Heterogeneous binding of high mobility group chromosomal proteins to nuclei

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Abstract
A dramatic difference is observed in the intracellular distribution of the high mobility group (HMG) proteins when chicken embryo fibroblasts are fractionated into nucleus and cytoplasm by mass enucleation of cytochalasin-B-treated cells, or by differential centrifugation of mechanically disrupted cells. Nuclei (karyoplasts) obtained by cytochalasin B treatment of cells contain > 90% of the HMG 1, while enucleated cytoplasts contain the remainder. A similar distribution between karyoplasts and cytoplasts is observed for the H1 histones, and the nucleosomal core histones as anticipated. The presence of these proteins, in low amounts, in the cytoplast preparation can be accounted for by the small percentage of unenucleated cell present. The nuclei isolated from mechanically disrupted cells contain only 30-40% of the total HMG 1 and 2, the remainder being recovered in the cytosol fraction. No histone is observed in the cytosol fraction. Unlike the higher MW HMG, most of the HMG 14 and 17 sediment with the nuclei after cell lysis by mechanical disruption. The distribution of HMG is unaffected by incubating cells with cytochalasin B and mechanically fractionating, rather than enucleating them. The dramatic difference in HMG 1 distribution observed, using the 2 fractionation techniques, cannot be explained by a cytochalasin-B-induced redistribution. On reextraction and sedimentation of isolated nuclei obtained by mechanical cell disruption, only 8% of the HMG 1 is released to the supernate. The majority of the HMG 1 originally isolated with these nuclei, representing 35% of the total HMG 1, is steadily bound, as is all the HMG 14 and 17. The remaining 65% of the HMG 1 and 2 is unsteadily bound, and leaks to the cytosol fraction under the conditions of mechanical disruption. Apparently the unstably bound HMG form a protein pool capable of equilibrating between cytoplasm and stably found HMG.

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