Role Of Non-histone Proteins Research Articles

Role of nonhistone proteins in the chromosomal events of mitosis.

This review is concerned with the role of chromosomal nonhistone proteins in three important aspects of mitotic events: chromosome condensation, sister chromatid separation at the metaphase:anaphase transition, and interactions between the chromosomes and cytoskeleton that occur during construction of the mitotic spindle and cleavage furrow. Emphasis will be given to the potential roles of topoisomerase II and the chromosome passenger proteins in these events. Other important aspects of mitotic events such as the regulation of the G2-->M transition, the structural changes that affect the nuclear envelope and other organelles during mitosis, and the mechanism of chromosome movement will not be considered here. Despite long histories of often elegant experimentation, all three of our chosen subjects remain areas of lively, ongoing controversy. Thus, although recent advances appear to have taken us many steps closer to an understanding of the underlying mechanisms, we suspect that final answers will be some time in coming.

Distribution and function of non-histone proteins in human chromosomes

The role of non-histone proteins in the structure and movements of mitotic chromosomes remains poorly understood. We describe here experiments aimed at characterization of the distribution of two very different classes of these proteins. The first is composed of integral components of the centromere (or primary constriction). The second class consists of proteins that we have termed “chromosome passenger proteins”. These proteins are chromosomal during most of the cell cycle, but appear to be associated with the cytoskeleton during anaphase and telophase.The centromere regions of chromosomes perform three essential functions in mitosis. (1) They form the site of attachment of the chromosomes to the mitotic spindle. (2) They contain the mechanochemical motor molecules that are responsible for the movements of the chromosomes along microtubules. (3) They regulate the pairing of sister chromatids during mitosis. The first two of these mitotic functions are properties of a disk-shaped structural specialization, the kinetochore, which is located at the surface of the centromeric heterochromatin.

Metaphase Chromosome Structure: The Role of Nonhistone Proteins

Metaphase Chromosome Structure: The Role of Nonhistone Proteins

Role of nonhistone proteins in metaphase chromosome structure

In this paper, we show that HeLa metaphase chromosomes still possess a highly organized structure retaining the familiar metaphase morphology following removal of virtually all the histones and most of the nonhistone proteins. The structure is stabilized by a relatively small number of nonhistones, which we call scaffolding proteins. These results are based on a method which allows the removal of the histones, and most of the nonhistone proteins, by competition with polyanions such as dextran sulfate and heparin. The histone-depleted chromosomes sediment in sucrose gradients as a broad peak between 4000 to 7000S. These structures are dissociated by mild trypsin or chymotrypsin treatment, or by 4 M urea, but are stable in 2 M NaCl and insensitive to treatment with RNAase A. The histone-depleted chromosomes have a DNA to protein ratio of about 6:1; gel electrophoresis reveals the presence of about 30 nonhistone proteins and the virtual absence of histones. These experiments suggest that nonhistone proteins exist in metaphase chromosomes which maintain the DNA chain in a highly folded conformation. Structural studies support this conclusion. Analysis by fluorescence microscopy of histone-depleted chromosomes stained with ethidium bromide shows that each chromatid is still paired with its sister chromatid, and consists of a central structure surrounded by a halo of DNA. The length of the central structure in each chromatid is about 2–3 times longer than the chromatid length in the original chromosome.

The premelting of nucleoprotein: role of non-histone proteins.

In native nucleoprotein, the premelting structural changes of DNA are not observed by circular dichroism measurements. In order to determine which protein fraction of chromatin is responsible for the absence of premelting we have examined a series of nucleoproteins depleted of different protein fractions by treatment with sodium chloride or sodium deoxycholate.The premelting reappears as soon as non-histone proteins are removed or in residual complexes from which the two slightly lysine-rich histone fractions (F2a2+F2b) have been removed. On the other hand, it is shown that histone F1 alone is not able to suppress the premelting phenomenon. It is thus concluded that the absence of premelting is a property of native nucleoprotein where interactions between the different proteins complexed with DNA can occur and especially between the non-histone proteins and the two slightly lysine-rich histone fractions.

Involvement of DNA in the acceptor mechanism for uterine oestradiol receptor.

WHEN oestradiol enters a responsive cell, it binds to a receptor protein and is transported into the nucleus where it is attached to chromatin1–3. The “acceptor” property of the chromatin is specific in that chromatin or nuclei of responsive cells accept more receptor than do preparations from non-responsive cells1,4,5. We know that the non-histone chromatin proteins provide this specificity but the way in which this is achieved is in doubt. Spelsberg et al.4–6 indicate that a non-histone protein is the acceptor in the progesterone/chick oviduct system (“active” role of non-histone proteins) whereas King et al.1,7 proposed, on the basis of their receptor-DNA binding studies in the oestradiol/rat uterus system, that DNA is the acceptor and that the non-histone proteins determine the specificity by controlling which regions of the DNA are accessible to the receptor (“passive” role of the non-histone proteins). Here we present further evidence that DNA is an integral part of the acceptor mechanism and discuss these results in relation to the progesterone data.

Nonhistone chromosomal proteins in synchronized HeLa cells.

Chromatin was isolated from synchronized HeLa cells at different stages of the cell division cycle and fractionated into DNA, histones, and nonhistone proteins. Electrophoresis of the nonhistone proteins in sodium dodecyl sulfate-polyacrylamide gels revealed a highly reproducible pattern of 22 bands, having estimated molecular weights of 15,000-180,000, with 85% (by mass) over 40,000. The amounts of some nonhistone proteins varied during the cell cycle by as much as 50%, while others remained at a constant level. One group of nonhistone proteins (molecular weight 75,000) was greatly reduced just before the start of DNA replication (S-phase), then returned to normal levels in the mid-S phase. These results are discussed with regard to the possible role of nonhistone proteins in regulating chromosome structure and function.