Epidermal growth factor receptor is a cellular receptor for human cytomegalovirus

Human cytomegalovirus (HCMV) is a widespread opportunistic herpesvirus that causes severe and fatal diseases in immune-compromised individuals, including organ transplant recipients and individuals with AIDS. It is also a leading cause of virus-associated birth defects and is associated with atherosclerosis and coronary restenosis. HCMV initiates infection and intracellular signalling by binding to its cognate cellular receptors and by activating several signalling pathways including those mediated by mitogen-activated protein kinase, phosphatidylinositol-3-OH kinase, interferons, and G proteins. But a cellular receptor responsible for viral entry and HCMV-induced signalling has yet to be identified. Here we show that HCMV infects cells by interacting with epidermal growth factor receptor (EGFR) and inducing signalling. Transfecting EGFR-negative cells with an EGFR complementary DNA renders non-susceptible cells susceptible to HCMV. Ligand displacement and crosslinking analyses show that HCMV interacts with EGFR through gB, its principal envelope glycoprotein. gB preferentially binds EGFR and EGFR-ErbB3 oligomeric molecules in Chinese hamster ovary cells transfected with erbB family cDNAs. Taken together, these data indicate that EGFR is a necessary component for HCMV-triggered signalling and viral entry.     

Identification of gamma-tubulin, a new member of the tubulin superfamily encoded by mipA gene of Aspergillus nidulans

Microtubules, which are essential for mitosis and many other cytoskeletal functions, are composed primarily of alpha- and beta-tubulin. The properties of microtubules are due, in part, to proteins other than tubulins that are part of, or interact with, microtubules and the identification and characterization of such proteins is important to understanding how microtubules function. Analyses of mutations at the mipA (microtubule interacting protein) locus of Aspergillus nidulans have suggested that the product of mipA interacts specifically, probably physically, with beta-tubulin in vivo and is involved in microtubule function. We have cloned and sequenced the wild-type mipA gene as well as complementary DNA copies of its messenger RNA. Comparisons of the predicted product of mipA with tubulins from diverse organisms reveal that mipA is a previously undiscovered member of the tubulin superfamily of genes; the only member yet discovered that does not encode alpha- or beta-tubulin. We propose that the product of mipA be called gamma-tubulin.     

Neural adhesion molecule L1 as a member of the immunoglobulin superfamily with binding domains similar to fibronectin

Diverse glycoproteins of cell surfaces and extracellular matrices operationally termed 'adhesion molecules' are important in the specification of cell interactions during development, maintenance and regeneration of the nervous system. These adhesion molecules have distinct functions involving different cells at different developmental stages, but may cooperate when expressed together. Families of adhesion molecules which share common carbohydrate domains do exist, despite the structural and functional diversity of these glycoproteins. These include the Ca2+-independent neural adhesion molecules: N-CAM, myelin associated glycoprotein (MAG) and L1. L1 is involved in neuron-neuron adhesion, neurite fasciculation, outgrowth of neurites, cerebellar granule cell migration, neurite outgrowth on Schwann cells and interactions among epithelial cells of intestinal crypts. We show here that in addition to sharing carbohydrate epitopes with N-CAM and MAG, L1 is also a member of the immunoglobulin superfamily. It contains six C2 domains and also shares three type III domains with the extracellular matrix adhesion molecule fibronectin.     

Cloning by functional expression of a member of the glutamate receptor family

We have isolated a complementary DNA clone by screening a rat brain cDNA library for expression of kainate-gated ion channels in Xenopus oocytes. The cDNA encodes a single protein of relative molecular mass (Mr) 99,800 which on expression in oocytes forms a functional ion channel possessing the electrophysiological and pharmacological properties of the kainate subtype of the glutamate receptor family in the mammalian central nervous system.     

On the nature of partial agonism in the nicotinic receptor superfamily

Partial agonists are ligands that bind to receptors but produce only a small maximum response even at concentrations where all receptors are occupied. In the case of ligand-activated ion channels, it has been supposed since 1957 that partial agonists evoke a small response because they are inefficient at eliciting the change of conformation between shut and open states of the channel. We have investigated partial agonists for two members of the nicotinic superfamily-the muscle nicotinic acetylcholine receptor and the glycine receptor-and find that the open-shut reaction is similar for both full and partial agonists, but the response to partial agonists is limited by an earlier conformation change ('flipping') that takes place while the channel is still shut. This has implications for the interpretation of structural studies, and in the future, for the design of partial agonists for therapeutic use.     

Identification and characterization of a novel member of the nerve growth factor/brain-derived neurotrophic factor family

The survival and functional maintenance of vertebrate neurons critically depends on the availability of specific neurotrophic factors. So far, only two such factors, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) have been characterized and shown to have the typical features of secretory proteins. This characterization has been possible because of the extraordinarily large quantities of NGF in some adult tissues, and the virtually unlimited availability of brain tissue from which BDNF was isolated. Both NGF and BDNF promote the survival of distinct neuronal populations in vivo and are related in their primary structure, suggesting that they are members of a gene family. Although there is little doubt about the existence of other such proteins, their low abundance has rendered their identification and characterization difficult. Taking advantage of sequence identities between NGF and BDNF, we have now identified a third member of this family, which we name neurotrophin-3. Both the tissue distribution of the messenger RNA and the neuronal specificity of this secretory protein differ from those of NGF and BDNF. Alignment of the sequences of the three proteins reveals a remarkable number of amino acid identities, including all cysteine residues. This alignment also delineates four variable domains, each of 7-11 amino acids, indicating structural elements presumably involved in the neuronal specificity of these proteins.     

Proteolytic turnover of the Gal4 transcription factor is not required for function in vivo

Transactivator-promoter complexes are essential intermediates in the activation of eukaryotic gene expression. Recent studies of these complexes have shown that some are quite dynamic in living cells owing to rapid and reversible disruption of activator-promoter complexes by molecular chaperones, or a slower, ubiquitin-proteasome-pathway-mediated turnover of DNA-bound activator. These mechanisms may act to ensure continued responsiveness of activators to signalling cascades by limiting the lifetime of the active protein-DNA complex. Furthermore, the potency of some activators is compromised by proteasome inhibition, leading to the suggestion that periodic clearance of activators from a promoter is essential for high-level expression. Here we describe a variant of the chromatin immunoprecipitation assay that has allowed direct observation of the kinetic stability of native Gal4-promoter complexes in yeast. Under non-inducing conditions, the complex is dynamic, but on induction the Gal4-promoter complexes 'lock in' and exhibit long half-lives. Inhibition of proteasome-mediated proteolysis had little or no effect on Gal4-mediated gene expression. These studies, combined with earlier data, show that the lifetimes of different transactivator-promoter complexes in vivo can vary widely and that proteasome-mediated turnover is not a general requirement for transactivator function.     

X-linkage of steroid sulphatase in the mouse is evidence for a functional Y-linked allele

In the human there is an X-linked gene affecting steroid sulphatase (STS) activity which, when deficient, is associated with X-linked congenital ichthyosis. The gene (STS) is located on the distal tip of the short arm and is only partially inactivated when it is on the inactive X-chromosome. In the mouse, the genetics of STS are not clear; the results of one study using XX:X0 oocyte comparisons indicated X-linkage, but three other studies using STS variants have produced segregation data compatible with autosomal linkage of murine STS. Here we present the results of STS assays of crosses of deficient C3H/An male mice to normal X0 animals which demonstrate X-linkage of STS in the mouse and indirectly indicate the existence of a functional STS allele on the Y-chromosome which undergoes obligatory recombination during meiosis with the X-linked allele.