Mutant avian erythroblastosis virus with restricted target cell specificity.

Avian erythroblastosis virus (AEV) induces a fatal erythroblastosis within 2 weeks of intravenous injection in chicks in virtually 100% of cases. In chicks injected intramuscularly, sarcomas frequently develop at the site of injection before the animals die from erythroblastosis. In vitro, AEV transforms both erythroblasts, derived from bone marrow cultures, and fibroblasts. These effects have been shown to be a general property of AEV and not of separate leukaemia- and sarcoma-inducing forms of the virus. AEV is defective for replication and can be propagated only in the prewence of helper virus. Its transformation specificity is independent of the helper virus used. It is not clear whether AEV has two different genes controlling transformation of the two types of target cell or whether it has only one gene coding for both. To investigate this question, we looked for mutants of AEV unable to transform one of the two types of target cell. We now describe such a mutant, which is defective for erythroblast transformation but which can still transform fibroblasts.     

Identification of a form of the avian erythroblastosis virus erb-B gene product at the cell surface

Avian erythroblastosis virus (AEV) induces both erythroblastosis and fibrosarcoma in chickens. The viral oncogene responsible for these diseases, erb, is divided into two regions, erb-A and erb-B, although recent evidence suggests that it is primarily the erb-B gene product that is responsible for the transforming activity. The erb-B gene product has been reported previously to be a membrane glycoprotein of 68,000 molecular weight (MW), gp68erb -B. However, we show here that gp68erb -B is an intracellular precursor which is modified further to a 74,000 MW protein, gp74erb -B. By the criteria of resistance to digestion with endoglycosidase H, subcellular fractionation and inhibition of biosynthesis by the ionophore monensin, gp74erb -B appears to be located at the cell surface. Recently, a comparison of the erb-B sequence with that of the epidermal growth factor (EGF) receptor has shown that these two genes are highly homologous, and that erb-B appears to represent a truncated form of this growth factor. In light of these data the identification of gp74erb -B at the plasma membrane suggests that this may be the functionally important form of the erb-B gene product.     

Protein kinase C reduces Mg2+ block of NMDA-receptor channels as a mechanism of modulation.

The roles of N-methyl-D-aspartate (NMDA) receptors and protein kinase C (PKC) are critical in generating and maintaining a variety of sustained neuronal responses. In the nociceptive (pain-sensing) system, tissue injury or repetitive stimulation of small-diameter afferent fibres triggers a dramatic increase in discharge (wind-up) or prolonged depolarization of spinal cord neurons. This central sensitization can neither be induced nor maintained when NMDA receptor channels are blocked. In the trigeminal subnucleus caudalis (a centre for processing nociceptive information from the orofacial areas), a mu-opioid receptor agonist causes a sustained increase in NMDA-activated currents by activating intracellular PKC. There is also evidence that PKC enhances NMDA-receptor-mediated glutamate responses and regulates long-term potentiation of synaptic transmission. Despite the importance of NMDA-receptors and PKC, the mechanism by which PKC alters the NMDA response has remained unclear. Here we examine the actions of intracellularly applied PKC on NMDA-activated currents in isolated trigeminal neurons. We find that PKC potentiates the NMDA response by increasing the probability of channel openings and by reducing the voltage-dependent Mg2+ block of NMDA-receptor channels.     

Loss of heterozygosity in three embryonal tumours suggests a common pathogenetic mechanism

Children with the Beckwith-Wiedemann syndrome have a greatly increased potential for the specific development of the embryonal tumours hepatoblastoma, rhabdomyosarcoma and Wilms' tumour. Data obtained with molecular probes suggest that the association between these disparate, rare tumour types reflects a common pathogenetic mechanism that entails the somatic development of homozygosity for a mutant allele at a locus on human chromosome 11.     

Chemical modification of DNA polymerase phosphoprotein from avian myeloblastosis virus.

Fractionation of purified avian myeloblastosis virus DNA polymerase, after phosphorylation in vitro, revealed the presence of a small acidic proten, a phosphate acceptor polypeptide with high specific activity. Its presence in the phosphorylated form with the polymerase resulted in as much as a 10-fold increase in the rate of DNA synthesis. Its presence in the dephosphorylated form with the polymerase had no effect in the rate of DNA synthesis.     

Glycosyl-phosphatidylinositol linkage as a mechanism for cell-surface expression of immunoglobulin D.

The B-cell antigen receptor of the IgM and IgD class is a multimeric complex consisting of the membrane-bound form of the immunoglobulin molecule and two other proteins, Ig-alpha and Ig-beta. The Ig-alpha and Ig-beta proteins form a disulphide-linked alpha/beta heterodimer and are encoded by the mb-1 (ref 9, 10) and B29 genes, respectively. Surface expression of the membrane-bound IgM molecule requires assembly with the alpha/beta heterodimer. The IgD molecule, however, can be expressed on the cell surface in an alpha/beta-dependent and -independent form. We show here that in the alpha/beta-independent form the IgD molecule is anchored in the plasma membrane through a glycosyl-phosphatidylinositol linker. In the presence of the alpha/beta heterodimer, most of the otherwise glycosyl-phosphatidylinositol-linked IgD molecule is expressed on the cell surface as transmembrane proteins.