Histone proteolysis

Another speculation has been put forth. Because the turnover of methyl groups such as the modification on lysine residues is low, ubiquitination of histone N-termini might serve as a signal for proteolysis of methylated histones such that dynamic regulation of the chromatin is possible. Since no histone demethylases have been identified, ubiquitin-mediated proteolysis might be a way to reverse the effects of histone methylation. ... 

ADP-ribosylation has also been implicated as a proteolytic antagonist during embryonic development [231]. Following fertilization in sea urchin, sperm-specific histones are degraded by the sperm-histone-selective (SpH) protease and subsequently replaced by cleavage stage histone variants. During this process, the maternal replacement histones are protected from proteolysis by ADP-ribosylation. 

Fig. 2. NAPI facilitates H2A, H2B release from nucleosomes that are on positively coiled DNA (A) but not negatively coiled DNA (B). The positively coiled DNA (6.0 kb) with a superhelical density of + 0.05 and negatively coiled DNA (6.0 kb) with a superhelical density of -0.05 were reconstituted with lysine, arginine-labeled histones H3, H4, H2A, H2B by NaCl dialysis from 2.0 M to 1.2 M to 0.6 M to 0.1 M NaCl over a 14 h period. The samples were incubated with NAPI at 35 °C for 5 min and applied to a 5-20% sucrose/100 mM NaCl/40 mM Tris, pH 7.8 gradient. After sedimentation at 200,000 X g for 5 h, fractions were collected and the distribution of DNA (bottom panel) was determined on agarose gel and the distribution of protein (top panel) on SDS-PAGE followed by fluorography. These data are unpublished observations (V. Levchenko and V. Jackson). The deg-H2A is degraded H2A in which a 15 amino acid peptide of the C terminal has been proteolytically removed. When H2A, H2B is no longer present in a nucleosome, the C terminal region is sensitive to proteolysis [126] from a protease which is a minor contaminate in the NAPI preparation.

Carnitine is synthesized from lysine and methionine by the pathway shown in Figure 14.2 (Vaz and Wanders, 2002). The synthesis of carnitine involves the stepwise methylation of a protein-incorporated lysine residue at the expense of methionine to yield a trimethyllysine residue. Free trimethyllysine is then released by proteolysis. It is not clear whether there is a specific precursor protein for carnitine synthesis, because trimethyllysine occurs in a number of proteins, including actin, calmodulin, cytochrome c, histones, and myosin. 

Table 2. Proteolysis of histone H-1 and poly(ADP-ribose) polymerase by pyrophosphate, nucleotide-stimulated protease system...

Surowy CS, Berger NA (1983) Nucleotide-stimulated proteolysis of histone H-1. Proc Natl Acad Sci USA 80 5510-5514... 

To determine which proteins contained (ADP-ribose) , fraction V was analyzed by acetic acid/urea polyacrylamide gel electrophoresis. As shown in Fig. 2A, the major portion of radioactivity migrated with histone HI and radioactivity was also found with staining bands of the four HMG proteins. Following 3-ABm treatment for 16 h, ADP-ribosylation of HMG 14 and 17 was almost completely inhibited, while that of HMG 1 and 2 and histone HI decreased less. The reduced ADP-ribosylation cannot be attributed to differences in protein extraction or proteolysis since Coomassie blue staining patterns were very similar and 3-ABm treatment did not affect the incorporation of labeled lysine into HMG proteins and histone HI (Fig. 2B). ADP-ribosylation... 

In a global analysis of histone H2A and H2B variants derived from Jurkat cells by MS, nine histone H2A and 11 histone H2B subtypes were identified, some of which had only been postulated before at the DNA level [391]. This was achieved by combining MS with HPLC separations and enzymatic proteolysis using endoproteinase Glu-C, endoproteinase Arg-C, and trypsin. With regard to modification status, e.g., the two main H2A variants, H2A.o and H2A.C, as well as H2A.1 were foimd either acetylated at Lys-5 or phos-phorylated at Ser-1. For the replacement histone H2A.z, acetylation at Lys-4 and Lys-7 was observed. The main histone H2B variant, H2B.a, was found acetylated at Lys-12, -15, and -20. In an other study, a direct, top-down MS approach was applied to identify H2B isoforms isolated from asynchronous HeLa cells using ESI-FT-ICR MS and BCD for MS/MS analysis of intact protein molecular ions (Fig. 22) [59]. These cells were foimd to express H2B.A, H2B.B, H2B.E, H2B.F, H2B.J, H2B.K, H2B.Q, and H2B.T. 

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