A procedure for the preparation of chromatin and chromatin proteins from cultured mammalian epithelial cells

Expression and purification of recombinant human histones

Nucleosomes reconstituted from bacterially expressed histones are useful for functional and structural analyses of histone variants, histone mutants, and histone post-translational modifications. In the present study, we developed a new method for the expression and purification of recombinant human histones. The human histone H2A, H2B, and H3 genes were expressed well in Escherichia coli cells, but the human histone H4 gene was poorly expressed. Therefore, we designed a new histone H4 gene with codons optimized for the E. coli expression system and constructed the H4 gene by chemically synthesized oligodeoxyribonucleotides. The recombinant human histones were expressed as hexahistidine-tagged proteins and were purified by one-step chromatography with nickel-nitrilotriacetic acid agarose in the presence of 6 M urea. The H2A/H2B dimer and the H3/H4 tetramer were refolded by dialysis against buffer without urea, and the hexahistidine-tags of the histones in the H2A/H2B dimer and the H3/H4 tetramer were removed by thrombin protease digestion. The H2A/H2B dimer and the H3/H4 tetramer obtained by this method were confirmed to be proficient in nucleosome formation by the salt dialysis method. The human CENP-A gene, the centromere-specific histone H3 variant, contains 28 minor codons for E. coli. A new CENP-A gene optimized for the E. coli expression system was also constructed, and we found that the purified recombinant CENP-A protein formed a nucleosome-like structure with histones H2A, H2B, and H4.

Association of platelet-derived growth factor-induced protein with nuclear material

Platelet-derived growth factor (PDGF) has been proposed to initiate the cell-cycle traverse of density-arrested BALB/c-3T3 cells by rendering quiescent cells 'competent' to respond to 'progression' factors contained in platelet-poor plasma (PPP). PDGF-treated cells remain competent for many hours following PDGF removal; subsequent addition of PPP triggers G0-G1 traversal and entry into S phase. Numerous observations suggest that the competent state reflects the existence of a stable PDGF-induced 'second signal' as opposed to a persistent association of PDGF with cells. Several unique proteins have been shown to be synthesized in response to PDGF; of these, the dose-dependent production of 'pI' (molecular weight 29,000) was found to correlate closely with the induction of competence. We report here the further characterization of pI in terms of its time-dependent synthesis, intracellular location and stability. Electrophoretic analysis of nuclear and non-nuclear extracts of PDGF-treated BALB/c-3T3 cells demonstrated that pI is synthesized during the first 6 h of G0-G1, is associated with nuclear material and is stable for 9-12 h. These findings are consistent with the proposed role of pI as the cellular mediator of PDGF.

Topography of the total protein population from cultured cells upon fractionation by chemical extractions

Chemical extractions are proposed as a major tool for a fractionation of cellular proteins. As a model system, proteins from cultured hamster lens cells have been divided by independent extractions into seven subcellular fractions, corresponding to water-soluble proteins and the proteins from membranes, microfilaments (and other deoxycholate-soluble proteins), intermediate filaments, microtubules, polysomes and nuclei respectively. The latter two fractions have been subfractionated yielding ribosomal proteins, the elongation and initiation factors of the protein-synthesis machinery, chromatin proteins and non-chromatin proteins. The protein compositions of the fractions have been analyzed by one-dimensional and two-dimensional gel electrophoresis. This resulted in an almost complete topography of the proteins detected on two-dimensional gels of total-cell lysates. Comparison of two-dimensional patterns of proteins from the total-cell lysate and proteins from hamster erythrocytes or from liver, muscle or brain tissue showed that the different cell types have only few proteins in common. Two proteins are common to all of these cell types, namely actin and a 68-kDa protein. The latter protein was, like actin, vimentin and the tubulin subunits, also present in most cell fractions. Evidence is presented that this protein is identical to a 68-kDa heat-shock protein.