Nuclear migration and positioning within cells are critical for many developmental processes and are governed by the cytoskeletal network. Although mechanisms of nuclear-cytoskeletal attachment are unclear, growing evidence links a novel family of nuclear envelope (NE) proteins that share a conserved C-terminal SUN (Sad1/UNC-84 homology) domain. Analysis of Caenorhabditis elegans mutants has implicated UNC-84 in actin-mediated nuclear positioning by regulating NE anchoring of a giant actin-binding protein, ANC-1. Here, we report the identification of SUN1 as a lamin A-binding protein in a yeast two-hybrid screen. We demonstrate that SUN1 is an integral membrane protein located at the inner nuclear membrane. While the N-terminal domain of SUN1 is responsible for detergent-resistant association with the nuclear lamina and lamin A binding, lamin A/C expression is not required for SUN1 NE localization. Furthermore, SUN1 does not interact with type B lamins, suggesting that NE localization is ensured by binding to an additional nuclear component(s), most likely chromatin. Importantly, we find that the luminal C-terminal domain of SUN1 interacts with the mammalian ANC-1 homologs nesprins 1 and 2 via their conserved KASH domain. Our data provide evidence of a physical nuclear-cytoskeletal connection that is likely to be a key mechanism in nuclearcytoplasmic communication and regulation of nuclear position.The nuclear envelope (NE) is a double-membrane structure that separates chromatin from the cytoplasm, thereby allowing regulation of DNA replication and gene expression in eukaryotic cells. Nuclear pore complexes span the double membrane and regulate the passage of molecules between the cytoplasm and the nucleus (16). The outer nuclear membrane (ONM) is contiguous with, and biochemically similar to, the endoplasmic reticulum (ER). In contrast, the inner nuclear membrane (INM) contains a unique set of integral membrane proteins. Both nuclear pore complexes and INM proteins are anchored by association with the nuclear lamina, a network of lamin intermediate filaments that underlies the INM. The lamina, together with the associated INM proteins, provides structural support for the NE and sites for attachment of chromatin to the nuclear periphery (reviewed in reference 11).Most mammalian cells express two classes of lamin protein, types A and B (reviewed in reference 26). A-type lamins, the major isoforms of which are lamins A and C, are alternative splice products of the LMNA gene (8, 23). B-type lamins are mainly composed of lamins B1 and B2, which are encoded by separate genes (LMNB1 and LMNB2, respectively). A-and B-type lamins differ in their patterns of expression. While type B lamins are found in all nucleated somatic cells, type A lamins are absent in early embryos, their expression correlating with terminal differentiation (33). This has led to the suggestion that A-type lamins, although not essential for individual cell survival, are involved in determining differentiation patterns, possibly through effects on c...

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Transcription of herpes simplex virus (HSV) immediate-early (IE) genes does not occur in sensory neurons latently infected with the virus or following infection of neuronal cell lines. In neuronal cell lines this inability results from the weak activity of the viral IE promoters, which is caused by a neuron-specific repressor factor that binds specifically to the TAATGARAT motif in these promoters and to related octamer elements. Cells expressing this repressor contain an additional octamer-binding protein that is absent from permissive cells. We identify this factor as the lymphocyte- and neuron-specific octamer-binding protein Oct-2 and show that Oct-2 mRNA is also present in dorsal root ganglion neurons, the natural site of HSV latency in vivo. Moreover, artificially elevated expression of Oct-2 can repress the IE promoter. The potential role of Oct-2 in the initiation and maintenance of in vivo latent infection with HSV is discussed.

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We have isolated several new clones of human ribosomal DNA. Each clone contains part of the external transcribed spacer, a complete 18 S-rRNA gene, the internal transcribed spacers, a complete 28 S-rRNA gene and a short downstream flanking region. We present a detailed map of the human ribosomal transcription unit with the locations of numerous useful restriction sites. In particular, a unique NheI site in the 5.8 S-rRNA gene enabled this gene to be mapped with respect to the 18 S-rRNA and 28 S-rRNA genes. The human 45 S-rRNA coding region is approx. 13,000 nucleotide residues long, of which the external transcribed spacer comprises approx. 3700 nucleotide residues and the first and second internal transcribed spacers comprise approx. 1070 and 1200 nucleotide residues respectively. A partial survey for sites of variation between clones has revealed a single point of variation among 18 S-rRNA gene sequences (a T/C variation at position 140), several sites of length variation in the regions of the transcribed spacers closely flanking the 18 S-rRNA genes, and some sites of length variation among 28 S-rRNA genes. Most of these sites of variation are associated with simple sequence tracts and are in regions that are known to undergo relatively rapid evolutionary divergence. In particular, the sites of variation among 28 S-rRNA genes occur in G + C-rich tracts whose lengths vary among vertebrates and that can be correlated with extensive hairpin structures previously observed by electron microscopy. Each of the clones so far surveyed in detail differs from the others in one or more respects.