Complete murine cDNA sequence, genomic structure, and tissue expression of the high mobility group protein HMG-I(Y)

High-mobility-group (HMG) proteins and histone H1 subtypes expression in normal and tumor tissues of mouse

Exhaustive extraction of mouse tissues with perchloric acid has been used together with reverse-phase HPLC and electrophoresis to quantify the amounts of chromosomal proteins HMG17, HMG14 and HMGI, relative to histone H1. Normal lung and thymus contain approximately 3% HMG17/HMG14 but only approximately 2% HMGI. In tumor tissues (Lewis lung carcinoma and lymphoma NQ35), the amount of HMG17/HMG14 is not greatly altered but HMGI levels rise considerably, reaching 10% in Lewis lung carcinoma. HMGI synthesis does not replace HMG17/HMG14 proteins, suggesting that HMGI proteins contribute to the structure of chromatin regions in a manner distinct from those of HMG17/HMG14. Ion-spray mass spectrometry has been used to determine the molecular masses of H1 subtypes from the same four mouse tissues. In addition to the six known species H1 zero, H1a, H1b, H1c, H1d and H1e, a newly defined subtype of mass 21,756 Da from Lewis lung carcinoma, named H1L was identified. Several phosphorylated H1 subtypes have also been defined by mass spectrometry. The combined use of reverse-phase HPLC and electrophoresis permitted quantification of these seven histone H1 subtypes in the four mouse tissues. Increased phosphorylation of H1 subtypes in tumors parallels the phosphorylation of HMGI proteins which are present in great amounts, showing that both are involved as post-translational-modified forms in the structure of the chromatin of neoplastic systems.

The high mobility group protein HMG I(Y) is required for NF-kappa B-dependent virus induction of the human IFN-beta gene

In this paper, we show that both NF-kappa B and the high mobility group protein I(Y) (HMG I(Y)) are required for virus induction of the human interferon-beta (IFN-beta) gene. NF-kappa B binds to the terminal regions of a 10 bp regulatory sequence through contacts in the major groove. while HMG I(Y) recognizes the central region of the same sequence through contacts in the minor groove. Mutations that interfere with binding of either protein decrease the level of virus induction, and activation of the gene can be blocked by either NF-kappa B or HMG I(Y) antisense RNA. HMG I(Y) stimulates the binding of NF-kappa B to the IFN-beta promoter, and it may also function as a promoter-specific accessory factor for NF-kappa B transcriptional activity.

Sequence-specific binding of HMG-I(Y) to the proximal promoter of the gp91-phox gene

Screening of a cDNA expression library with a CCAAT-box element derived from the myelomonocyte-specific gp91-phox promoter resulted in the isolation of three independent HMG-I(Y) cDNA clones. Filter binding competition studies reveal that HMG-Y binds to this promoter element in a sequence-specific manner and exhibits a gradient of binding affinities for various A/T-rich sequences. Two adjacent A/T-rich regions within the gp91-phox promoter CCAAT-box element are required for maximal binding. In addition, competition experiments demonstrate that the binding affinity of HMG-Y is influenced by sequences that flank A/T-rich core binding sites.

Chromosomal localization of the murine gene and two related sequences encoding high-mobility-group I and Y proteins

HMG-I and its isoform HMG-Y are members of the abundant high-mobility-group of nonhistone chromatin proteins; they bind to A + T-rich regions of chromosomal DNA and are expressed at high levels in rapidly dividing, undifferentiated mammalian cells. HMG-I and HMG-Y are alternatively spliced products of a single functional gene, designated Hmgi in the mouse. Here, we report the occurrence of at least three distinct Hmgi-related loci in the mouse. Only one of these loci was present in all of the 10 mouse strains examined; therefore, this locus most likely represents the transcriptionally active, functional gene, Hmgi. Genetic linkage analysis of interspecific and intersubspecific backcrosses showed that Hmgi is located in the t-complex region of mouse Chromosome 17. Two additional Hmgi-related sequences, Hmgi-rs1 and Hmgi-rs2, were found only in certain mouse strains and probably represent pseudogenes. Hmgi-rs1 is located on Chromosome 11; it was present in all of the standard laboratory inbred mouse strains examined but was absent in wild-derived inbred strains of Mus spretus, M. musculus castaneus, and M. m. molossinus. Hmgi-rs2 was found only in M. m. castaneus and is located on Chromosome 6. Hmgi genes have not been previously mapped in any species, but the location of the probable functional gene on murine Chromosome 17 suggests that the homologous gene in humans is located on Chromosome 6.

Synergistic effect of histone H1 and nucleolin on chromatin condensation in mitosis: role of a phosphorylated heteromer

Repeated motifs, rich in basic residues, are characteristic of both the N-terminal domain of the nucleolus-specific protein, nucleolin, and the second half of the C-terminal domain of histone H1. These repeats are also the target for phosphorylation by the mitosis-specific p34cdc2 kinase. We have previously shown that synthetic peptides [(KTPKKAKKP)2 for histone H1 and (ATPAKKAA)2 for nucleolin] corresponding to these two repeated motifs are able to act in synergy to induce DNA hypercondensation (Erard et al., 1990). In order to determine the molecular basis of this synergistic interaction, we have studied the condensation of the homopolymer poly(dA).poly(dT) in the presence of the two synthetic peptides. Circular dichroism has been used to monitor the psi (+)-type condensation and has revealed that phosphorylation enhances the synergistic effect of the two peptides. Analysis of different combinations of the two peptides suggests that there is a direct interaction between them which is stabilized by phosphorylation. Furthermore, there is a striking correlation between the degree of homopolymer condensation and the stability of the heteromeric complex. Phosphorylation takes place on the threonine residues on the repeat motifs within a region which is likely to adopt a beta-turn structure. Circular dichroism and infrared spectroscopy provide evidence that phosphorylation stabilizes the beta-turn structure of both peptides, and computer modeling shows that this may be due to steric hindrance imposed by the phosphate group. We suggest that phosphorylated nucleolin and histone H1 interact through their homologous domain structured in beta-spirals in order to condense certain forms of DNA during mitosis.

Comparison of multiple forms of the high mobility group I proteins in rodent and human cells. Identification of the human high mobility group I-C protein

The class I of the high mobility group (HMG) proteins is formed by phosphoproteins which are associated with AT-rich DNA sequences in the nucleus. Three HMGI proteins have previously been described in proliferating rodent cells (HMG Y, HMG I and HMGI-C). All three proteins exhibit microheterogeneity. The microheterogeneity of mouse HMG Y has been investigated in detail and shown to be due to phosphorylation of the protein which is sensitive to alkaline-phosphatase treatment. HMG I is similarly modified. Human cells have up to now only been found to contain HMG Y and HMG I. A search for the third protein, HMGI-C, in human cells was carried out and the protein was found in a hepatoma cell line, but not in normal or transformed T-cells. This HMGI-C protein was found to be modified by phosphorylation, part of which was found to be phosphatase insensitive. An unexpected additional finding in this study was that human cells contain two HMG17 proteins which differ in their N-terminal primary sequences.

High-mobility-group proteins P1, I and Y as substrates of the M-phase-specific p34cdc2/cyclincdc13 kinase

All dividing cells entering the M phase of the cell cycle undergo the transient activation of an M-phase-specific histone H1 kinase which was recently shown to be constituted of at least two subunits, p34cdc2 and cyclincdc13. The DNA-binding high-mobility-group (HMG) proteins 1, 2, 14, 17, I, Y and an HMG-like protein, P1, were investigated as potential substrates of H1 kinase. Among these HMG proteins, P1 and HMG I and Y are excellent substrates of the M-phase-specific kinase obtained from both meiotic starfish oocytes and mitotic sea urchin eggs. Anticyclin immunoprecipitates, extracts purified on specific p34cdc2-binding p13suc1-Sepharose and affinity-purified H1 kinase display strong HMG I, Y and P1 phosphorylating activities, demonstrating that the p34cdc2/cyclincdc13 complex is the active kinase phosphorylating these HMG proteins. HMG I and P1 phosphorylation is competitively inhibited by a peptide mimicking the consensus phosphorylation sequence of H1 kinase. HMG I, Y and P1 all possess the consensus sequence for phosphorylation by the p34cdc2/cyclincdc13 kinase (Ser/Thr-Pro-Xaa-Lys/Arg). HMG I is phosphorylated in vivo at M phase on the same sites phosphorylated in vitro by H1 kinase. P1 is phosphorylated by H1 kinase on sites different from the sites of phosphorylation by casein kinase II. The three thermolytic phosphopeptides of P1 phosphorylated in vitro by purified H1 kinase are all present in thermolytic peptide maps of P1 phosphorylated in vivo in proliferating HeLa cells. These phosphopeptides are absent in nonproliferating cells. These results demonstrate that the DNA-binding proteins HMG I, Y and P1 are natural substrates for the M-phase-specific protein kinase. The phosphorylation of these proteins by p34cdc2/cyclincdc13 may represent a crucial event in the intense chromatin condensation occurring as cells transit from the G2 to the M phase of the cell cycle.

Expression of mRNAs encoding mammalian chromosomal proteins HMG-I and HMG-Y during cellular proliferation

The high mobility group chromosomal proteins HMG-I and HMG-Y are closely related isoforms that are expressed at high levels in rapidly dividing, undifferentiated mammalian cells. We analyzed HMG-I/Y mRNA levels at various cell cycle stages in murine NIH/3T3 fibroblasts partially synchronized by seeding from quiescent, contact-inhibited cultures. Flow microfluorometric analysis of DNA content demonstrated a comparable degree of synchronization in such seeded NIH/3T3 cell populations as is obtained by serum deprivation or other means and has the added advantage of avoiding the use of possibly detrimental inhibitors or metabolic starvation to induce such synchrony. We show that HMG-I/Y mRNA levels gradually increase in NIH/3T3 cells during the first 16 h after seeding (G0/G1 to late S phase), but thereafter remain constant, in contrast to the cell cycle-regulated expression of the histone H3 gene. Although there is a 6-fold increase in HMG-I/Y expression during the transition from quiescent to proliferating NIH/3T3 cells, there is a much greater difference in expression (15- to 50-fold) among different cell types, possibly related to their state of differentiation. The HMG-I/Y mRNAs appear to be very stable; there was no decrease in their levels 6 h after actinomycin D transcription termination. The proportion of HMG-I to HMG-Y mRNAs was greater in the human than in the murine cells examined, appeared to be greater in proliferating than in quiescent cells, and did not always correspond with the HMG-I to HMG-Y protein ratio.

Netropsin, distamycin and berenil interact differentially with a high-affinity binding site for the high mobility group protein HMG-I

Netropsin, distamycin, berenil and the chromosomal protein HMG-I share the ability to bind preferentially to AT-rich regions of DNA. We studied the binding behaviour of the chemical agents towards a high-affinity binding site for HMG-I by DNase I and MPE footprinting and analyzed their ability to challenge HMG-I-DNA complexes by competition experiments. Significant differences in the binding affinities and in the efficiencies to abolish HMG-I-DNA complexes were observed for the three drugs. Netropsin proved to be the most avidly binding compound and the most efficient competitor raising the interesting possibility that netropsin affects cell growth by interfering with HMG-I-DNA interaction.

Characterization of the translocation breakpoint sequences in Philadelphia-positive acute lymphoblastic leukemia

We have previously described a patient in whom the breakpoint occurred within the first intron of the BCR gene and have cloned the 9q+ and 22q- junctions. We have now determined the nucleotide sequence around the breakpoints on both translocation products from this patient as well as the corresponding regions from the normal chromosomes 9 and 22. We have compared the sequence with that of the breakpoint regions in the Ph1-positive leukemic patients in order to check for the presence of conserved motifs. A + T-rich sequences and ALU repeat elements are the only sequence characteristics which appear to be very common around translocation regions. The chromosome 9 ABL sequences at or adjacent to the breakpoints present in the 22q- product show homology to the consensus ALU sequence while the chromosome 22 sequences do not, suggesting a non-homologous recombination mechanism. While no sequences are deleted, there is a two-base-pair "homology" at the junction. Therefore, staggered breaks followed by ligation and repair could be part of the mechanism involved in the process of translocation in some cases of Ph1-positive ALL.

Identification of sites on chromosomal protein HMG-I phosphorylated by casein kinase II

The amino acid sequence of a region on chromosomal protein HMG-I from human cells that is phosphorylated by casein kinase II has been determined. The sequence is: Leu-Glu-Lys-Glu-Glu-Glu-Glu-Gly-Ile-Ser-Gln-Glu-Ser(P)-Ser(P)-Glu-Glu-Gl u-Gln. It corresponds to the C-terminal residues 90-107 of HMG-I [(1989) Mol. Cell. Biol. 9, 2114-2123]. Sequence analysis of the native peptide (90-107) after treatment, which specifically converts phosphoserine residues to S-ethylcysteine, revealed that 70-80% of serine residues 102 and 103 were phosphorylated in vivo. Both residues were fully phosphorylated in vitro by incubation with casein kinase II. These results suggest that casein kinase II is involved in the regulation of HMG-I function in the cells.

Analysis of the HMGI nuclear proteins in mouse neoplastic cells induced by different procedures

Four malignant tumors induced in mouse by different experimental procedures were compared as regards their high-mobility-group (HMG) proteins. All tumors showed the complete set of three HMG proteins which we call HMGI-C, I-D, and I-E. The presence of the three HMGI proteins is a characteristic of the transformed phenotype regardless of whether the tumor was chemically, virally, or spontaneously derived. However, the level of expression of the HMGI proteins is not constant in the four tumors. Using reverse-phase HPLC, the individual HMGI proteins were isolated from the spontaneously derived tumor (Lewis lung carcinoma) and shown by amino acid analysis to be similar to those previously obtained from a tumor grown in nude mice by inoculation of in vitro-transformed cells.