Parathymosin Affects the Binding of Linker Histone H1 to Nucleosomes and Remodels Chromatin Structure

Abstract

Linker histone H1 is the major factor that stabilizes higher order chromatin structure and modulates the action of chromatin-remodeling enzymes. We have previously shown that parathymosin, an acidic, nuclear protein binds to histone H1 in vitro and in vivo. Confocal laser scanning microscopy reveals a nuclear punctuate staining of the endogenous protein in interphase cells, which is excluded from dense heterochromatic regions. Using an in vitro chromatin reconstitution system under physiological conditions, we show here that parathymosin (ParaT) inhibits the binding of H1 to chromatin in a dose-dependent manner. Consistent with these findings, H1-containing chromatin assembled in the presence of ParaT has reduced nucleosome spacing. These observations suggest that interaction of the two proteins might result in a conformational change of H1. Fluorescence spectroscopy and circular dichroism-based measurements on mixtures of H1 and ParaT confirm this hypothesis. Human sperm nuclei challenged with ParaT become highly decondensed, whereas overexpression of green fluorescent protein-or FLAG-tagged protein in HeLa cells induces global chromatin decondensation and increases the accessibility of chromatin to micrococcal nuclease digestion. Our data suggest a role of parathymosin in the remodeling of higher order chromatin structure through modulation of H1 interaction with nucleosomes and point to its involvement in chromatindependent functions.Eukaryotic DNA is packaged into chromatin (1), a structural framework that plays a key role in the regulation of the transcriptional status of genes and genetic loci (2-4). The basic level of chromatin organization is the nucleosome that consists of DNA wrapped around an octamer of histones (5). Linker histones (e.g. H1 and H5) seal the entry/exit of DNA and stabilize the folding of 10-nm nucleosomal arrays into higher order chromatin structures (6, 7). Binding of histone H1 to nucleosomal arrays affects the spacing of nucleosomes (8), restricts their mobility (9), and reduces the transient exposure of DNA on the nucleosome surface (10). Consistent with its structural role as a major determinant of chromatin folding, histone H1 was found to inhibit the action of enzymes that enhance the accessibility of DNA, such as histone acetyltransferases and ATP-dependent remodeling complexes. For example, changes in linker histone stoichiometry modulate the levels of core histone acetylation (11) and regulate the activity of all classes of ATP-dependent remodeling enzymes (12, 13). Collectively, these findings support the notion that H1 "locks down" chromatin regions and acts as a general inhibitor of transcription (14, 15). However, genetic studies have shown that, rather than being global repressors of transcription, linker histones affect the expression of specific genes (16). Multiple isoforms of linker histone H1 exist in mammalian somatic cells (17) and play differential roles in gene expression (18). Recently, two independent studies highlight the important role of H1 subtypes in dev...

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