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Abstract
High mobility group nucleosome-binding (HMGN) proteins belong to a family of nuclear proteins that bind to nucleosomes and enhance transcription from chromatin templates by altering the structure of the chromatin fiber. The intranuclear organization of these proteins is dynamic and related to the metabolic state of the cell. Here we report that approximately 50% of the HMGN proteins are organized into macromolecular complexes in a fashion that is similar to that of other nuclear activities that modify the structure of the chromatin fiber. We identify several distinct HMGN-containing complexes that are relatively unstable and find that the inclusion of HMGN in the complexes varies according to the metabolic state of the cell. The nucleosome binding ability of HMGN in the complex is stronger than that of the free HMGN. We suggest that the inclusion of HMGN proteins into metastable multiprotein complexes serves to target the HMGN proteins to specific sites in chromatin and enhances their interaction with nucleosomes.
Highlights
The nucleus of eukaryotic cells contains multiple activities that modify the structure of the chromatin fiber thereby affecting a variety of DNA-related activities such as transcription, replication, recombination, and DNA repair [1,2,3,4,5]
high mobility group nucleosomebinding (HMGN) Proteins Are Incorporated into Large Multiprotein Complexes—In the initial steps to determine whether HMGN1 and HMGN2 proteins are associated with other cellular components in macromolecular complexes, we fractionated HeLa S3 nuclear extracts on Zorbax GF450 or Superose 6 size exclusion columns
Quantitative analysis of the HMGN proteins in the two main chromatographic fractions indicates that in nuclear extracts prepared by extraction with 0.35 M NaCl, about 40% of the HMGN is in high molecular mass fractions
Summary
The nucleus of eukaryotic cells contains multiple activities that modify the structure of the chromatin fiber thereby affecting a variety of DNA-related activities such as transcription, replication, recombination, and DNA repair [1,2,3,4,5]. The proteins associated with the chromatin modifying activities may play a role in targeting them to specific modification sites. A macromolecular HMGN complex was detected in HeLa nuclear extract, suggesting that, like other chromatin modifying activities, HMGN proteins may function in the context of macromolecular complexes [18]. In this respect the mechanism of action of HMGN, and perhaps that of other structural chromatinbinding proteins, may be very similar to that of the histone acetyltransferases or the ATP-dependent chromatin remodeling activities. We suggest that HMGN proteins associate dynamically with multiple mac-
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