The Central Tryptic Fragment of Histones H1 and H5 Is a Fully Compacted Domain and Is the Only Folded Region in the Polypeptide Chain

Abstract

Scanning microcalorimetry was used to show that the globular structure in histones H1 [calf thymus] and H5 [chicken erythrocyte] melts reversibly over a relatively narrow temperature range as a single domain with enthalpy of melting very close to the van''t Hoff enthalpy. These proteins therefore exhibit macroscopic cooperativity like that of many well-studied small proteins. For histone H5 the observed melting temperature and melting enthalpy coincide with those previously obtained using intrinsic Cotton effects in the circular dichroism spectrum, but not with those from extrinsic Cotton effects or NMR chemical shifts. Calorimetric measurements on a central tryptic fragment from histones H1 and H5 show these fragments to have the same molar melting enthalpy as the parent histones. The folded structures of H1 and H5 are therefore located only within these fragments. For histone H1 this conclusion is supported by the finding that the C-terminal fragment (residues 122-213) shows no peak in the specific heat curve.