Low degree of ubiquitination of histone 2A in the dipteran Chironomus tentans.

A polyclonal antibody to ubiquitin has been prepared and shown to react with both ubiquitin and ubiquitinated histone 2A (uH2A). Applying this antibody in Western blotting experiments, we have observed that the salivary glands of Chironomus tentans contain an unusually low amount of uH2A (1% of histone 2A), while the amount of free ubiquitin is as abundant as in other animal cells, e.g. HeLa cells. The same low content of uH2A was also found in diploid epidermal cells of Chironomus origin suggesting that the low amount is not a characteristic of the polytene state of chromatin in salivary gland cells but rather a property of C. tentans as a species. The significance of the low degree of ubiquitination is discussed in relation to the information available on the organization of Chironomus chromatin into unusually large chromomeric entities.

Inhibition of transcription does not affect the total amount of ubiquitinated histone 2A in chromatin.

Using a polyclonal anti-ubiquitin antibody in Western blotting experiments, we detected three antibody-binding components in a HeLa cell extract: ubiquitin, a ubiquitin-histone 2A conjugate (uH2A) and a 17 kD protein, probably corresponding to an additional ubiquitin conjugate. Since ubiquitination of histone 2A (H2A) has been invoked in the transcription process, the amount of uH2A was studied after inhibition of ribosomal RNA (rRNA) synthesis with actinomycin D and of heterogeneous nuclear RNA (hnRNA) synthesis with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). The amount of uH2A did not change, suggesting that the overall level of ubiquitination of histone 2A is not directly coupled to on-going transcription of either rRNA or hnRNA. Since the uH2A content of protein coding genes constitutes a considerable portion of total chromatin uH2A, it seems also likely that there is no major change in the degree of ubiquitination on the templates of the protein-coding genes themselves upon cessation of transcription. It is proposed that the pattern of ubiquitination of histone 2A is established on a long-term basis and that it is related to the overall organization and distribution of the chromatin material in the interphase nucleus.

Amino acid sequence of a specific antigenic peptide of protein B23.

A specific antigenic peptide was obtained from protein B23 (Mr/pI = 37,000/5.1) after 30 min of digestion with staphylococcal V8 protease (10 micrograms/ml/mg protein B23). The antigenic peptide was purified by DEAE-cellulose chromatography and high pressure liquid chromatography on a reverse-phase C18 column. The antigenic peptide contains 14.7 and 18.7 mol% of glutamic acid and lysine, respectively. Amino acid sequence analysis showed that the peptide has 68 amino acids and is located on the carboxyl-terminal sequence of protein B23. The sequence is Ser-Phe-Lys-Lys-Gln-Glu-Lys-Thr-Pro-Lys-Thr-Pro- Lys-Gly-Pro-Ser-Ser-Val-Glu-Asp-Ile-Lys-Ala-Lys-Met-Gln-Ala-Ser-Ile-Glu- Lys-Gly- Gly-Ser-Leu-Pro-Lys-Val-Glu-Ala-Lys-Phe-Ile-Asn-Tyr-Val-Lys-Asn-Cys-Phe- Arg-Met- Thr-Asp-Gln-Glu-Ala-Ile-Gln-Asp-Leu-Trp-Gln-Trp-Arg-Lys-Ser-Leu-Cooh. Extensive digestion of the antigenic peptide with V8 protease, trypsin, or chymotrypsin results in loss of the antigenic activity. Three cloned cDNAs (hpB1, hpB2, and hpB7) which code for the 82 amino acids at the COOH terminus of protein B23 and the 3' non-translating sequence were identified and characterized. All three clones have identical nucleotide sequences coding for the antigenic portion of the protein (68 amino acids at the COOH terminus), the stop codon, and the 3' non-translated region. However, mutation of 6 nucleotide bases of one clone (hpB2) caused changes in 4 amino acids in the sequence just preceding the immunoreactive region. The result suggests the presence of at least 2 immunologically similar but distinct proteins which are both recognized by the anti-B23 antibody.

Isolation and characterization of a 125-kilodalton rapidly labeled nucleolar phosphoprotein.

A 125-kilodalton (kDa) phosphoprotein was isolated from nucleoli of Novikoff hepatoma cells in the presence of various inhibitors of proteases, alkaline phosphatase, and RNase. This protein was the most highly phosphorylated protein found thus far in the nucleolus. The half-life of [32P]phosphate in the 125-kDa phosphoprotein was approximately 60 min. Amino acid analysis of the protein showed it had a high serine content (15.5 mol %), a high glutamine plus glutamic acid content (15.5 mol %), and a high lysine content (10.3 mol %). Phosphoserine was the only phosphorylated amino acid identified. After alkaline hydrolysis of the 32P-labeled protein, ribonucleotides were found which accounted for approximately 8.5% of the [32P]phosphate. After cytidine 3',5'-[32P]diphosphate ([32P]pCp) labeling by RNA ligase, several oligoribonucleotide sequences were purified including GGGCOH and GGGGCOH. The binding of oligonucleotides to peptides was stable under denaturing fractionation conditions including 6 M urea treatment and incubation at 100 degrees C for 10 min in sodium dodecyl sulfate and beta-mercaptoethanol. Furthermore, when nucleotide-peptide complex was treated with ribonuclease T2 followed by snake venom phosphodiesterase, the junctional nucleotide pCp was released. These results suggest that one or more ribonucleotides are covalently bound to the 125-kDa phosphoprotein.

Characterization of high mobility group protein levels during spermatogenesis in the rat.

The distribution, quantitation, and synthesis of high mobility group (HMG) proteins during spermatogenesis in the rat have been determined. HMG1, -2, -14, and -17 were isolated from rat testes by Bio-Rex 70 chromatography combined with preparative gel electrophoresis. Amino acid analysis revealed that each rat testis HMG protein was similar to its calf thymus analogue. Tryptic peptide maps of somatic and testis HMG2 showed no differences and, therefore, failed to detect an HMG2 variant. Testis levels of HMG proteins, relative to DNA content, were equivalent to other tissues for HMG1 (13 micrograms/mg of DNA), HMG14 (3 micrograms/mg of DNA), and HMG17 (5 micrograms/mg of DNA). The testis was distinguished in that it contained a substantially higher level of HMG2 than any other rat tissue (32 micrograms/mg of DNA). HMG protein levels were determined from purified or enriched populations of testis cells representing the major stages of spermatogenesis; spermatogonia and early primary spermatocytes, pachytene spermatocytes, early spermatids, and late spermatids; and testicular somatic cells. High levels of HMG2 in the testis were due to pachytene spermatocytes and early spermatids (56 +/- 4 and 47 +/- 6 micrograms/mg of DNA, respectively). Mixtures of spermatogonia and early primary spermatocytes showed lower levels of HMG2 (12 +/- 3 micrograms/mg of DNA) similar to proliferating somatic tissues, whereas late spermatids had no detectable HMG proteins. The somatic cells of the testis, including isolated populations of Sertoli and Leydig cells, showed very low levels of HMG2 (2 micrograms/mg of DNA), similar to those in nonproliferating somatic tissues. HMG proteins were synthesized in spermatogonia and primary spermatocytes, but not in spermatids. Rat testis HMG2 exhibited two bands on acid-urea gels. A "slow" form comigrated with somatic cell HMG2, while the other "fast" band migrated ahead of the somatic form and appeared to be testis-specific. The "fast" form of HMG2 accounted for the large increase of HMG2 levels in rat testes. These results show that the very high level of HMG2 in testis is not associated with proliferative activity as previously hypothesized.

Purification and partial characterization of a 19kD/pI 4.5 nucleolar phosphoprotein.

Two-dimensional PAGE analysis of proteins associated with the slowly sedimenting "fibrillar" structures of HeLa nucleoli revealed a protein with a M of 19,000 and a pI of 4.5 which was highly labeled both with 32P-orthophosphate and 35S-methionine. The protein was isolated from Novikoff hepatoma nucleoli by extraction in 0.35 M NaCl and 5 mM DTT followed by chromatography in EDTA on DEAE-cellulose and Sephadex G-100. The protein was homogeneous with respect to two-dimensional PAGE, number of tryptic peptides and carboxyl terminal analysis. The protein contained an acidic/basic amino acid ratio of 2.1, 7 residues of methionine, 2 residues of cysteine, a blocked amino terminus and a carboxyl terminal lysylleucine.

Ubiquitin - protein conjugates.

The data available at present indicates there are three distinct functions of ubiquitin, two of which are related to protein conjugation. The first of these has been extensively studied by our laboratory and others interested in nucleosomes and changes in chromatin states. The ubiquitin-histone (Ub-2A, Ub-2B) conjugation reaction now appears to be a very dynamic process. In the deconjugation (lyase) reaction, both the histone 2A and the ubiquitin are left intact and in a form which makes possible ready reconjugation. Accordingly, this may be a mechanism for "moment-to-moment" Control of the genome. The second function in which ubiquitin is conjugated involves proteolytic activity. This activity is correlated with protein turnover. In this process, the ubiquitin-protein conjugate apparently serves as a "signal" for the protease cleavage of the protein. The released ubiquitin is also intact and is probably available for reconjugation. In the third function, ubiquitin was suggested to serve as a "hormone". The studies thus far have been carried out primarily on induction of T- and B-lymphocytes, reduction or delay of Coombs' positivity and reduction of spleen weight. The precise physiological role of this reported function is still unclear, particularly because the ubiquitin used was probably not the physiologically active form.

Protein A24 lyase activity in nucleoli of thioacetamide-treated rat liver releases histone 2A and ubiquitin from conjugated protein A24.

Isolated liver nucleoli from rats treated for 3 days with thioacetamide contained an enzyme activity which specifically degraded conjugate protein A24. Two-dimensional polyacrylamide gel electrophoresis indicated that the amount of protein A24 in chromatin decreased during incubation at 37 degrees C for 60 min with these nucleoli. Concomitantly, a marked increase was found in the content of free ubiquitin, the nonhistone component of protein A24. Incubation of 3H-labeled protein A24 with the thioacetamide-treated liver nucleoli resulted in the linear release of 3H-labeled histone 2A and 3H-free ubiquitin in the presence of phenylmethanesulfonyl fluoride (PMSF) for 2 h. Pretreatment of the nucleoli with trypsin or by heating at 80 degrees C for 10 min inhibited their ability to cleave protein A24. Protein A24 lyase catalyzes the reaction: protein A24 leads to histone 2A plus ubiquitin.

Chromatin conjugate protein A24 is cleaved and ubiquitin is lost during chicken erythropoiesis.

The content of protein A24, an adduct of histone 2A and ubiquitin, was studied during chicken erythropoiesis. The amount of protein A24 was negligible in transcriptionally inactive mature chicken erythrocyte nuclei and 6-fold higher in the transcriptionally active nuclei of erythroid cells from phenylhydrazine-treated chickens. The decreased amount of protein A24 in the mature cells was offset by the relatively increased amount of histone 2A. A loss of free ubiquitin was also noted. In contrast, the amounts of high mobility group proteins 1,2, and E were essentially constant. Inasmuch as cleavage of the ubiquitin---histone 2A bond of protein A24 and loss of ubiquitin accompanied transcriptional shutdown during erythropoiesis, the presence of protein A24 and ubiquitin in premature polychromatic erythrocytes may reflect the presence of potentially active and transcribing chromatin structures.

Presence of protein A24 in rat liver nucleosomes.

Two-dimensional gel profiles of the 0.2 M H2SO4-soluble proteins of monomer nucleosomal fractions were found to contain protein A24. Protein A24 is of interest because it is composed of histone 2A and "ubiquitin", apparently joined by an isopeptide linkage [Goldknopf, I.L. & Busch, H. (1977) Proc. Natl. Acad. Sci. USA 74, 864-868; Hunt, L.T. & Dayhoff, M.O. (1977) Biochem. Biophys. Res. Commun. 74, 650-655]. Monomer nucleosomal fractions were obtained by sucrose density gradient centrifugation of micrococcal nuclease digests of rat liver nuclei. As shown by their DNA size, the monomer fractions were highly purified. Proteins A24 and Bu, another protein of unknown characteristics, were found along with histones 1, 2A, 2B, 3, and 4 in the monomer fractions in relative amounts similar to those found in extracts from whole nuclei and chromatin. Other acid-soluble proteins found in the nuclear and chromatin extracts were essentially absent from the monomer fraction. Inasmuch as protein A24 and Bu were found in lesser amounts than the histones, it is suggested that they are associated with specialized subsets of nucleosomes.

Isopeptide linkage between nonhistone and histone 2A polypeptides of chromosomal conjugate-protein A24.

Chromosomal protein A24 has a unique structure inasmuch as it contains histone 2A and a nonhistone polypeptide the sequence of which has been partially determined. Comparative analysis of the ninhydrin-insensitive amino-terminal tryptic peptides of protein A24 and histone 2A and a quantitative analysis of their carboxyl-terminal amino acid indicated that protein A24 has two amino termini and one carboxyl terminus. The amino acid sequence analysis of tryptic peptide 17' of protein A24: (see text) showed it contains tryptic peptide 17 of histone 2A, Lys-Thr-Glu-Ser-His-His-Lys. Lysine 119, the amino terminus of this peptide, which is derived from the histone 2A portion of protein A24, is linked by an isopeptide bond to the carboxyl group of a glycine residue. Accordingly, the branched structure of protein A24 proposed is: (see text).

The NH2- and COOH-terminal amino acid sequence of nuclear protein A24.

The NH2- and COOH-terminal sequence of nuclear portein A24 has been determined by automatic Edman degradation and carboxypeptidase A and B digestion. Protein A24 is of interest because it is composed in part of histone 2A (Goldknofp, I.L., and Busch, H., (1975) Biochem, Biophys. Res. Commun. 65, 951-960). The sequence of the first 37 NH2-terminal residues is: Met-Gln-Ile-Phe-Val-Lys-Thr-Leu-Thr-Gly-Lys-Thr-Ile-Thr-Leu-Glu-Val-Glu-Pro-Ser-Asp-Thr-Ile-Glu-Asn-Val-Lys-Ala-Lys-Ile-Gln-Asp-Lys-Glu-Gly-Ile-Pro- This sequence is not homologous to any known histone sequence. It contains regions of internal homology (italics). The COOH-terminal amino acid sequence is the same as that of histone 2A, naely: -His-His-Lys-Ala-Lys-Gly-Lys-COOH.