Serological analysis of human deoxyribonucleic acid polymerases. Preparation and properties of antiserum to deoxyribonucleic acid polymerase I from human lymphoid cells

The Assembly of HTLV-1-How Does It Differ from HIV-1?

Retroviral assembly is a highly coordinated step in the replication cycle. The process is initiated when the newly synthesized Gag and Gag-Pol polyproteins are directed to the inner leaflet of the plasma membrane (PM), where they facilitate the budding and release of immature viral particles. Extensive research over the years has provided crucial insights into the molecular determinants of this assembly step. It is established that Gag targeting and binding to the PM is mediated by interactions of the matrix (MA) domain and acidic phospholipids such as phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). This binding event, along with binding to viral RNA, initiates oligomerization of Gag on the PM, a process mediated by the capsid (CA) domain. Much of the previous studies have focused on human immunodeficiency virus type 1 (HIV-1). Although the general steps of retroviral replication are consistent across different retroviruses, comparative studies revealed notable differences in the structure and function of viral components. In this review, we present recent findings on the assembly mechanisms of Human T-cell leukemia virus type 1 and highlight key differences from HIV-1, focusing particularly on the molecular determinants of Gag-PM interactions and CA assembly.

History of the discoveries of the first human retroviruses: HTLV-1 and HTLV-2

HTLV-1 was discovered in the US in 1979, and published in 1980. This was rapidly followed by four additional reports in early 1981 describing additional isolates, characterization of some of the HTLV-1 proteins, serological assays for specific antibodies indicative of HTLV-1 infection, and evidence for integrated DNA proviruses in infected cells. None of this early work was dependent upon or influenced by the subclassification of some T-cell malignancies as ATL (in Japan). Instead, I was stimulated by prior work from many investigators in the US and Europe on retroviruses which caused leukemia in animals and our discoveries were made possible by our technical approaches developed in the 1970s involving especially sensitive assays for RT as a surrogate marker for a retrovirus and our discovery of Il-2 which made it possible to culture human T cells. However, following our reports the same virus was isolated in Japan, and both groups provided evidence that HTLV-1 caused ATL, a subclassification of T-cell malignancies first recognized in Japan.

The discovery of the first human retrovirus: HTLV-1 and HTLV-2

I describe here the history leading up to and including my laboratory's discovery of the first human retrovirus, HTLV-I, and its close relative, HTLV-II. My efforts were inspired by early work showing a retroviral etiology for leukemias in various animals, including non-human primates. My two main approaches were to develop criteria for and methods for detection of viral reverse transcriptase and to identify growth factors that could support the growth of hematopoietic cells. These efforts finally yielded success following the discovery of IL-2 and its use to culture adult T cell lymphoma/leukemia cells.

Human retroviruses after 20 years: a perspective from the past and prospects for their future control

Among viruses the human retroviruses may be of special interest to immunologists, because they target cells of the immune system, particularly mature CD4+ T cells, impair their function and cause them to grow abnormally (human T-cell leukemia virus, HTLV) or to die (human immunodeficiency virus, HIV). Human retroviruses cause disease ranging from neurological disorders and leukemias (HTLV-1) to AIDS (acquired immunodeficiency virus) (HIV) and promote development of several types of malignancies (HIV). They share many common features, but their contrasts are greater, especially the far greater replication and variation of HIV associated with its greater genomic complexity. Both have evolved striking redundancy for mechanisms which promote their survival. Thus, HTLV has redundant mechanisms for promoting growth of provirus containing T cells needed for virus continuity, because it is chiefly through its cellular DNA provirus that HTLV replicates and not through production of virions. Conversely, HIV has redundancy in its mechanisms for promoting virion replication and escape from the host immune system. It is via these redundant mechanisms that they produce disease: leukemias from mechanisms promoting T-cell proliferation (HTLV-1) and AIDS from mechanisms promoting virus replication and T-cell death (HIV). The practical challenges for the future are clear. For HTLV-1, education and control of breastfeeding. For HIV, the formidable tasks now ahead in part demand new kinds of talent, talents that will foster greater insights into the development of therapy for the developing countries, new forms of less toxic therapies for all infected persons, a continued and expanded commitment to education, and a persistent 'never say die' commitment to the development of a truly preventive vaccine with all the scientific and nonscientific challenges that these objectives face.

Evidence for a cytoplasmic factor regulating murine leukemia virus DNA synthesis and its preliminary characterization

Postmitochondrial cytoplasmic extracts, prepared from uninfected NIH/3T3 cells as well as from chronically or exogenously infected with murine leukemia virus (MLV), were found to stimulate the endogenous reaction of purified MLV reverse transcriptase. No such stimulation was observed with the exogenous reaction of this enzyme, using poly (rA) oligo (dT) as an exogenous template-primer. While the stimulatory capacity of extracts from uninfected and chronically infected cells was comparable, that of the exogenously infected cells was much more powerful in this respect. The stimulatory activity could be destroyed by trypsin, indicating that it was excerted by a protein. In uninfected and chronically infected cells this protein was found to be of a short functional life time under conditions blocking continuous protein synthesis. However the mRNA coding for this factor was found in these cells to be stable. On the other hand, the increased stimulatory activity, observed in extract of exogenously infected cells, was independent on protein synthesis and therefore was attributed to a protein apparently introduced into the cells by the penetrating virions. Experiments with monospecific antibodies against MLV proteins suggested that p30 is an important accessory for reverse transcriptase activity and that the cytoplasmic stimulatory factor might be also related to p 30.

Regulation of human T-cell proliferation: T-cell growth factor and isolation of a new class of type-C retroviruses from human T-cells

The discovery, characterization, and purification of human T-cell growth factor (TCGF) has led to the establishment of continuously growing T-lymphoblast cell lines from normal people and from patients with certain T-cell neoplasias. In contrast to normal T-cells, neoplastic mature T-cells respond directly to TCGF, requiring no prior lectin or antigen in vitro activation. The transformed T-cell lines have phenotypic characteristics consistent with the neoplastic cells of their disease of origin. A novel retrovirus, human T-cell lymphoma-leukemia virus (HTLV), has been isolated from the fresh and cultured cells of two of these patients. Subsequent characterization of this virus has shown that it is not significantly related to any known animal retrovirus, is not an endogenous (genetically transmitted) virus of man, and so far has been associated only with fresh or cultured T-cells from patients with T-cell neoplasia. These results suggest that HTLV infected some mature T-cells of some people and that it might be involved in some neoplasias involving these cells.

Biochemical and immunological properties of the DNA polymerase and RNAase H activities of purified feline leukemia virus reverse transcriptase

Feline leukemia virus DNA polymerase was purified by ion-exchange and nucleic acid affinity chromatographies. The enzyme consists of a single polypeptide chain of mol. wt. approx. 72,000 as determined by both glycerol density gradient centrifugation and SDS-polyacrylamide gel electrophoresis. The preferred divalent cation for DNA synthesis is Mn2+ on a variety of template-primers, and its optimum concentration appears to be significantly lower than reported results of other mammalian type-C viral enzymes. The purified enzyme also contained RNAase H activity. Both DNA polymerase and RNAase H activities appear to reside on the same molecule as demonstrated by the copurification of both activities through various purification steps. The divalent cation requirement for maximum activity of RNAase H is also similar to that of the DNA polymerase. RNAase H without detectable polymerase activity was generated by a limited chymotrypsin digestion of the purified reverse transcriptase. This RNAase H activity was inhibited equally effectively as RNAase H in the intact reverse transcriptase by antisera prepared against reverse transcriptase of feline leukemia virus. These results indicate that the RNAase H catalytic activity of reverse transcriptase is distinct from the polymerase portion of the molecule. Since the RNAase H activity appears to be more stable, the measurement of RNAase H activity with a proper antibody might be useful for assaying tumor cells for the presence of the viral enzyme.

Characterization of a type-C virus produced by co-cultures of human leukemic bone-marrow and fetal canine thymus cells

The putative human helper virus SKA-21/A204V, isolated by Nooter et al. in 1977 from human leukemic bone-marrow cells following co-culture with normal fetal canine thymus cells, Cf2th, has been characterized with respect to its major viral core protein, reverse transcriptase, and nucleic acid sequences. The results of these analyses show that this virus is not distinguishable from the woolly monkey type-C virus, SSAV-1, by the techniques employed.

DNA polymerases of anucleated cells. Isolation and characterization of two DNA polymerases from human platelets

Two different DNA polymerases have been purified and characterized from human platelets. In the mitochondrial fraction a unique activity of the polymerase gamma type has been found. The same enzyme is found in the extramitochondrial supernatant. A second DNA polymerase, called 'cytoplasmic' DNA polymerase has been found in the 10000 x g supernatant of human platelets. The following properties of the latter DNA polymerase from human platelets are identical to those of DNA polymerase alpha from normal cells: DEAE-cellulose and phosphocellulose chromatography, size, thermal stability, phosphonoacetic acid and ethidium bromide inhibition. However, some of its properties, like high resistance to N-ethylmaleimide and the lack of DNA polymerization using synthetic RNA primers, are those of DNA polymerase beta.

Surface antibodies of human myelogenous leukaemia leukocytes reactive with specific type-C viral reverse transcriptases

Purified immunoglobin G (IgG) from patients with chronic myelogenous leukaemia specifically neutralised RT from feline leukaemia virus while purified IgG from other types of leukaemias and from normal blood cells were less reactive and in some cases preferentially reacted with RT from horizontally transmitted primate type-C viruses (simian sarcoma virus-gibbon ape leukaemia virus group). This indicates the presence of a heterogeneous immune response to RT or to an RT-like molecule in humans.

Studies on DNA alpha-polymerase of mouse myeloma: partial purification and comparison of three molecular forms of the enzyme

Activity of DNA alpha-polymerase in extracts from MOPC-104E was not associated with a single protein molecule, but with several molecular species that differed in isoelectric point. The three most abundant of these enzyme species were first separated from other DNA polymerases and then resolved from each other by repeated chromatography on diethylaminoethylcellulose columns. Next, with the use of glycerol gradient centrifugation and DNA-cellulose column chromatography, the three species were further purified to a state representing more than 5000-fold purification over the crude extract. These three highly purified enzyme species exhibited very similar catalytic properties, and the main activity of each species sedimented at the same rate (6-7S) in glycerol gradients containing 0.5 M KCl. Analysis of the polypeptide content of each species revealed that polypeptides of about 150 000 and 60 000 daltons cofractionated with the DNA polymerase activity. The multiple alpha-polymerase species did not appear to result from in vitro proteolytic cleavage, since multiple species were observed in extracts prepared under several different types of conditions, including the presence of the protease inhibitors, phenylmethanesulfonyl fluoride, or trasylol. The three species were recovered in about the same relative amounts from both the nuclear and cytoplasmic fractions of MOPC-104E, and it appeared that multiple species of alpha-polymerase were also present in extracts from fetal bovine liver.

Further characterization of a DNA polymerase activity in mouse sperm nuclei

The presence of a nuclear DNA polymerase in mouse sperm from adult testes has been confirmed and the properties of this enzyme further investigated. This activity was shown to be greatly enhanced by treating the spermatozoa with methanol or ethanol before incubation in the reaction medium or by their addition in small amounts to this medium. It was protected against degradation by nuclear proteases by adding soybean trypsin inhibitor and was stimulated by ATP. It was found to be Mg2+ dependent (optimum concentration: 7.5 mM), DNA dependent, and all four deoxynucleoside triphosphates were needed for optimal reaction. The radioactive acid-precipitable product of polymerization was not eliminated by organic solvents, nor by pronase, ribonuclease or by nuclease S1; however, it was converted to a large extent to acid-soluble products by pancreatic deoxyribonuclease. Since it was only partially solubilized by Triton X-100, it therefore did not appear to be preferentially associated with the nuclear membranes. The activity recovered after incubation depended also on the pH (optimum at pH 8.3) and did not work well in a medium for DNA polymerase alpha. The temperature for maximum incorporation of nucleotides was found to be 32 degrees C and, under our conditions, the reaction was linear for 30 min. The DNA polymerase activity was inhibited by low and high concentrations of KCl. It was not lowered by N-ethylmaleimide or p-hydroxymercuribenzoate; urea slightly stimulated the reaction and this stimulation was reversed by subsequent treatment with N-ethylmaleimide. Actinomycin D (40 mug/ml), ethidium bromide (25--50 muM), netropsin (5--50 mug/ml), and spermidine (0.5--2.5 mM) lowered the polymerization of DNA precursors. The nuclear enzyme could shift from the endogenous template to activated exogenous calf thymus DNA, the resulting nuclear radioactivity being reduced. The endogenous DNP template ability was not increased by deoxyribonuclease activation according to the method of Aposhian and Kornberg (J. Biol. Chem. (1962) 237, 519--525) suggesting that the amount of DNA polymerase associated with chromatin was probably limiting the reaction. The DNA polymerase activity detected in mouse sperm nuclei has numerous properties of low molecular weight DNA polymerases (DNA polymerase beta) reported in several eukaryotic organisms.

Fluctuation in activity of the molecular forms of cellular DNA polymerase during infection by SV40

Infection of BSC-1 cells by SV40 brings about an increase of 7--11-fold in DNA polymerase activity, found in the nuclei and cytoplasm, respectively. The overall ratio between activites of DNA polymerase beta (3.1S) and DNA polymerase alpha (5.5S) remains fairly constant throughout infection. However,there is a large increase in DNA polymerase alpha2 (7.1S) in the cytoplasm, and its appearance in the nuclei late in infection. The addition of 1 M NaCl to infected cytoplasm,causes an aggregation of DNA polymerase alpha into a higher sedimenting form (9.8S), termed DNA polymerase alpha3. DNA polymerase alpha1, alpha2 and alpha3 are different molecular forms of the same enzyme, as can be seen by their similar inhibition by N-ethyl-maleimide, heparin and NaCl. However, this new activity, alpha3, is stimulated by dithiothreitol to a greater extent at pH 9.30 than at pH 7.94. The conformational changes induced in DNA polymerase and its increase in activity during infection with SV40 are discussed.

Type C RNA tumor virus isolated from cultured human acute myelogenous leukemia cells

Previously, type C RNA tumor virus-related components have been described in blood leukocytes from patients with acute myelogenous leukemia. These components, for example, reverse transcriptase, have been shown to be most closely related to those from two oncogenic subhuman primate type C viruses (woolly monkey sarcoma virus and gibbon ape leukemia virus). Now, we report the continuous production of budding type C viruses with the same characteristic reverse transcriptase by three separate culturings of leukocytes from a single bleeding from a patient with acute myelogenous leukemia. These isolations were made possible by the discovery of a source of conditioned media which sustains exponential growth of human myelogenous leukemia cells in liquid suspension culture.