Human telomeres contain two distinct Myb-related proteins, TRF1 and TRF2

Human telomeres are composed of long arrays of TTAGGG repeats that form a nucleoprotein complex required for the protection and replication of chromosome ends. One component of human telomeres is the TTAGGG repeat binding factor 1 (TRF1), a ubiquitously expressed protein, related to the protooncogene Myb, that is present at telomeres throughout the cell cycle. Recent evidence has implicated TRF1 in the control of telomere length. TRF1 is proposed to be an inhibitor of telomerase, acting in cis to limit the elongation of individual chromosome ends. Here we report the cloning of TRF2, a distant homologue of TRF1 that carries a very similar Myb-related DNA-binding motif. Like TRF1, TRF2 was ubiquitously expressed, bound specifically to duplex TTAGGG repeats in vitro and localized to all human telomeres in metaphase chromosomes. TRF2 was shown to have an architecture similar to that of TRF1 in that it carries a C-terminal Myb motif and a large TRF1-related dimerization domain near its N terminus. However, the dimerization domains of TRF1 and TRF2 did not interact, suggesting that these proteins exist predominantly as homodimers. While having similar telomere binding activity and domain organization, TRF2 differed from TRF1 in that its N terminus was basic rather than acidic, and TRF2 was much more conserved than TRF1. The results indicate that the TTAGGG repeat arrays at the ends of human and mouse chromosomes bind to two related proteins. Because TRF1 and TRF2 showed significant differences, we suggest that these factors have distinct functions at telomeres.

Identification of XLerk, an Eph family ligand regulated during mesoderm induction and neurogenesis in Xenopus laevis

We have isolated and characterized the first Xenopus transmembrane Eph ligand, XLerk (Xenopus Ligand for Eph Receptor Tyrosine Kinases). While this ligand has 72% identity with the closest mammalian family member, Lerk-2, it is the cytoplasmic domain of this molecule that is the most conserved domain with 95% identity. XLerk exists as a maternally expressed mRNA, however, expression of transcripts and protein increase during gastrulation and again in the late swimming tadpole stage. In the adult, XLerk is expressed at low levels in most adult tissues with increased levels observed in the kidney, oocytes, ovary and testis. While low levels of XLerk expression are observed in the adult brain, in situ hybridization analysis demonstrates prominent expression in the developing olfactory system, retina, hindbrain, cranial ganglia, and somites. Furthermore, we have shown that XLerk transcripts are significantly elevated during mesoderm induction caused by activin and FGF, but not during noggin-induced neuralization. These results suggest a role for XLerk in the developing mesenchymal and nervous tissue.

Oral lymphoma in HIV infection

OBJECTIVE

This study reports the main clinical and histological features, and the treatment and outcomes of six HIV-associated oral lymphomas.

SUBJECTS

During the period 1991-1995, six cases of lymphoma presenting in the oral cavity were diagnosed in this clinic. All six patients were men ranging in age from 36 to 62 years with CD4 counts below 100 cells mm-3.

RESULTS

In five patients, oral lymphoma was the first AIDS defining disease. Four lesions occurred on the gingiva, one on the palate and one on the tongue. The duration of symptoms prior to presentation ranged from 2 weeks to 14 months. In all but one patient, the lesions presented as red, exophytic masses and were diagnosed clinically as Kaposi's sarcoma. None of the five lesions presenting on the palate or gingiva showed radiographic evidence of bone destruction. Metastatic work-up failed to diagnose lymphoma elsewhere in the five cases where this was performed. Histological diagnoses included two small, non-cleaved (Burkitt's type) lymphomas, two immunoblastic lymphomas, one large cell lymphoma and one small, non-cleaved (non-Burkitt's) lymphoma.

CONCLUSION

These findings highlight the uncommon clinical presentation of this disease and despite its relative rarity, demonstrate the need to maintain a high degree of suspicion in any unusual oral lesion in this group of patients.

TRF1 is a dimer and bends telomeric DNA

TRF1 is a mammalian telomeric protein that binds to the duplex array of TTAGGG repeats at chromosome ends. TRF1 has homology to the DNA-binding domain of the Myb family of transcription factors but, unlike most Myb-related proteins, TRF1 carries one rather than multiple Myb-type DNA-binding motifs. Here we show that TRF1 binds DNA as a dimer using a large conserved domain near the N-terminus of the protein for TRF1-TRF1 interactions. Dimerization was observed both in a complex with DNA and in the yeast two-hybrid assay. TRF1 dimers were found to require both Myb repeats for the formation of a stable complex with DNA, indicating a parallel between the DNA-binding mode of TRF1 and other Myb-related proteins. TRF1 was found to have a number of biochemical similarities to Rap1p, a distantly related DNA-binding protein that functions at telomeres in yeast. Rap1p and TRF1 both require two Myb motifs for DNA binding and both factors bind along their cognate telomeric sequences without showing strong cooperative interactions between adjacent proteins. Furthermore, TRF1 was found to bend its telomeric site to an angle of -120 degrees. Since Rap1p similarly distorts telomeric DNA, we propose that DNA bending is important for the function of telomeres in yeast and mammals.

Comparison of the human and mouse genes encoding the telomeric protein, TRF1: chromosomal localization, expression and conserved protein domains

Mammalian chromosome ends contain long arrays of TTAGGG repeats that are complexed to a telomere specific protein, the TTAGGG repeat binding factor, TRF1. Here we describe the characterization of genes encoding the human and mouse TRF1 proteins, hTRF1 and mTRF1. The mTRF1 cDNA was isolated based on sequence similarity to the hTRF1 cDNA and the mTRF1 mRNA was shown to be ubiquitously expressed as a single 1.9 kb polyadenylated transcript in mouse somatic tissues. High levels of a 2.1 kb transcript were found in testes. In vitro translation of the mTRF1 cDNA resulted in a 56 kDa protein that binds to TTAGGG repeat arrays. mTRF1 displayed the same sequence specificity as hTRF1, preferring arrays of TTAGGG repeats as a binding substrate over TTAGGC and TTGGGG repeats. Expression of an epitope-tagged version of mTRF1 showed that the protein is located at the ends of murine metaphase chromosomes. In agreement, conceptual translation indicated that mTRF1 and hTRF1 are similarly-sized proteins with nearly identical C-terminal Myb-related DNA binding motifs. In addition, comparison of the predicted mTRF1 and hTRF1 amino acid sequences showed that the acidic nature of the N-terminus of TRF1 is conserved and revealed a highly conserved novel domain of approximately 200 amino acids in the middle of the proteins. However, other regions of the proteins are poorly conserved (<35% identity) and the overall level of identity of the mTRF1 and hTRF1 amino acid sequences is only 67%. The TRF1 genes are not syntenic; the hTRF1 gene localized to human chromosome 8 band q13 while the mTRF1 gene localized to mouse chromosome 17 band E3. The data indicate that the genes for mammalian telomeric proteins evolve rapidly.

Structure, subnuclear distribution, and nuclear matrix association of the mammalian telomeric complex

Mammalian telomeres are composed of long arrays of TTAGGG repeats complexed with the TTAGGG repeat binding factor, TRF. Biochemical and ultrastructural data presented here show that the telomeric DNA and TRF colocalize in individual, condensed structures in the nuclear matrix. Telomeric TTAGGG repeats were found to carry an array of nuclear matrix attachment sites occurring at a frequency of at least one per kb. The nuclear matrix association of the telomeric arrays extended over large domains of up to 20-30 kb, encompassing the entire length of most mammalian telomeres. TRF protein and telomeric DNA cofractionated in nuclear matrix preparations and colocalized in discrete, condensed sites throughout the nuclear volume. FISH analysis indicated that TRF is an integral component of the telomeric complex and that the presence of TRF on telomeric DNA correlates with the compact configuration of telomeres and their association with the nuclear matrix. Biochemical fractionation of TRF and telomeric DNA did not reveal an interaction with the nuclear lamina. Furthermore, ultrastructural analysis indicated that the mammalian telomeric complex occupied sites throughout the nuclear volume, arguing against a role for the nuclear envelope in telomere function during interphase. These results are consistent with the view that mammalian telomeres form nuclear matrix-associated, TRF-containing higher order complexes at dispersed sites throughout the nuclear volume.

A human telomeric protein

Telomeres are multifunctional elements that shield chromosome ends from degradation and end-to-end fusions, prevent activation of DNA damage checkpoints, and modulate the maintenance of telomeric DNA by telomerase. A major protein component of human telomeres has been identified and cloned. This factor, TRF, contains one Myb-type DNA-binding repeat and an amino-terminal acidic domain. Immunofluorescent labeling shows that TRF specifically colocalizes with telomeric DNA in human interphase cells and is located at chromosome ends during metaphase. The presence of TRF along the telomeric TTAGGG repeat array demonstrates that human telomeres form a specialized nucleoprotein complex.

Glycoprotein cytoplasmic domain sequences required for rescue of a vesicular stomatitis virus glycoprotein mutant

We have used transient expression of the wild-type vesicular stomatitis virus (VSV) glycoprotein (G protein) from cloned cDNA to rescue a temperature-sensitive G protein mutant of VSV in cells at the nonpermissive temperature. Using cDNAs encoding G proteins with deletions in the normal 29-amino-acid cytoplasmic domain, we determined that the presence of either the membrane-proximal 9 amino acids or the membrane-distal 12 amino acids was sufficient for rescue of the temperature-sensitive mutant. G proteins with cytoplasmic domains derived from other cellular or viral G proteins did not rescue the mutant, nor did G proteins with one or three amino acids of the normal cytoplasmic domain. Rescue correlated directly with the ability of the G proteins to be incorporated into virus particles. This was shown by analysis of radiolabeled particles separated on sucrose gradients as well as by electron microscopy of rescued virus after immunogold labeling. Quantitation of surface expression showed that all of the mutated G proteins were expressed less efficiently on the cell surface than was wild-type G protein. However, we were able to correct for differences in rescue efficiency resulting from differences in the level of surface expression by reducing wild-type G protein expression to levels equivalent to those observed for the mutated G proteins. Our results provide evidence that at least a portion of the cytoplasmic domain is required for efficient assembly of the VSV G protein into virions during virus budding.

Gnathotrichus sulcatus: synergistic response to enantiomers of the aggregation pheromone sulcatol

In laboratory and field bioassays, Gnathotrichus sulcatus responded to sulcatol (6-methyl-5-hepten-2-ol) only when both enantiomers were present. Response was greater to racemic sulcatol than to a mixture (65 : 35) of S-(+) and R-(-) enantiomers, the naturally occurring isomeric ratio. Enantiomer-specific active sites on receptor proteins in the same or different cells are implicated.