Structural Studies of the Non-Histone Chromosomal Proteins HMG-T and H6 from Trout Testis

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Abstract
Trout testis contains 2 proteins of the high mobility group H6 and HMG-T that have been implicated in the structure of active chromatin. Protein H6 was studied by high resolution proton NMR and by circular dichroism and showed no evidence of secondary or tertiary structure formation in free solution. At low ionic strength protein H6 binds to DNA by a weak interaction in the N-terminal region between residues 10 and 35. Protein H6 therefore behaves structurally like the homologous calf-thymus high-mobility-group proteins HMG-14 and HMG-17. Salt addition to solutions of protein HMG-T results in secondary and tertiary structure formation that is accompanied by aggregation. Circular dichroism shows that the helical content of protein HMG-T (.apprxeq. 9%) is very much less than that of the homologous calf thymus proteins HMG-1 and HMG-2. At low ionic strength protein HMG-T precipitates DNA due to the formation of large scale aggregates that disperse only when the protein is released at .apprxeq. 0.35 M NaCl. The NMR spectrum of the aggregated state does not show the presence of a large number of free acidic residues, in contrast to spectra of soluble complexes of HMG-1 and DNA at the same ionic strengths. HMG-T lacks the highly acidic domain found in HMG-1 (and HMG-2) that remains free from DNA under these conditions, and the entire chain of HMG-T binds to DNA. There are therefore major structural differences between HMG-T and the homologous calf thymus proteins.