissociation constant of 2 x 10(-8) M With negati ve cooperativity, and the last one (or two) H1's were bound cooperativ ely with K(D) = 1.8 X 10(-7) M. Regulation of the pH or the concentrat ion of some other ion may be used in vivo to alter the interactions be tween HMG1 and -2, H1 histone, and DNA.