The cell-cycle regulated proliferating cell nuclear antigen is required for SV40 DNA replication in vitro

Cell-free extracts prepared from human 293 cells, supplemented with purified SV40 large-T antigen, support replication of plasmids containing the SV40 origin of DNA replication. A cellular protein (Mr approximately 36,000) that is required for efficient SV40 DNA synthesis in vitro has been purified from these extracts. This protein is recognized by human autoantibodies and is identified as the cell-cycle regulated protein known as proliferating cell nuclear antigen (PCNA) or cyclin.     

Two homologous protein components of hepatic gap junctions

Gap junctions consist of closely packed pairs of transmembrane channels, the connexons, through which materials of low relative molecular mass diffuse from the cell to neighbouring cells. In liver, connexons consist of six protein subunits which, until now, were believed to be identical. However, besides the major polypeptide of relative molecular mass (Mr) 28,000 (and see refs 4 and 6), a component of Mr 21,000 (21K) has been repeatedly observed in liver. The amino-terminal sequence (18 residues) of this less abundant protein shows that it is related to, but distinct from, the Mr 28K protein. Immuno-staining and immuno-precipitation show both proteins to be in the same gap junctional plaques. Thus, it seems that hepatic gap junction channels (and by extension possibly others) are composed of two (or more) homologous proteins.     

Localization of the gene for familial adenomatous polyposis on chromosome 5

Colorectal cancer is the second most common cancer in the United Kingdom and other developed countries in the West. Although it is usually not familial, there is a rare dominantly inherited susceptibility to colon cancer, familial adenomatous polyposis (FAP; also often previously called familial polyposis coli). During adolescence affected individuals develop from a few hundred to over a thousand adenomatous polyps in their large bowel. These are sufficiently likely to give rise to adenocarcinomas to make prophylactic removal of the colon usual in diagnosed FAP individuals. Adenomas may occur elsewhere in the gastrointestinal tract and the condition is often associated with other extracolonic lesions, such as epidermoid cysts, jaw osteomata and fibrous desmoid tumours. Adenomata have been suggested to be precancerous states for most colorectal tumours. Knudson has suggested that the mutation for a dominantly inherited cancer susceptibility may be the first step in a recessive change in the tumour cells, and that the same gene may be involved in both familial and non-familial cases of a given tumour. Following up a case report of an interstitial deletion of chromosome 5 in a mentally retarded individual with multiple developmental abnormalities and FAP, we have now shown that the FAP gene is on chromosome 5, most probably near bands 5q21-q22.     

Electrogenic glutamate uptake is a major current carrier in the membrane of axolotl retinal glial cells

Glutamate is taken up avidly by glial cells in the central nervous system. Glutamate uptake may terminate the transmitter action of glutamate released from neurons, and keep extracellular glutamate at concentrations below those which are neurotoxic. We report here that glutamate evokes a large inward current in retinal glial cells which have their membrane potential and intracellular ion concentrations controlled by the whole-cell patch-clamp technique. This current seems to be due to an electrogenic glutamate uptake carrier, which transports at least two sodium ions with every glutamate anion carried into the cell. Glutamate uptake is strongly voltage-dependent, decreasing at depolarized potentials: when fully activated, it contributes almost half of the conductance in the part of the glial cell membrane facing the retinal neurons. The spatial localization, glutamate affinity and magnitude of the uptake are appropriate for terminating the synaptic action of glutamate released from photoreceptors and bipolar cells. These data challenge present explanations of how the b-wave of the electroretinogram is generated, and suggest a mechanism for non-vesicular voltage-dependent release of glutamate from neurons.     

T-cell receptors of human suppressor cells

Cells which can suppress the immune response to an antigen (TS cells) appear to be essential for regulation of the immune system. But the characterization of the TS lineage has not been extensive and many are sceptical of studies using uncloned or hybrid T-cell lines. The nature of the antigen receptor on these cells is unclear. T cells of the helper or cytotoxic lineages appear to recognize their targets using the T-cell receptor (TCR) alpha beta-CD3 complex. TCR beta-gene rearrangements are also found in some murine and human suppressor cell lines but others have been shown not to rearrange or express the beta-chain or alpha-chain genes. We previously established TS clones derived from lepromatous leprosy patients which carry the CD8 antigen and recognize antigen in the context of the major histocompatibility complex (MHC) class II molecules in vitro. We here report the characterization of additional MHC-restricted TS clones which rearrange TCR beta genes, express messenger RNA for the alpha and beta chains of the TCR and express clonally unique CD3-associated TCR alpha beta structures on their cell surface but do not express the gamma chain of the gamma delta TCR on the cell surface. We conclude that antigen recognition by at least some human CD8+ suppressor cells is likely to be mediated by TCR alpha beta heterodimers.     

Novel source of 1,2-diacylglycerol elevated in cells transformed by Ha-ras oncogene

Genes involved in the transduction of signals required for normal cell proliferation commonly appear to be subverted in the neoplastic process. One such group is the highly conserved family of ras genes, which have been detected as transforming genes in a wide variety of naturally occurring tumours. By analogy with other known G proteins, the p21 proteins encoded by ras genes may act as regulatory proteins in the transduction of signals that lead to DNA synthesis. A major pathway involved in the DNA synthesis induced by growth factors is mediated by phosphatidylinositol turnover: cleavage of phosphoinositides by phospholipase C produces 1,2-diacylglycerol, and inositol phosphates. The former acts as an essential cofactor for protein kinase C (ref. 4), and inositol-(1,4,5)-triphosphate mobilizes Ca2+ from non-mitochondrial intracellular stores. We demonstrate a reproducible increase in 1,2-diacylglycerol, in the absence of a detectable increase in inositol phosphates, in transformed cells containing Ha-ras oncogenes and with different membrane targeting signals for the ras p21 protein. These findings suggest that a source other than phosphoinositides exists for the generation of 1,2-diacylglycerol and that the Ha-ras oncogene specifically activates this novel pathway for 1,2-diacylglycerol production.     

Ligand binding to the beta-adrenergic receptor involves its rhodopsin-like core

Recently the genes for several hormone receptors that interact with guanine nucleotide binding proteins (G proteins) have been cloned, including the hamster beta 2-adrenergic receptor (beta 2AR), a human beta AR, the turkey erythrocyte beta AR and the porcine muscarinic acetylcholine receptor (MAR). All these receptors share some amino-acid homology with rhodopsin, particularly in 7 hydrophobic stretches of residues that are believed to represent transmembrane helices. To determine whether differences in ligand specificity result from the divergence in the sequences of the hydrophilic regions of these receptors, we have expressed in mammalian cells genes for the wild-type hamster and human beta AR proteins, and a series of deletion mutant genes of the hamster beta 2AR. The pharmacology of the expressed receptors indicates that most of the hydrophilic residues are not directly involved in the binding of agonists or antagonists to the receptor. In addition, we have identified a mutant receptor that has high agonist affinity but does not couple to adenylate cyclase.     

Sequence and functional expression of the GABA A receptor shows a ligand-gated receptor super-family

Amino-acid sequences derived from complementary DNAs encoding the alpha- and beta-subunits of the GABA/benzodiazepine receptor from bovine brain show homology with other ligand-gated receptor subunits, suggesting that there is a super-family of ion-channel-containing receptors. Co-expression of the in vitro-generated alpha-subunit and beta-subunit RNAs in Xenopus oocytes produces a functional receptor and ion channel with the pharmacological properties characteristic of the GABAA receptor.