Reduction of a conserved Cys is essential for Myb DNA-binding

The human c-Myb gene product is a regulator of transcription with intrinsic DNA-binding activity located in two of three aminoterminal repeats R2R3. Three purified recombinant c-Myb proteins, a 42 kD protein corresponding to the amino-terminal half (HM42), and two proteins representing R2R3 or R1R2R3 (HMR23 and HMR123) have been analyzed either as purified proteins or present in bacterial extracts in gelshift analyses using a high-affinity DNA oligonucleotide. The purified proteins are inactive in DNA-binding unless supplemented with a reducing agent such as dithiothreitol (DTT) in vitro. Alternatively a cellular nuclear extract (Nex) from HeLa cells strongly activates the binding. This effect is dose-dependent and sensitive to heat. The Nex does not lead to changes in the Myb-DNA mobility shift assay excluding a direct association of the complex with a cellular component. Site-directed mutagenesis of the aminoacid residue 130, a single conserved cysteine in HMR23 to serine almost completely abolishes DNA binding. Oxidation by diamide or alkylation by N-ethylmaleimide (NEM) of the Myb-proteins in vitro inhibit their interaction with DNA whereby the diamide effect is reversible by addition of excess of DTT. Nex prepared from COS cells transfected with c-myb leads to Myb-DNA interaction which is not responsive to DTT but sensitive to NEM and diamide. Our data indicate that the reduced cysteine of Myb is essential for its DNA-binding and that Myb function may be regulated by a reduction-oxidation mechanism.

Molecular cloning, expression and in vitro functional characterization of Myb-related proteins in Xenopus

Two cDNAs encoding Myb-related proteins have been cloned from Xenopus laevis and they have been termed Xmyb1 and Xmyb2. The Xmyb1 cDNA clone codes for an open reading frame of 733 amino acids and exhibits a high degree of similarity over the entire predicted protein sequence with the human B-Myb protein. Xmyb2 is a partial cDNA clone encoding three copies of amino-terminal tandem repeat elements typical for the Myb DNA-binding domain. The predicted protein sequence is most closely related to the human A-Myb gene product. In vitro translation of two deletion mutants of Xmyb1, truncated in the 3'-portion of the open reading frame, results in protein products which cross-react with polyvalent as well as monoclonal antibodies directed against the human c-Myb protein. The same two XMyb1 proteins, which both contain the complete set of aminoterminal repeats, specifically bind to the c-Myb-specific DNA binding sequence as evidenced by electrophoretic mobility shift analysis in vitro. RNA expression profiles of Xmyb1 and -2 are very different from each other; Xmyb1 is present throughout oogenesis and early Xenopus embryogenesis; in adult tissue it is primarily detected in blood. In contrast, Xmyb2 is expressed at only very low levels during oogenesis, not detectable in embryonic RNA preparations, and in adult tissue it is predominantly expressed in testis, with only a very low level seen in blood.

Heterogeneous expression of c-myb protein in human leukemia detected by simultaneous two color flow cytometric analysis

The expression of c-myb mRNA and protein was analyzed in fresh leukemic cells by Northern-blot analyses and by immunofluorescent staining using monoclonal c-myb specific antibodies. Staining of the cells was evaluated by flow cytometry. The results demonstrate c-myb mRNA expression predominantly in acute lymphocytic leukemia (ALL, 4/4 cases), acute myeloic leukemia (AML, 17/17) and chronic myeloic leukemia (CML, 12/12) but rarely in chronic lymphocytic leukemia (CLL, 1/17). Immunofluorescent analyses revealed expression of c-myb protein in the nucleus of ALL (5/7) and AML (9/9) with a good correlation of c-myb-positive cells and with the number of proliferating (Ki67-positive) blast cells.

Direct interaction of CREB protein with 21 bp Tax-response elements of HTLV-ILTR

The three 21-bp repeats (Tax-responsive elements) of the long terminal repeat (LTR) of the human T-cell leukemia virus (HTLV-I) mediates the response of the Tax protein. All three Tax-responsive elements (TREs) contain a TGACG motif, reminiscent of the CREB/ATF-binding site TGACGTCA. DNA-affinity chromatography with the 5'-TRE resulted in a previous study in proteins of about 32, 36 to 42, 50 and 110 kDa. Here we demonstrate that the 42 kDa protein is the cAMP-response element-binding (CREB) protein. This is shown by phosphorylation of the proteins eluted from the DNA-affinity column with protein kinase A (PKA) in vitro and subsequent indirect immunoprecipitation with a CREB-specific antiserum raised against an internal CREB-specific peptide. This method allows detection of phosphorylated proteins by autoradiography with high sensitivity and is superior to metabolic labeling. One of the phosphorylated proteins co-migrates with immuno-affinity-purified CREB protein--also phosphorylated in vitro--and competes with the peptide antigen, which proves the specificity of the reaction. The purified CREB protein leads to specific DNA-protein complexes in DNA mobility-shift analyses with all three TREs. Comparison of these TRE-CREB complexes with those formed by nuclear extracts from the HTLV-I-transformed T-cell line C81-66-45 indicates that additional cellular factors contribute to the complexes, especially to the middle TRE. This is also shown by using CREB-depleted instead of complete nuclear extracts for DNA mobility-shift assays. Antibodies against CREB but not Tax affect the mobility of the DNA-protein complex.

Cleavage of the HIV-1 p66 reverse transcriptase/RNase H by the p9 protease in vitro generates active p15 RNase H

The reverse transcriptase/RNase H of HIV-1 is composed of a p66/p51 heterodimer when analyzed from virus particles. A recombinant reverse transcriptase (RT)/RNase H which after purification consisted mainly of p66 was analyzed as substrate of the purified recombinant HIV-1 protease p9 in vitro. The p66 protein if treated with the protease is processed to a stable p66/p51 heterodimer. A p15 protein is a prominent cleavage product which was identified as the carboxyterminal portion of p66 by means of a monoclonal antibody. It exhibits RNase H activity when tested by activated gel analysis. Presence of SDS during the incubation allowed complete degradation of p66 depending on the conditions, which indicates that conformation of a substrate is relevant for cleavage by the HIV-1 protease. A synthetic heptapeptide AET-FYVD derived from the region between RT and RNase H is cleaved efficiently in vitro by the HIV-1 protease at the F'Y junction, and may mimick a natural cleavage site. P66/p51 heterodimers exhibit higher RT and RNase H activities than p66 when renatured from polyacrylamide gels.

Mutations of a conserved residue within HIV-1 ribonuclease H affect its exo- and endonuclease activities

The human immunodeficiency virus 1 (HIV-1) reverse transcriptase (RT) is a protein of 66 kDa, p66, which contains two domains, an amino-terminal DNA polymerase and an RNase H at the carboxy terminus of the molecule. In order to characterize the mode of action of the RNase H, two previously described mutant enzymes were used, with substitutions in the highly conserved histidine 539, which was mutated to the neutral amino acid asparagine and to the negatively charged aspartate. The purified wild-type (wt) and mutant (mt) enzyme activities are analyzed here using RNA-DNA hybrids consisting of in vitro transcribed RNA that harbors the polypurine tract (PPT) from HIV-1 and DNA oligonucleotides complementary to the PPT or to other regions of the RNA. Analysis of the radioactively labeled RNA of these model hybrids after RNase H treatment indicates that both, wt and mt enzymes, are capable of cleaving the RNA in an endonucleolytic manner. The mt enzymes exhibit a severely reduced exonuclease activity. They are more sensitive towards salt and competition with excess of unlabeled hybrid, suggesting a reduced substrate binding affinity. DNA elongation by the RT is coupled with RNA hydrolysis by the 3'-5' exonuclease of the wt RNase H. The RNase Hmt of the mt enzymes, however, does not exhibit such processive 3'-5' exonuclease activity during DNA synthesis but gives rise to sporadic endonucleolytic cuts, whereas the RT is not affected. The endonuclease activities of the RNase H mt enzymes exhibit cleavage preferences in the absence or presence of DNA synthesis different from those of the wt enzyme. They cannot recognize specific sequences required to generate a PPT-primer and therefore cannot initiate plus-strand DNA synthesis in vitro at the 3' end of the PPT, which is essential for viral replication.

Interaction of the v-and c-Myb proteins with regulatory sequences of the human c-myc gene

Eight c-Myb-binding sites have been identified in the regulatory region of the human c-myc gene using gel retardation and DNAase I footprint assays with purified bacterially expressed full-length and carboxy-terminally truncated c-Myb proteins. These binding sites exhibit different affinities whereby strong binding correlates better with conservation of the palindromic sequences, AACXGTT or AACGTT, than the previously described consensus sequence. Flanking AT-rich sequences further increase the binding affinity. The c-Myb-binding sites are arranged in pairs consisting of one high- and one low-affinity binding site. Binding of the Myb proteins to these sites is non-cooperative. The v-Myb protein protects two nucleotides fewer than the c-Myb protein. Co-transfection of reporter CAT genes, containing upstream human c-myc sequences including exon 1, with c-Myb-expressing constructs resulted in positive transactivation, which was eightfold with full-length Myb and 14-fold with the truncated Myb. This result suggests that the Myb protein could participate in regulation of human c-myc gene expression.

Coupling of reverse transcriptase and RNase H during HIV-1 replication

The retroviral replicating enzyme reverse transcriptase (RT) is associated with an RNase H which specifically hydrolyzes RNA in RNA-DNA hybrids. The RNase H exhibits endo- as well as exonuclease activity when analyzed with in vitro synthesized viral RNA hybridized to a shorter synthetic DNA oligonucleotide. In the presence of deoxyribonucleotides the RT synthesizes DNA in a concerted action with the RNase H, which simultaneously hydrolyzes the RNA template. Mutants of the RNase H derived by site-directed mutagenesis of a conserved histidine 539 are preferentially impaired in their exo- and less in their endonuclease activity. A specific polypurine-rich oligonucleotide created at the polypurine tract (PPT), serves as a primer for plus-strand DNA synthesis. Initiation of DNA synthesis at the PPT-primer can be demonstrated for the wt enzyme in vitro in contrast to the mt enzymes which do not recognize this sequence. A replication model based on these results is presented.

Mutations within the RNase H domain of human immunodeficiency virus type 1 reverse transcriptase abolish virus infectivity

The C-terminal region of human immunodeficiency virus (HIV) reverse transcriptase (RT) contains the domain responsible for RNase H activity. To determine the importance of this RNase H domain, specific changes in the C-terminal region of a recombinant RT expressed in Escherichia coli were introduced by amino acid substitutions and specific deletions. The enzyme activities of purified wild-type and mutant RT/RNase H proteins, standardized for protein content, were compared by filter assays and thermal inactivation kinetics. A point mutation of His 539----Asn produced an enzyme with a marked thermolabile RNase H function (nine-fold increase in inactivation), whereas RT function was only marginally more labile than that of the wild-type (two-fold). A second mutation, His 539----Asp, impaired both enzyme activities to a similar degree (four- to five-fold). A C-terminal deletion of 19 amino acids (aa) (aa 540 to 558) and a C-terminal truncation of 21 aa (aa 540 to 560) reduced RT as well as RNase H activity. A 130 aa deletion enzyme exhibited no RNase H activity and insufficient RT activity to allow inactivation studies. Two mutants, the 19 aa deletion and His----Asn, were introduced into proviral HIV-1 DNA clones to determine whether changes in enzyme activity, particularly RNase H activity, affected virus infectivity. Both mutants were non-infectious, indicating that the C-terminal 19 to 21 amino acids and His 539 of the RT/RNase H protein are essential for HIV replication. These results are consistent with the assumption that RNase H is essential for the infectivity of HIV-1.

Analysis of DNA sequences present in complexes of v-Myc and cellular DNA

We have isolated cellular DNA sequences from the myelocytomatosis virus-transformed quail fibroblastic cell-line (MC29-Q8) by indirect immunoprecipitation of v-Myc-DNA complexes and subsequent cloning of the DNA. The v-Myc-DNA complexes were obtained from isolated nuclei pretreated with 150 mM salt and indirect immunoprecipitation of the p110 Gag-Myc protein with the IgG of a Gag-specific monoclonal antibody. A non-specific monoclonal IgG was used as control to account for non-specific interactions. The DNA from the precipitates was isolated, cloned and characterized. 21 positive and 13 control clones with inserts ranging in size from 25 to 330 nucleotides were sequenced and analyzed for sequence homologies to known DNA-motives. Some of the sequences over-represented in the specific DNA fragments corresponded to elements which have been previously described to promote DNA amplification. Six of these DNA fragments were tested for their ability to promote DNA amplification by insertion into a plasmid containing the HSV-1 tk gene with a truncated promoter. These constructs were transfected into mouse Ltk- cells which grow under HAT selection only upon amplification of the tk-carrying plasmids. Two of the six DNA fragments showed the capability to amplify their plasmids in cis and create stable cellular clones. Copy numbers of the amplified plasmids in these Ltk+ clones ranged from 470 to 680 whereby the amplified sequences were integrated as large clusters of head-to-tail tandems. The data presented here suggest that the Myc protein may be involved in DNA amplification and therefore may play a role in DNA replication.

Interaction of HIV-1 ribonuclease H with polypurine tract containing RNA-DNA hybrids

The mode of action of the RNase H activity from HIV-1 was analyzed with a purified recombinant p66/p51 reverse transcriptase RT/RNase H protein and RNA-DNA hybrid consisting of RNA harboring the polypurine tract (ppt) and three complementary synthetic DNA oligonucleotides. Upon incubation of this preformed RNA-DNA hybrid with the p66/p51 RT/RNase H, a cleavage pattern is observed that indicates endonucleolytic RNase H activity with some sequence specificity for the next to last nucleotide of the 3'-end of the ppt RNA and one cut within the ppt. The RNase H avoids cleavage of G or A stretches. During RNA-directed DNA synthesis the RNA is hydrolyzed in a concerted action of RT and RNase H whereby the RNase H exhibits endonuclease as well as 3'-5'-exonuclease activity. The distance between the active centers of the RT and RNase H corresponds to 18 base pairs of the RNA-DNA hybrid. Plus-strand DNA-directed DNA synthesis initiates exactly at the next to last nucleotide of the 3'-end of the ppt RNA by means of the RNase H activity.

Transcriptional activation by human c-myb and v-myb genes

The human c-myb proto-oncogene is the cellular progenitor of the viral v-myb oncogene and codes for a 75 kD protein involved in growth regulation and differentiation in a number of cells. Fusion proteins in which human c-myb sequences are linked to the DNA binding domain of the yeast transcriptional activator GAL4 can activate transcription from a reporter gene which carries the chloramphenicol acetyl transferase (CAT) gene linked in cis to a repeat of the GAL4 binding site. Deletions of carboxyterminal sequences allowed the identification of the domain responsible for transcriptional activation, which is located between amino acid residues 275 to 327. Deletion of this activator domain results in abrogation of the transcriptional activation. The GAL4-v-myb fusion protein can also activate transcription whereas no transactivation by GAL4-c-myb is observed, indicating that a carboxyterminal domain of c-myb which is absent from v-myb apparently negatively regulates transcriptional activation. Dimer formation which is required for transactivation by GAL4 fusion proteins can, when GAL4 is truncated, be mediated by a region of the c-myb protein upstream of the transactivator domain possibly including the transactivator domain itself but not a putative leucine zipper located downstream of this region.

Transcriptional down-regulation of c-myc expression by protein synthesis-dependent and -independent pathways in a human T lymphoblastic tumor cell line

We show that in the human T lymphoblastic tumor cell line Molt4 c-myc mRNA and protein expression is down-regulated after exposure to dimethyl sulfoxide, to phorbol myristate acetate, or to the calcium ionophore A23187, which raises the intracellular calcium concentration. A block to RNA elongation is largely responsible for decreased c-myc transcription. Although negative regulation by dimethyl sulfoxide takes place even when protein synthesis is inhibited by cycloheximide, the phorbol myristate acetate effect is blocked to some extent only by cycloheximide. The calcium ionophore-induced c-myc suppression, however, strictly requires de novo protein synthesis. Therefore, two different negative regulatory pathways are involved in c-myc regulation: one which is independent and one which depends on de novo protein synthesis. The latter one appears to be mediated by a rapidly calcium-dependent induced gene product.

Tax-independent binding of multiple cellular factors to Tax-response element DNA of HTLV-I

The human T-cell leukemia virus type I (HTLV-I) promoter contains three copies of imperfect repeats of a 21-base pair sequence designated here as TRE (Tax-response element) that is responsive to the virally encoded transactivator protein Tax. We have identified and separated four nuclear proteins from C81-66-45 cells, an HTLV-I immortalized Tax-expressing human T-lymphocyte line (Salahuddin et al., 1983), that interact with the TRE-DNA, none of which are identical with the Tax-protein. The proteins identified have molecular weights of about 32, 36 to 42, 50 and 110 kD. Four different methods were used to identify the proteins. First, from different cell lines three or all four of the nuclear proteins were specifically cross-linked by UV irradiation to the radioactively labeled TRE-DNA fragment. Second, TRE-DNA binding proteins sedimented through a glycerol density gradient at rates corresponding to proteins of native molecular weights of 35 to 50 kD and 110 kD. Third, only the 50 kD protein was retained on a biotinylated DNA-streptavidin matrix when the DNA fragment contained the TRE-DNA. Fourth, extensive purification by several cycles of TRE-DNA affinity chromatography resulted in the 32, 36 to 42 and 110 kD proteins and to less extent the 50 kD factor. Two abundant proteins of 75 and 80 kD were competed out by poly[d(I-C)] in all reactions. The cAMP-response element CRE, TGACGTCA, present in the 21 base-pair sequence, appears to be essential for specific protein-TRE-DNA interactions because mutation of the two G's destroys this complex. This result suggests that the cAMP response element binding protein, CREB, is involved in the protein-TRE-DNA complex and in mediating the Tax response.

In vitro inhibition of HIV-1 proteinase by cerulenin

Retroviruses encode proteinases necessary for the proteolytic processing of the viral gag and gag-pol precursor proteins. These enzymes have been shown to be structurally and functionally related to aspartyl proteinases such as pepsin and renin. Cerulenin is a naturally occurring antibiotic, commonly used as an inhibitor of fatty acid synthesis. Cerulenin has been observed to inhibit production of Rous sarcoma virus and murine leukaemia virus by infected cells, possibly by interfering with proteolytic processing of viral precursor proteins. We show here that cerulenin inhibits the action of the HIV-1 proteinase in vitro, using 3 substrates: a synthetic heptapeptide (SQNYPIV) which corresponds to the sequence at the HIV-1 gag p17/p24 junction, a bacterially expressed gag precursor, and purified 66 kDa reverse transcriptase. Inhibition of cleavage by HIV-1 proteinase required preincubation with cerulenin. Cerulenin also inactivates endothiapepsin, a well-characterised fungal aspartyl proteinase, suggesting that the action of cerulenin is a function of the common active site structure of the retroviral and aspartic proteinases. Molecular modelling suggests that cerulenin possesses several of the necessary structural features of an inhibitor of aspartyl proteinases and retroviral proteinases. Although cerulenin itself is cytotoxic and inappropriate for clinical use, it may provide leads for the rational design of inhibitors of the HIV proteinase which could have application in the chemotherapy of AIDS.

HIV-1 protease: mutagenesis of asparagine 88 indicates a domain required for dimer formation

Considerable interest exists in the HIV-1 protease for biochemical studies as a potential therapeutic target of acquired immunodeficiency syndrome. We have produced the retroviral enzyme in E. coli from a synthetic gene encoding the protease that was constructed by assembling six overlapping and complementary oligonucleotides into the vector pKK223-3. When expressed in E. coli, the recombinant protease was able to correctly process the HIV-1 core protein p24 from a beta-galactosidase-gag fusion protein and to use a heptapeptide as a substrate for proteolytic cleavage. A single base pair mutation was identified in a recombinant that resulted in the substitution of lysine for asparagine at position 88 and a significant loss of enzyme activity. Through site-directed mutagenesis, the Asn88 was changed to five other residues representative of all classes of amino acids. The correlation between enzyme activity and amino acid substitution suggests that the protease domain surrounding position 88 affects the protein's potential for forming an active homodimeric protein and hence, indicates a biochemical interaction that could be inhibited by novel antiviral compounds.

Inhibition of human immunodeficiency virus type 1 RNase H by sulfated polyanions

The reverse transcriptase (RT) activity of human immunodeficiency virus type 1 and other retroviruses is closely associated with a hybrid-degrading RNase H activity which is essential for retroviral replication. We have analyzed the effect of sulfated polysaccharides on human immunodeficiency virus type 1 recombinant RT and RNase H activities in vitro. Heparin, dextran sulfates, and xylan polysulfate were found to be much more potent inhibitors of RNase H than of RT and exhibit 50% infective doses of 0.04 to 0.1 micrograms/ml (corresponding to 0.1 to 25 nM) which is up to 5,000-fold more efficient than that for RT. Inhibitors of RNase H activity are attractive as antiviral drugs.

Distribution of c-myc, c-myb, and Ki-67 antigens in interphase and mitotic human cells evidenced by immunofluorescence staining technique

Using specific antibodies and the immunofluorescence staining technique we found a similar subcellular distribution pattern of the cellular proto-oncogene proteins c-myc and c-myb in interphase and mitotic HL60 and Molt4 cells. Antibodies against c-myc as well as those against c-myb protein gave rise to a nuclear staining excluding the nucleoli. In mitotic cells both proteins are apparently not associated with the chromatin of the condensed chromosomes, but appear diffusely distributed throughout the cytoplasm. In contrast, immunostaining using the proliferation marker antibody Ki-67 yielded in both cell lines several prominent specks in the nucleus and a weak finely dispersed staining throughout the nucleoplasm. No fluorescence was detectable in the cytoplasm. In dividing cells Ki-67 immunofluorescence was found to be associated with the surface of the chromosomes. The functional significance of the different localizations of the proteins is discussed in light of what is currently known about nuclear antigens.

Isolation and characterization of the tax protein of HTLV-I

The transactivator protein tax of the human T-cell leukemia virus type I, HTLV-I, is responsible for transactivation of gene expression of viral and cellular genes and is involved in the onset of adult T-cell leukemia, ATL. Genetic deletion studies have implicated a region of the HTLV-I LTR designated as tax-acceptor region, TAR, which is the target of the tax protein. Using antibodies against a tax carboxyterminal synthetic decapeptide the tax protein was purified from an HTLV-I immortalized human T-lymphocyte cell line by immunoaffinity chromatography. The tax protein, purified to apparent homogeneity binds to double-stranded DNA irrespective of its origin from either a nuclear or cytoplasmic fraction of the HTLV-I immortalized cell-line - both of which harbor similar quantities of tax protein. The tax protein binds less to single-stranded DNA and not to single-stranded RNA in vitro. It also binds to DNA-cellulose and heparin-Sepharose. Nuclease treatment of isolated nuclei does not release the tax protein under conditions which release known DNA-binding proteins, such as the myb protein. Transactivation by the tax protein presumably involves host-cell factors, since it does not recognize specific DNA sequences.

An analysis of the inhibition of replication of HIV and MuLV by some 3'-blocked pyrimidine analogs

Some 3'-blocked pyrimidine analogs were synthesized and tested as inhibitors of replication of human immunodeficiency virus (HIV) and Moloney-murine leukemia virus (MuLV). The analogs were of 3 kinds: (1) analogs of 3'-azido-3'-deoxythymidine (AZT) in which the C-5 CH3 of the base was exchanged for H (AZU) or C2H5 (AZEU); (2) 3'-fluoro-3'-deoxythymidine (FLT) and analogs thereof, in which the C-5 CH3 of the base was exchanged for H (FLU), C2H5 (FLEU) or nC3H7 (FLPU); (3) the threo analogs of AZT (AZT increases) and AZU (AZU increases). All analogs were less active inhibitors of HIV replication than AZT, except FLT, which was as active as AZT. The 3'-fluoro analogs and AZEU did not inhibit MuLV replication at non-cytotoxic concentrations. Oral administration of FLT to MuLV-infected mice result in antiviral effects only at toxic drug levels. AZU and FLU were less potent inhibitors of HIV replication than AZT or FLT, but the 2'-deoxy uridine analogs were less cytotoxic to human embryonic fibroblasts than the thymidine analogs. The 5'-triphosphates of AZU, AZT, AZEU, FLT and FLEU were tested as inhibitors of the HIV- and MuLV-reverse transcriptases. Ranking of the Ki/Km values for HIV-RT resulted in the following order of potency of the 5'-triphosphates AZT = FLT greater than AZU greater than AZEU greater than FLEU. The 5'-triphosphates of AZEU, FLT and FLEU did not inhibit the MuLV-RT, which explains, in part, the lack of effect of these analogs against MuLV replication. The threo forms (azido "up") of AZU and AZT were less active inhibitors of HIV replication than the erythro forms (azido "down"). A 15N-NMR and 1H-NMR study showed that the furanose moieties of analogs with the azido function "up" assume a conformation distinct from that of the analogs with azido "down". This is due to intramolecular stabilisation of the "N" conformer in the threo ("up") diastereomer, due to interaction of the azido functions with the nucleobase and possibly the OH group of C-5' of the furanose. As discussed, this conformation might explain the decreased biological activity of threo forms compared with the erythro forms.

Expression of p64c-myc and neuroendocrine properties define three subclasses of small cell lung cancer

Twelve human small cell lung cancer (SCLC) cell lines and 6 non-SCLC cell lines were analysed with respect to expression of the c-myc, c-myb, and c-raf1 protooncogenes at the protein level. Analysis of p64c-myc protein expression in 12 SCLC cell lines resulted in the observation that it is present at high levels not only in cells with low, but also in those with moderate neuroendocrine differentiation. Neuroendocrine differentiation was based on parameters such as growth rate, colony formation, L-Dopa decarboxylase (DDC) activity, bombesin, and neurotensin described before. Surprisingly, in two cell lines with low neuroendocrine differentiation but without c-myc protein expression (SCLC-86M1 and NCI-H526) p75c-myb expression was observed which may therefore be able to substitute for the p64c-myc protein. Analysis of p74c-raf1 expression did not result in correlation with any growth or differentiation parameter since it was expressed at low levels in 11 out of 12 cases. We conclude that SCLC in vitro can be classified in three rather than two previously defined subclasses. In addition to the classic subclass with slow growth, high neuroendocrine differentiation, and absent or very low p64c-myc expression and the variant subclass with fast growth, absent to very low neuroendocrine differentiation, and high p64c-myc expression, we suggest a third subclass designated as transitional with moderate growth, moderate neuroendocrine differentiation, and high p64c-myc expression. Data on a small number of non-SCLC cell lines tested showed that high levels of p64c-myc correlate with high in vitro growth rates. This indicates that high p64c-myc levels may be associated with high proliferative activity, and lack of differentiation in lung cancer in general. The p74c-raf1 protein was found in all non-SCLC cell lines. Whether this classification of SCLC cell lines is applicable to SCLC in vivo remains to be determined.

Mapping of a small phosphopeptide at the carboxyterminus of the viral myb protein by monoclonal antibodies

Several myb-specific monoclonal antibodies were produced and their antigen recognition sites characterized using a series of bacterially expressed truncated myb proteins. The monoclonal antibodies were used for analysing the in vivo phosphorylation site of the oncogene protein from avian myeloblastosis virus (AMV), p48v-myb. The p48v-myb protein labeled metabolically with [32P]orthophosphate was isolated from the AMV-transformed chicken myeloblast cell line BM-2 by immunoaffinity chromatography. Phosphoamino acid analysis indicated that it was phosphorylated mainly on serine and to a lesser extent (less than 5%) on threonine residues. Indirect immunoprecipitation of phosphopeptides from trypsin-digested [32P]-labeled purified p48v-myb protein by use of the myb-specific monoclonal antibodies allowed the mapping of a small phosphopeptide at the carboxyterminus of p48v-myb.

Synthetic peptides as substrates and inhibitors of human immune deficiency virus-1 protease

Retroviruses code for a virus-specific protease which is essential for polyprotein processing and viral infectivity. The human immune deficiency virus-1 protease is an aspartic protease of 9 kDa which was synthesized by recombinant DNA technology and arises by autocatalytic processing from a polyprotein precursor which has recently been demonstrated by use of a protease-specific monoclonal antibody. The protease was shown to form dimers. Here we demonstrate that synthetic peptides can be used as both model substrates as well as inhibitors for investigation of the protease. 14 synthetic peptides, 7-18 amino acids in length, containing putative protease cleavage sites of the viral polyprotein gag and pol precursors, have been analyzed with the partially purified protease by the use of high performance liquid chromatography. In seven cases, where cleavage was observed, the length of the peptides did not significantly influence the cleavage efficiencies, heptapeptides being large enough as model substrates. No cleavage was observed with a protein preparation purified in parallel from control bacteria not expressing the human immune deficiency virus-1 protease. The protease was not only able to cut next to a proline but also between other peptides indicating that the proline is not a prerequisite. Three peptides with either reduced bonds at the cleavage site or a substitution by statin were inhibitory while another uncleaved substrate was not. The usefulness of small model substrates for characterization of the protease is further demonstrated by determination of a kinetic optimum pH (3.5-5.5) and incubation temperature (37 degrees C).