Conformation and Domain Structure of the Non-histone Chromosomal Proteins, HMG 1 and 2. Isolation of Two Folded Fragments from HMG 1 and 2

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Abstract
Proteins HMG 1 and 2 [calf] were digested with trypsin and 2 major products, stable to further digestion between 8 min and 2 h, were purified (peptides A and B). Peptide B from HMG 1 was identified as residues 12-75 and peptide A as residues 94 96-169 by amino acid analyses and Edman degradations. Peptide B spontaneously folds with the formation of 51% helix and exhibits the majority of the perturbed NMR resonances characteristic of folded intact HMG 1. Peptide B is stably folded in the presence of 150 mM NaCl between pH 3 and 10, like intact MHG 1. Peptide A forms 30% .alpha.-helix and also exhibits tertiary folding but is denatured by pH 10. The 11 N-terminal residues removed by trypsin contain both sites of post-synthetic acetylation (residues 2 and 11), a situation very similar to that found with core histones. HMG 1 and 2 may consist of 4 structural domains, viz: residues 1-11, residues 12 to approximately 75, residues 94-169 and the very acidic region beyond residue 169. The instability of peptide A may mean that it is not a truly independent domain. No structural similarities to histone H1 are therefore observed in HMG 1 and 2.