Histones amino acid composition

The above general scheme based on comparisons of the most evolutionarily conserved GHl domain does not reveal the rich microheterogeneity of linker histones which stems from differences between the less well conserved basic tails. Such variants, often referred to as somatic subtypes, occur in plants (for review see Ref. [80]) and animals, both invertebrates and vertebrates (for review see Refs. [81-83]). For example in mammals five somatic subtypes represent the major form of HI HF-1, HF-2, HF-3, HF-4, and HI a, according to the nomenclature proposed in Ref. [82]. The N- and C-terminal tails of the subtypes differ in length, the amino acid composition and the frequency and distribution of phosphorylation sites. The testis specific Hit can be considered the most diverged subtype of the major form. 

Duerre, J.A. and Chakrabarty, S. (1975) Methylated basic amino acid composition of histones from the various organs from the rat. J. Biol. Chem. 250, 8457-8461. 

Found in the chromatin of all eukaryotic cells, histones have molecular weights between 11,000 and 21,000 and are very rich in the basic amino acids arginine and lysine (together these make up about one-fourth of the amino acid residues). All eukaryotic cells have five major classes of histones, differing in molecular weight and amino acid composition (Table 24-3). The H3 histones are nearly identical in amino acid sequence in all eukaryotes, as are the H4 histones, suggesting strict conservation of their functions. For example, only 2 of 102 amino acid residues differ between the H4 histone molecules of peas and cows, and only 8 differ between the H4 histones of humans and yeast. Histones HI, H2A, and H2B show less sequence similarity among eukaryotic species. 

What feature of the amino acid composition of histones enables them to interact strongly with DNA ... 

Quantitation. Archaeal histones are enriched in arginine and lysine residues, and the methods of protein quantitation based on Coomassie Brilliant Blue G250 dye binding are inaccurate because the dye preferentially binds to these basic residues. Most archaeal histones also lack tyrosine or tryptophan residues, precluding A280 measurement. Therefore, reliable quantitation of archaeal histone solutions requires a determination of the total amino acid composition after acid hydrolysis. ... 

Table 1. Amino acid composition of ADP-ribosylated non-histone acceptor proteins purified from mouse testis nuclei ...

Although the nuclear membrane s proteins have been fractionated by gel electrophoresis and their amino acid composition has been studied, their function remains unknown. However, it is certain that they are not histones. Although the quantitative content of phospholipids in the nuclear membrane is not certain, it is estimated that 50-60% of the phospholipids found in the nucleus are associated with the membrane. They include cardiolipin, phosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl inositol, phosphatidyl choline, sphingomyelin, and lysophosphatidyl choline. Of course, as in most cellular membranes, their function remains to be determined. 

Histones can be separated by chromatography using amberlite resins, carboxymethyl-cellulose, and se-phadex. The proteins have been eluted with different solutions, depending upon the investigator. The elution may be complete or incomplete. When incomplete, there is of course the possibility that specific histones with important biological properties have been lost. The histone preparations that are put on the column are usually heterogeneous, and several fractions are obtained. Because several laboratory groups have studied the amino acid composition of the various fractions, a comparison may be made between fractions obtained by different methods. 

Many laboratories have succeeded in fractionating histones by selective extraction and differential precipitation. The selective extraction procedure is based on the difference in the amino acid composition of the various types of histones. For example, a histone-rich lysine can be obtained by extraction from whole thymus with 5% perchloric acid followed by precipitation with 10% trichloroacetic acid. If the histones found in the perchloric acid extract are passed through a carboxymethyl-cellulose column and eluted from the column with borate buffer at pH 9, then different components called 1, 2, and 3 can be recovered. 

These brief considerations of the various histone fractionation methods indicate that different types of histones are obtained, depending upon the method used. Sometimes a histone isolated by one method has an amino acid composition homologous to a histone isolated by another procedure. Often a procedure will yield a type of histone that cannot be obtained by other procedures. Whether the different fractions are representative of native histones or whether they are the result of degradation remains to be established. 

Chromatographic analysis on IRC 50 of histones prepared from various tissues (kidney, liver, thymus) of guinea pigs revealed no difference in the amino acid composition of the histone populations. Furthermore, when the chromatographic behavior, mobility on electrophoresis, or amino acid composition of histones obtained from a variety of mammalian tissues were compared, no consistent differences were discovered. 

Differences between histone populations are not exclusively related to the amino acid composition of the histone. Differences in metabolism or in the rate of synthesis and, consequently, in the amount of histones may be responsible for changes in the rate and type of genetic information transferred. After biosynthesis, histones can undergo four different metabolic alterations phosphorylation, acetylation, methylation, and thiolization. 

Histones are the basic proteins associated with the DNA of chromosomes of virtually all eucaryotic organisms. These proteins are remarkably similar In all organisms (amino acid sequence etc.). Five types of histones, classified according to their amino acid composition, are known (Table 9.1). Their sequences have all been determined, and they show that the basic amino acids are concentrated in one half of the molecule. The fact that they differ so little from one species to another suggests that they are indispensable for maintenance of eucaryotic chromosome structure. They are present In about equimolar amounts. 

The electrophoretic heterogeneity of the histones is mainly associated with differences in the amino acid composition of their fractions. However, fractionation on Sephadex, based on separation... 

Investigations have shown (Hnilica et al, 1962 Laurence et al., 1963 Hnilica, 1966 etc.) that histones from some tissues (tumors, liver, spleen, thymus) gave a practically identical pattern of their fractions and identical amino acid composition of the fractions. However, this still does not provide an adequate basis for the conclusion that all forms of tissue specificity in general are absent between all tissues. Considerable evidence has now been obtained to show that specificity of this type does in fact exist, although it is not very well defined. 

Mauritzen and Stedman (1959, 1960) found slight but definite tissue differences in the amino acid composition of nuclear histones isolated from different organs. These differences concerned only some amino acids (aspartic and glutamic acids, alanine, leucine, isoleucine, and valine). They postulate that histones may be tissue-specific proteins. The relative quantities of the same histone fractions also differ appreciably from one tissue to another. 

The biosynthetic development of protamine was first studied by analysis of amino-acid composition and terminal groups of proteins in the nuclei fraction from the testis of rainbowtrout (Salmo irideus) at different stages of spermatogenesis, as shown in Table X-2. These studies showed that in the immature stage DNA is bound to basic proteins of the histone type, which are gradually replaced as maturation proceeds by basic proteins of the protamine type, so that in the fully matured sperm heads DNA is bound only to protamine [Ando and Hashimoto, 1958 (1 2, 3) Felix 1958 Felix, I960]. 

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