Molecular cloning of an intronless gene for the hamster centromere antigen CENP-B

Featured structure-activity relationships for some tri- and tetrachlorobiphenyls in human CYP2E1-activated mutagenicity - Impact of the extent of ortho-chlorination

Polychlorinated biphenyls (PCBs) as a group of persistent organic pollutants are confirmed human carcinogens; however, their mutagenicity remains mostly unknown. We have reported the mutagenicity of some PCBs with one to four chlorines in mammalian cells expressing human CYP2E1. To further explore the structural requirements for the mutagenicity of PCBs, eight tri- and tetrachlorobiphenyls untested before were investigated for the induction of gene mutations and micronuclei in a V79-derived cell line expressing both human CYP2E1 and sulfotransferase (SULT) 1A1 (V79-hCYP2E1-hSULT1A1), with SULT1A1 activity inhibited by pentachlorophenol, a potent SULT1 inhibitor. 2,2',6-Tri-, 2,3',6-tri, 2,4',6-tri-, and 2,2',5-trichlorobiphenyls (PCBs 19, 27, 32, and 18, respectively) induced micronuclei and gene mutations in V79-hCYP2E1-hSULT1A1 cells, at potencies slightly higher than 2,6-dichlorobiphenyl, but one order of magnitude below that by 2,3,3'- and 2,3,4'-trichlorobiphenyls as reported recently; in the parental V79-Mz cells, they were nonmutagenic and weak in micronuclei induction. Among the four tetrachlorobiphenyls with varying number of ortho chlorines, 2,3,3',4'-tetrachlorobiphenyl (PCB 56) induced both micronuclei and gene mutations in V79-hCYP2E1-hSULT1A1 cells with a potency greater than the above compounds; however, 2,2',3,3'-tetrachlorobiphenyl was equivocal and 2,2',3,6'-tetra- and 2,2',6,6'-tetrachlorobiphenyls inactive in V79-hCYP2E1-hSULT1A1 cells. Immunofluorescent staining of micronuclei formed by PCBs 32 and 56 in V79-hCYP2E1-hSULT1A1 cells with centromere protein B antibodies indicated that they were predominantly whole chromosomes, implying aneugenic potentials. This study suggests that tri- and tetrachlorobiphenyls with a single ortho chlorine can be most mutagenic under activation by human CYP2E1, and greater numbers of ortho chlorines may cause a drastic decline in the activity, especially for tetrachlorobiphenyls.

Convergent domestication of pogo-like transposases into centromere-binding proteins in fission yeast and mammals

The mammalian centromere-associated protein B (CENP-B) shares significant sequence similarity with 3 proteins in fission yeast (Abp1, Cbh1, and Cbh2) that also bind centromeres and have essential function for chromosome segregation and centromeric heterochromatin formation. Each of these proteins displays extensive sequence similarity with pogo-like transposases, which have been previously identified in the genomes of various insects and vertebrates, in the protozoan Entamoeba and in plants. Based on this distribution, it has been proposed that the mammalian and fission yeast centromeric proteins are derived from "domesticated" pogo-like transposons. Here we took advantage of the vast amount of sequence information that has become recently available for a wide range of fungal and animal species to investigate the origin of the mammalian CENP-B and yeast CENP-B-like genes. A highly conserved ortholog of CENP-B was detected in 31 species of mammals, including opossum and platypus, but was absent from all nonmammalian species represented in the databases. Similarly, no ortholog of the fission yeast centromeric proteins was identified in any of the various fungal genomes currently available. In contrast, we discovered a plethora of novel pogo-like transposons in diverse invertebrates and vertebrates and in several filamentous fungi. Phylogenetic analysis revealed that the mammalian and fission yeast CENP-B proteins fall into 2 distinct monophyletic clades, each of which includes a different set of pogo-like transposons. These results are most parsimoniously explained by independent domestication events of pogo-like transposases into centromeric proteins in the mammalian and fission yeast lineages, a case of "convergent domestication." These findings highlight the propensity of transposases to give rise to new host proteins and the potential of transposons as sources of genetic innovation.

Titin as a chromosomal protein

We identified titin as a chromosomal protein using a human autoimmune scleroderma serum. We cloned the corresponding gene in the fruitfly, Drosophila melanogaster. We have demonstrated that titin is not only expressed and localized in striated muscle but is also distributed uniformly on condensed mitotic chromosomes using multiple antibodies directed against different domains of both Drosophila and vertebrate titin. Titin is a giant sarcomeric protein responsible for the elasticity of striated muscle. Titin may also function as a molecular scaffold during myofibril assembly. We hypothesize that titin is a component of chromosomes that may function to determine chromosome structure and provide elasticity, playing a role similar to that proposed for titin in muscle. We have identified mutations in Drosophila Titin (D-Titin) and are characterizing phenotypes in muscle and chromosomes.

Isolation of a species-specific satellite DNA with a novel CENP-B-like box from the North African rodent Lemniscomys barbarus

A species-specific satellite DNA (Lb-MspISAT) was isolated from the North African rodent Lemniscomys barbarus. This DNA is highly homogeneous in the sequence of different repeats and shows no internal repetitions. Filter and in situ hybridizations demonstrated that it is tandemly repeated at the centromeres of all chromosomes of the complement. A 19-bp CENP-B-like motif was found in Lb-MspISAT which conserves 12 of the 17-bp of the human CENP-B box, but only 5 of the 9-bp of the canonical sequence that is necessary to bind the CENP-B protein. Compared with the human CENP-B box, nucleotide substitutions and insertions increase the palindromic structure of this motif. The possibilities that it may be involved in centromeric function or in homogenization of the Lb-MspISAT sequence are discussed.

Centromeric protein B null mice are viable with no apparent abnormalities

The centromere protein B (CENP-B) is a centromeric DNA/binding protein. It recognizes a 17-bp sequence motif called the CENP-B box, which is found in the centromeric region of most chromosomes. It binds DNA through its amino terminus and dimerizes through its carboxy terminus. CENP-B protein has been proposed to perform a vital role in organizing chromatin structures at centromeres. However, other evidence does not agree with this view. For example, CENP-B is found at inactive centromeres on stable dicentric chromosomes, and also mitotically stable chromosomes lacking alpha-satellite DNA have been reported. To address the biological function of CENP-B, we generated mouse null mutants of CENP-B by homologous recombination. Mice lacking CENP-B were viable and fertile, indicating that mice without CENP-B undergo normal somatic and germline development. Thus, both mitosis and meiosis are able to proceed normally in the absence of CENP-B.

Human autoantibodies reveal titin as a chromosomal protein

Assembly of the higher-order structure of mitotic chromosomes is a prerequisite for proper chromosome condensation, segregation and integrity. Understanding the details of this process has been limited because very few proteins involved in the assembly of chromosome structure have been discovered. Using a human autoimmune scleroderma serum that identifies a chromosomal protein in human cells and Drosophila embryos, we cloned the corresponding Drosophila gene that encodes the homologue of vertebrate titin based on protein size, sequence similarity, developmental expression and subcellular localization. Titin is a giant sarcomeric protein responsible for the elasticity of striated muscle that may also function as a molecular scaffold for myofibrillar assembly. Molecular analysis and immunostaining with antibodies to multiple titin epitopes indicates that the chromosomal and muscle forms of titin may vary in their NH2 termini. The identification of titin as a chromosomal component provides a molecular basis for chromosome structure and elasticity.

Centromere protein B null mice are mitotically and meiotically normal but have lower body and testis weights

CENP-B is a constitutive centromere DNA-binding protein that is conserved in a number of mammalian species and in yeast. Despite this conservation, earlier cytological and indirect experimental studies have provided conflicting evidence concerning the role of this protein in mitosis. The requirement of this protein in meiosis has also not previously been described. To resolve these uncertainties, we used targeted disruption of the Cenpb gene in mouse to study the functional significance of this protein in mitosis and meiosis. Male and female Cenpb null mice have normal body weights at birth and at weaning, but these subsequently lag behind those of the heterozygous and wild-type animals. The weight and sperm content of the testes of Cenpb null mice are also significantly decreased. Otherwise, the animals appear developmentally and reproductively normal. Cytogenetic fluorescence-activated cell sorting and histological analyses of somatic and germline tissues revealed no abnormality. These results indicate that Cenpb is not essential for mitosis or meiosis, although the observed weight reduction raises the possibility that Cenpb deficiency may subtly affect some aspects of centromere assembly and function, and result in reduced rate of cell cycle progression, efficiency of microtubule capture, and/or chromosome movement. A model for a functional redundancy of this protein is presented.

A helix-turn-helix structure unit in human centromere protein B (CENP-B)

CENP-B has been suggested to organize arrays of centromere satellite DNA into a higher order structure which then directs centromere formation and kinetochore assembly in mammalian chromosomes. The N-terminal portion of CENP-B is a 15 kDa DNA binding domain (DBD) consisting of two repeating units, RP1 and RP2. The DBD specifically binds to the CENP-B box sequence (17 bp) in centromere DNA. We determined the solution structure of human CENP-B DBD RP1 by multi-dimensional 1H, 13C and 15N NMR methods. The CENP-B DBD RP1 structure consists of four helices and has a helix-turn-helix structure. The overall folding is similar to those of some other eukaryotic DBDs, although significant sequence homology with these proteins was not found. The DBD of yeast RAP1, a telomere binding protein, is most similar to CENP-B DBD RP1. We studied the interaction between CENP-B DBD RP1 and the CENP-B box by the use of NMR chemical shift perturbation. The results suggest that CENP-B DBD RP1 interacts with one of the essential regions of the CENP-B box DNA, mainly at the N-terminal basic region, the N-terminal portion of helix 2 and helix 3.

Nucleotide specificity at the boundary and size requirement of the target sites recognized by human centromere protein B (CENP-B) in vitro

Human centromere protein B (CENP-B) has a sequence-specific DNA binding activity. We previously reported several CENP-B binding motifs by analysing synthetic oligonucleotides as well as alphoid DNA isolated from the human genomic library. Here, we examined the size requirement and nucleotide specificity of human CENP-B binding sequences in vitro. We synthesized three sets of mixed oligonucleotides containing diverged authentic binding sites (CTTCGTTGGAAACGGGA) in which certain pairs of nucleotides (underlined) were degenerated. Each oligonucleotide with a defined sequence was separately introduced into a plasmid and mixed with GST-fused recombinant CENP-B. The DNA-protein complex formed was affinity purified with glutathione Sepharose. Any nucleotide substitutions at the positions 1, 2 and 17 did not significantly influence the recovery, while the substitutions at positions 3, 4 and 16 did, suggesting that the internal 14-bp motif (TCGTTGGAAACGGG) constituted the minimum requirement. However, it showed a lower affinity to CENP-B, compared with the authentic motif. The inclusion of T at the 5' end greatly increased the affinity, and the further addition of A or T at the 3' end (TTCGTTGGAAACGGGA/T) offered affinity similar to the authentic motif. The first nucleotide of the 17-bp authentic binding motif may not be essential for CENP-B binding.

A novel centromere monospecific serum to a human autoepitope on the histone H3-like protein CENP-A

Centromere autoantibodies are commonly found in the serum of patients with some systemic autoimmune diseases. Previous studies have shown that a major human centromere autoantigen is the histone H3-like protein CENP-A. Although the human cDNA has been cloned, native CENP-A has been neither isolated nor expressed in Escherichia coli, and specific antibodies to this chromatin-associated centromere protein are not available yet. In this report, a highly charged peptide on CENP-A (residues 3-17) was used to generate a monospecific antibody that reacts by immunoblots with the 17 kDa centromeric protein. Immunofluorescence analysis showed reactivity of this anti-CENP-A serum in several but not all mammalian culture cells analyzed, suggesting that the sequence of this histone-like centromere protein could be more variable throughout evolution than originally thought. Selective extractions of human placenta nuclear proteins and immunoblot analysis indicated that CENP-A behaves in a similar way to the core histone polypeptides after nuclease digestion of chromatin. Also, immunoblot analysis demonstrated that the CENP-A peptide used as immunogen is a target region on the CENP-A molecule in several but not all CREST patients analyzed with high titers of autoantibodies to the centromere. Lastly, we found that in Jurkat cells induced to apoptosis, CENP-A remains associated with the centromere, in contrast to other human autoantigens studied during apoptosis.

The distribution of binding sites for centromere protein B (CENP-B) is partly conserved among diverged higher order repeating units of human chromosome 6-specific alphoid DNA

We previously reported the isolation of alphoid satellite clones from a human genomic library using a DNA immunoprecipitation with centromere protein B (CENP-B). Here, we have characterized the distribution of CENP-B-binding sites on the 3-kb BamHI repeats of the cos2 clone. Using in situ hybridization, this alphoid satellite was located primarily at the centromeric region of chromosome 6. The functional binding sites were mapped by precipitating the restriction fragments with recombinant CENP-B in vitro. One repeat (2B3-11) consisted of 19 copies of alphoid monomer, eight of which possessed the binding sites, while another (2B3-9) consisted of 18 copies of the monomer, seven of which possessed the binding sites. The distribution of the sites was well conserved between them, except for the terminus. A similar analysis with the remaining 6-kb region suggested the presence of a continuous 1-kb region with regular spacing of EcoRI sites and the CENP-B-binding sites. When the nucleotide sequence of 2B3-11 was compared with that of another chromosome 6-specific alphoid repeat (p308) that had been described previously, this 1-kb region was highly conserved between them. The distribution of the CENP-B binding sites and the order of alphoid monomers might define the folding of alphoid repeats in the centromeric region.

Cloning, mapping, and tissue distribution of a human homologue of the mouse jerky gene product

Inactivation of the jerky gene by insertion of a transgene into the mouse genome results in epileptic seizures in transgenic mice. This finding indicates that the jerky gene plays an important role in inducing epilepsy syndromes in mice. We report here our efforts in cloning, chromosomal mapping, and analysis of tissue distribution of a novel human gene, the HHMJG, a homologue to the mouse jerky gene product. We have successfully identified a full length cDNA clone encoding a novel human protein homologous to the mouse jerky gene product. The finding was based on the result of an analysis of EST (expressed sequence tag) sequences of a clone from a human tonsil cDNA library. A 4.0 kb mRNA species of the HHMJG is abundantly expressed in the majority of human tissues examined, including brain and skeletal muscle. However, in the testes, two mRNA species of the HHMJG, approximately 2.0 and 4.0 kb, are abundantly expressed. Sequence analysis of the HHMJG cDNA indicates that it encodes a putative protein of 51 kD, which shares significant sequence homology to not only the mouse jerky gene product but also some nuclear regulatory proteins, such as centromere binding protein-B. The predicted nuclear localization of the HHMJG product suggests that this protein may function as a nuclear regulatory protein. The result of human chromosomal mapping shows that the HHMJG is located on human chromosome 11q21. Our identification of the HHMJG cDNA provides a potential gene candidate to further investigate the biological significance and clinical implications of the HHMJG in human epilepsy.