A comparison of the high mobility group non-histone chromatin protein HMG 2 in chicken thymus and erythrocytes

Production of functional chick liver HMG 2a protein in Escherichia coli

An efficient Escherichia coli system for the production of a variant form of high-mobility group-2a protein (HMG 2a), having the additional 5 amino acid residues (Ala-Pro-Thr-Leu-Glu) at the NH2-terminal, has been constructed. cDNA encoding HMG 2a was ligated with the Omp A signal peptide sequence and was inserted into an inducible bacterial expression vector pSH-L. After the plasmid introduced into E. coli was expressed by temperature shift, the recombinant product was purified by trichloacetic acid precipitation followed by Bio-Rex 70 column chromatography. The purified product showed the expected NH2-terminal sequence and the superhelical activity of circular DNA similar to the authentic HMG 2a isolated from chick liver.

High mobility group protein, HMG-1, contains insignificant glycosyl modification

High mobility group protein-1 (HMG-1) is a ubiquitous, highly conserved, and abundant nuclear protein. Recent findings suggest that HMG-1 may serve as a DNA chaperone protein and play a role in the regulation of transcription. There is a mounting interest in elucidating the mechanism by which HMG-1 protein takes part in these activities. HMG-1 has been reported to undergo an extensive array of posttranslational modifications, including glycosylation. We extend the earlier findings on the glycosylation of HMG-1 by quantitating the amount of carbohydrate on HMG-1 from calf thymus and chicken erythrocytes isolated by 2 different purification procedures. In addition, 2 different developmental stages (embryonic and adult) were examined in the chicken erythrocytes. The glycosyl composition was quantitated using the Dionex HPAE-PAD II system. Furthermore, the presence of O-linked GlcNAc on HMG-1 was determined by the enzymatic incorporation of 3H-galactose into HMG-1 protein. Contrary to earlier reports, less than 0.5 mol of total monosaccharides (Fuc, Man, GalNH2, GlcNH2, Gal) were detected per mole of HMG-1 protein, regardless of the source of the protein or the method of isolation. In addition, less than 0.002 mol of O-linked GlcNAc per mole of HMG-1 protein was detected. Thus, insignificant amount of glycosylation was found on HMG-1 protein. Because O-linked GlcNAc modification of proteins is believed to be a reversible posttranslational event, more definitive studies will need to be conducted before ruling out that the function of HMG-1 protein is not regulated by glycosylation.

Developmental changes in the expression of HMG 2a protein

The levels of HMG 2a chromosomal protein and its mRNA change during the post-hatched development of chicks were investigated. The contents of both HMG 2a and 2b proteins of liver, heart, brain, muscle and gizzard were abundant in the newly hatched chicks but their contents decreased significantly in those tissues of the 70-day-old chicks. The HMG 2a mRNA levels of liver, heart and brain in 70-day-old chick decreased to about 40% of those mRNA in the newly hatched chicks while the HMG 2a mRNA levels of muscle and gizzard in the 70-day-old chicks increased 5- and 3-fold, respectively. These results suggest that the decrease in the HMG 2a protein contents of the muscle and gizzard in the 70-day-old chicks may be largely due to the stimulation of HMG 2a protein degradation or the reduction of HMG 2a mRNA translation.

Tissue-specific pattern of nonhistone high mobility group proteins in various organs of the chicken

Electrophoretic analysis of tissue-specific differences of nonhistone high mobility group (HMG) proteins from nuclei of various organs of the chicken revealed that in organs with a higher proportion of replicating cells (thymus, Bursa Fabricii, spleen) the relative amount of HMG-17 is considerably higher than that of HMG-14; however, in transcriptionally active organs with a very small proportion of replicating cells (glandular stomach, liver) HMG-14 and HMG-17 are present at roughly equal and low amounts. In glandular stomach, liver and spleen, the relative contents of both HMG-1 and HMG-2 are markedly lower than in thymus and Bursa Fabricii. Moreover, the total amount of HMG proteins is higher in those organs which contain replicating lymphocytes.

Isolation of a chicken HMG2 cDNA clone and evidence for an HMG2-specific 3'-untranslated region

HMG1 and HMG2 (high-mobility group proteins) are two of the most abundant nonhistone chromosomal proteins in higher eukaryotes. Mammalian HMG1 cDNA sequences have the unusual feature of being conserved not only over their coding regions, but also over large segments of their 3'-untranslated regions (3' UTRs) as well. In contrast, the only reported mammalian HMG2 cDNA clone has a distinct 3' UTR. We now report the isolation of a chicken HMG2 cDNA clone and show that it is markedly similar to the mammalian HMG2 cDNA clone over both its coding regions and 3' UTRs. We therefore infer that the 3' UTRs of the HMG1 and HMG2 genes are subject to distinct evolutionary pressures. Our data, along with published data, also serve to highlight 26 amino acid positions where HMG1 and HMG2 are distinctly conserved, and we note that trout HMG-T conforms to the HMG1 paradigm at most of these diagnostic positions.

Molecular cloning of chick liver HMG 2a cDNA and developmental expression of HMG 2a mRNA

The cDNA clone encoding HMG 2a of chick liver was isolated from a lambda gt11 expression library using polyclonal antibodies. DNA sequence analysis revealed an open reading frame of 201 amino acids. Comparison of the nucleotide sequences of cDNA coding for chick liver HMG 2a with pig thymus HMG 2 and human monocytic leukemia cell HMG 2 showed 70% homology. 2.0 kb and 1.2 kb mRNAs were found in newly hatched chick liver and decreased during postnatal development of chicks.

Characterization of HMG2 complement changes in chicken tissues

Variations in the content of nonhistone proteins high mobility group 2a (HMG2a) and HMG2b have been determined in several cell types of chicken. HMG2a was found to accumulate during erythrocyte maturation. HMG2b is the major HMG2 subtype in testis and reaches the highest proportion, detected so far, in spermatid cells obtained by centrifugal elutriation. In hepatocytes HMG2b is barely detectable and HMG2a is the major subtype. In conclusion, the pattern of HMG2 composition is different in three quiescent and terminally differentiated cell types, no correlation between the state of cell proliferation and HMG2 composition can be established.

Monoclonal-antibody-based enzyme-linked immunosorbent assay for HMG 2b nonhistone protein in chick liver

Monoclonal antibodies were prepared against the high mobility group (HMG) protein 2b from chick liver chromatin and a monoclonal-antibody-based enzyme-linked immunosorbent assay (ELISA) was developed for chick HMG 2b. The sensitivity of the assay is about ten times that of the previously described radioimmunoassay and solid-phase enzyme immunoassay for HMG proteins. With the use of ELISA technique, the amount of HMG 2b (micrograms protein/mg DNA) in the livers of 1-day old and 70-day old chicks was found to be 2.56 +/- 0.4, and 1.20 +/- 0.2, respectively. The age-dependent change in the level of HMG proteins probably reflects changes in the functional state of chromatin during ageing.

High mobility group (HMG) proteins in the tammar wallaby Macropus eugenii: quantitative variations between tissues and testis-specific co-extracted proteins

1. Tammar wallaby (Macropus eugenii, Marsupialia) proteins with similar electrophoretic mobilities to calf non-histone chromosomal proteins HMG 1, 2, 14 and 17 are perchloric acid extracted from whole tissues (liver, kidney, spleen, brain and testis) and purified liver nuclei (using PCA or 0.35 M NaCl). 2. Tammar and calf HMG 1 have similar amino acid compositions. 3. Two testis-specific basic proteins co-extracting with HMG-like proteins from both tammar and red kangaroo (Megaleia rufa) are found in whole testis, purified testis nuclei, but not epididymis. 4. Tammar HMG 2 separates into two components on both acid urea and SDS gels. The larger, more basic protein, HMG 2b, is relatively abundant in proliferating tissues (testis, spleen).

Heterogeneity of high-mobility-group protein 2. Enrichment of a rapidly migrating form in testis

A determination of the absolute amounts of high-mobility-group proteins 1 and 2 (HMG1 and HMG2) in rat tissues demonstrated that amounts of HMG2 were low in non-proliferating tissues, somewhat higher in proliferating and lymphoid tissues, but were extremely elevated in the testis. This increase was due to a germ-cell-specific form of HMG2 with increased mobility relative to somatic HMG2 on acid/urea/polyacrylamide-gel electrophoresis. To determine if the findings in the rat were a general feature of spermatogenesis, testis (germinal), spleen (lymphoid), and liver (non-proliferating) tissues from various vertebrate species were examined for their relative amounts of HMG1 and HMG2, and for HMG2 heterogeneity. Bull, chimpanzee, cynomologus monkey, dog, gopher, guinea pig, hamster, mouse, opossum, rabbit, rat, rhesus monkey, squirrel and toad (Xenopus) tissues were analysed. Nearly all species showed relatively high contents of HMG2 in testis tissue, whereas HMG1 contents were similar in all species and tissues. Ten of thirteen species showed a rapidly migrating HMG2 subtype in testis tissue, separable by acid/urea/polyacrylamide-gel electrophoresis. Xenopus, which lacks HMG2 in somatic tissues, showed an HMG2-like protein in testis tissue. Although the rapidly migrating HMG2 subtype in species other than rat was not testis-specific, it was always enriched in the testis. This study indicates that increased amounts of HMG2 and the enrichment of a rapidly migrating HMG2 subtype are general features of spermatogenic cells.

Microheterogeneity of the mammalian high mobility group (HMG) proteins 1 and 2 investigated by reverse-phase high performance liquid chromatography

Microheterogeneity within the high mobility group (HMG)-1 and HMG-2 groups of nonhistone chromatin proteins has been investigated using reverse-phase high-performance liquid chromatography (RP-HPLC) under conditions (acetonitrile elution with 0.1% trifluoroacetic acid (TFA) as the counter ion) which separate proteins primarily on the basis of differences in their overall hydrophobicity. RP-HPLC proved to be a fast and efficient means for separating multiple subspecies of both the HMG-1 and HMG-2 proteins from both crude nuclear extracts and from ion-exchange column "purified" protein samples obtained from different types of mammalian cell nuclei. In crude nuclear extracts at least eight different HMG-2 protein species (two major and six minor), but only one major HMG-1 species, could be resolved by RP-HPLC. Three of the minor HMG-2 protein species could be isolated in "pure" form from crude extracts in one RP-HPLC step whereas under the same conditions the two major HMG-2 peaks (as well as the other minor species) were contaminated with either HMG-1 or HMG-3 (a degradation product of HMG-1). In crude extracts the major HMG-1 fraction always seems to be contaminated with one of the HMG-2 subfractions. RP-HPLC analysis of apparently "pure" protein preparations isolated by ion-exchange chromatography techniques revealed that "pure" HMG-1 can be resolved into at least three different protein species and "pure" HMG-2 into at least four different species. Amino acid analyses of different resolvable forms of the HMG proteins were not inconsistent with the suggestion that at least some of these may be primary sequence variants of the individual proteins, but other possibilities also exist.

Concentrations of high-mobility-group proteins in the nucleus and cytoplasm of several rat tissues

Nuclear and cytoplasmic fractions were isolated from various tissues of the rat by a nonaqueous technique. The high-mobility-group (HMG) proteins were extracted from these fractions with acid and separated by one- and two-dimensional PAGE. The concentrations of high-mobility-group proteins HMG1, HMG2, and HMG17 in the nucleus and cytoplasm were then estimated from the staining intensities of the electrophoretic bands. The cytoplasmic concentrations of these proteins were very low--usually less than 1/30 of those present in the corresponding nuclear fractions. For the tissues studied (liver, kidney, heart, and lung), the concentrations of HMG proteins in the nucleus did not differ significantly from one tissue to another. Averaged over the four tissues investigated, there were 0.28 molecule of HMG1, 0.18 molecule of HMG2, and 0.46 molecule of HMG17 per nucleosome. These values are considerably higher than those that have been reported previously.

Comparison of the high-mobility-group chromosomal proteins in rainbow-trout (Salmo gairdnerii) liver and testis

Chromatography and characterization of the proteins extracted by 5% (w/v) HClO4 from rainbow-trout (Salmo gairdnerii) liver and testis show that the two tissues present a characteristically different spectrum of high-mobility-group (HMG) proteins. A variant subfraction of HMG C is found in liver, but is not detectable in testis, where even the main fraction of HMG C is present in only very low quantity. A protein, F, which appears to be related to protein H6 has similarly been isolated only from liver and not from testis. Quantification of the HMG proteins in total 5%-HClO4 extracts of trout liver and testis nuclei shows that, in relation to DNA, levels of HMG T1 and T2, and D are more than 2-fold, and C, 20-fold higher in liver than in testis. However, these differences do not result merely from the sequential withdrawal of HMG proteins at the same time that histones are replaced by protamines in the developing spermatid, since in testis, at some stages of maturation, levels of H6 are almost 2-fold higher than in liver. The implications of these findings for the function of HMG proteins are discussed.

Production of HMG-3 by limited trypsin digestion of purified high-mobility-group nonhistone chromatin proteins

Three isolated nonhistone proteins (HMG-1, HMG-2 and HMG-E) have been purified from chicken erythrocyte chromatin without exposure to overt denaturing conditions, and subjected to limited proteolysis. When treated with trypsin, the three proteins exhibited similar patterns of degradation, as judged by SDS and acid/urea gel electrophoresis. In particular, the first product, P1 (a relatively stable intermediate in each digestion), was a protein analogous to HMG-3, a principal degradation product in preparations of calf thymus high-mobility-group proteins. At least in the case of HMG-E, the products formed by tryptic attack on P1 are the two individual DNA binding domains of HMG-E. P1 derived from HMG-E and one of the individual DNA binding domains of HMG-E were purified by chromatography on columns containing DNA-cellulose or phosphocellulose. The properties of these two portions of HMG-E are consistent with our recently postulated three-domain structure for HMG-1 and its homologs (Reeck, G.R., Isackson, P.J. and Teller, D.C. (1982) Nature 300, 76-78). Thus, P1 consists of two DNA-binding domains of approximately equal molecular weight covalently linked together. From chromatography on DNA-cellulose columns, it is clear that P1 binds to DNA more tightly than does HMG-E. The highly acidic C-terminal domain of HMG-E (which is removed by trypsin in generating P1) thus counteracts the DNA binding of the two other domains of HMG-E (at least in the protein's interaction with purified DNA).

Isolation from trout liver of a methionine-containing H1 subfraction, THImet

A trout H1 subfraction, THImet, which contains one residue of methionine and a blocked N-terminal amino acid, has been isolated from trout liver. Cyanogen bromide cleavage of THImet and amino acid analysis and polyacrylamide gel electrophoresis of the cleavage products show that it contains a residue of methionine at about the same distance from the N terminus as mammalian H1o and avian H5. THImet is shown to be similar to, but not identical with, the protein derived from trout erythrocytes generally accepted as the equivalent in fish of avian H5.

Ionic interactions between proteins in nonequilibrium pH gradient electrophoresis: histones affect the migration of high mobility group nonhistone chromatin proteins

In two-dimensional gel electrophoresis of the high mobility group (HMG) proteins, it has proved necessary to use nonequilibrium pH gradient electrophoresis (NEPHGE) in the first dimension rather than isoelectric focusing, because of the basic character of most of the HMG proteins [D. Tyrell, P. J. Isackson, and G. R. Reeck (1982) Anal. Biochem. 119, 433-439]. In this paper it is reported that in samples that contain histones, the mobilities of HMG proteins (particularly HMG-1, HMG-2, and HMG-E) are severely distorted in NEPHGE. This presumably results from formation of complexes between histones and HMG proteins through ionic interactions. Analysis of HMG proteins by NEPHGE/sodium dodecyl sulfate-gel electrophoresis is thus precluded in samples containing histones. Our results raise the possibility of similar artifacts occurring in NEPHGE (or isoelectric focusing) analysis of other proteins with regions of high charge density.

High mobility group nonhistone chromosomal proteins of the developing sea urchin embryo

The high mobility group or HMG proteins are nonhistone chromosomal proteins that have been found in relatively high amounts in nuclei of many tissues. A number of studies have shown that some of these proteins are preferentially associated with actively transcribed regions of the genome and may play a role in maintaining these regions in an active state. In this study, we undertook an investigation of the high mobility group proteins from the sea urchin, Stronglyocentrotus purpuratus. Initially the putative sea urchin HMGs were extracted from isolated nuclei of hatching blastula-stage embryos with 5% perchloric acid (PCA). The major proteins in this extract were characterized according to their electrophoretic mobility, amino acid composition, and association with isolated deoxyribonucleoprotein particles. The results indicate there is only one "major" sea urchin HMG protein, termed P2 in this paper. An estimate of the amount of P2 in relation to the inner histones, however, was low compared to what has been found for other HMG proteins. Of the other major 5% PCA-extractable proteins, one was identified as the cleavage stage H1. Another protein apparently resulted from H3 contamination in the 5% PCA extract, and the fourth major protein did not have all the characteristics of an HMG. In particular, it was not found associated with nucleosomal particles. The HMG proteins from other developmental stages were then examined. Five percent PCA extracts of nuclei from unfertilized eggs, 2-cell, 16-cell, hatching blastula, gastrula, and pluteus stages were analyzed on SDS- and acetic acid-urea gels. This analysis indicated that P2 exists in two different forms differing slightly in charge. The less basic form was found in the egg, 2-cell and 16-cell extracts. At the hatching blastula stage, both forms were present and by pluteus stage, the more basic form predominated. It appears that P2 is undergoing a developmental change from a less to more basic form. The presence of P2 in the 5% PCA extract of egg nuclei is proof that P2 does not initially appear sometime during embryogenesis but is already in the egg nucleus prior to fertilization.

Is high mobility group protein 17 phosphorylated in vivo? Re-examination of the HeLa cell cycle data

When in vivo [32P] phosphate labeled HMG proteins from unsynchronized HeLa cells are separated by electrophoresis in acid-urea polyacrylamide gels, as opposed to separation in SDS-polyacrylamide, HMG 17 does not show any 32P incorporation. Likewise, no 32P radioactivity was found in HMG 17 protein isolated at different stages of the cell cycle from synchronized cells. By contrast, HMG 14 reveals a previously reported (Bhorjee, J.S. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6944-6948) cell cycle stage-specific dependent phosphorylation with maximum 32P radioactivity in the G2 phase relative to G1. Furthermore, HMG 14 is resolved into multiple electrophoretic forms as phosphoprotein in the acid-urea system. The results presented seriously question the data on the in vivo phosphorylation of HMG 17, and suggest that these be reevaluated.

Heterogeneity of proteins resembling high-mobility-group protein HMG-T in trout testes nuclei

Two proteins, HMG-T1 and HMG-T2, with electrophoretic mobilities and compositions similar to those of protein HMG-T, were isolated from trout testes nuclei. The isoelectric points of proteins HMG-T1, HMG-T2 and HMG-T differ. The first 20 residues of protein HMG-T2 have been sequenced and differ from protein HMG-T by only one residue.

The isolation, characterization and partial sequences of the chicken erythrocyte non-histone chromosomal proteins HMG14 and HMG17. Comparison with the homologous calf thymus proteins

Non-histone chromosomal proteins HMG14 and HMG17 were isolated from chicken erythrocyte nuclei. The proteins were characterized by amino acid analysis and by N-terminal sequence analyses. Comparison with the corresponding data for the calf thymus proteins shows that 11% of the residues in HMG14 protein and 5% of the residues in HMG17 protein differ between the two species. Proteins HMG14 and HMG17 therefore do not appear to exhibit the evolutionary stability shown by the nucleosome core histones. Detailed evidence for the amino acid sequence data has been deposited as Supplementary Publication SUP 50101 (4 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies may be obtained on the terms given in Biochem. 4. (1978) 169, 5.