MTSS1, a novel target of DNA methyltransferase 3B, functions as a tumor suppressor in hepatocellular carcinoma

DNA methyltransferase 3B (DNMT3B) mediates gene silencing via epigenetic mechanisms during hepatocellular carcinoma (HCC) progression. We aimed to identify novel targets of DNMT3B and their potential regulatory mechanisms in HCC. Metastasis suppressor 1 (MTSS1) was one of the DNMT3B targets and selected for further study. DNMT3B overexpression was detected in 81.25% of clinical HCC specimens and was negatively associated with MTSS1 in HCC cells and clinical samples. The underlying mechanism by which DNMT3B silences MTSS1 was studied using a combination of methylation-specific polymerase chain reaction (PCR) and bisulfite genome sequencing, chromatin immunoprecipitation-PCR and luciferase reporter assays. We found that the MTSS1 promoter region was sparsely methylated, and the methylation inhibitors failed to abolish DNMT3B-mediated MTSS1 silencing. DNMT3B protein bound directly to the 5'-flanking region (-865/-645) of the MTSS1 gene to inhibit its transcription. The functional role of MTSS1 was investigated using in vitro and in vivo tumorigenicity assays. As a result, MTSS1 exerted tumor suppressor effects and arrested cells in the G2/M phase, but not the G1/S phase of the cell cycle when it was depleted or overexpressed in HCC cells. Taken together, MTSS1, a novel target of DNMT3B, is repressed by DNMT3B via a DNA methylation-independent mechanism. MTSS1 was further characterized as a novel tumor suppressor gene in HCC. These findings highlight how DNMT3B regulates MTSS1, and such data may be useful for the development of new treatment options for HCC.

Selection and characterization of HIV-1 variants resistant to the (+) and (-) enantiomers of 2'-deoxy-3'-oxa-4'-thiocytidine (dOTC)

Human immunodeficiency virus type 1 (HIV-1) variants were selected for resistance against the (+) and (-) enantiomers of a novel nucleoside analogue, 2'-deoxy-3'-oxa-4'-thiocytidine (dOTC), using the infectious molecular clone HXB2D grown in the MT-4 line of human T cells. The variants selected with (+) dOTC were approximately 6-7-fold less sensitive than wild-type virus to this drug. Cloning and sequencing of the complete reverse transcriptase (RT) coding region of these variants identified the M1841 mutation and further selection with virus containing the M1841 substitution led to the appearance of an M184V mutation. In contrast, selection experiments performed with (-) dOTC yielded variants capable of growing in drug concentrations as high as 100 microM, but phenotypic analysis of these viruses revealed near wild-type 50% inhibitory concentration (IC50) values for this compound. Site-directed mutagenesis experiments in which the M1841 and M184V mutations were introduced into HXB2D confirmed the importance of these mutations when viruses were grown in MT4 cells. However, wild-type IC50 values in regard to both (-) and (+) dOTC were obtained when these recombinant viruses were grown in cord blood mononuclear cells (CBMC). Clinical isolates of HIV-1 resistant to lamivudine and containing the M184V substitution also displayed low-level resistance to both (-) and (+) dOTC when grown in CBMC. Finally, cell-free RT assays were performed in the presence of either (-) dOTC triphosphate, (+) dOTC triphosphate, or the triphosphate of a racemic mixture of (+) and (-) dOTC with wild-type and mutated M184V-containing recombinant RT. The data demonstrate chain termination effects of these compounds with regard to both wild-type and mutated enzyme and that the latter was approximately twofold less sensitive than the former to these drugs.

Sensitivity and resistance to (+)-calanolide A of wild-type and mutated forms of HIV-1 reverse transcriptase

We have tested both wild-type and drug-resistant mutated, recombinant human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) molecules for sensitivity to each of two non-nucleoside RT inhibitors (NNRTI), (+)-calanolide A and nevirapine, in primer extension assays. We found that RT containing either the V106A or Y181C substitutions, associated with NNRTI resistance, displayed approximately 90-fold resistance to nevirapine but remained fully sensitive to (+)-calanolide A and that the Y181C mutation marginally enhanced susceptibility to the latter drug. In contrast, the Y188H substitution in RT resulted in about 30-fold resistance to (+)-calanolide A in these assays but did not result in diminished sensitivity to nevirapine. Tissue culture results indicated that the combination of (+)-calanolide A and nevirapine possessed an additive to weakly synergistic effect in blocking replication of HIV-1 in tissue culture. These results suggest that (+)-calanolide A and nevirapine might have rationale as a combination therapy for HIV disease.

Identification and characterization of two G protein-coupled receptors for neuropeptide FF

The central nervous system octapeptide, neuropeptide FF (NPFF), is believed to play a role in pain modulation and opiate tolerance. Two G protein-coupled receptors, NPFF1 and NPFF2, were isolated from human and rat central nervous system tissues. NPFF specifically bound to NPFF1 (K(d) = 1.13 nm) and NPFF2 (K(d) = 0.37 nm), and both receptors were activated by NPFF in a variety of heterologous expression systems. The localization of mRNA and binding sites of these receptors in the dorsal horn of the spinal cord, the lateral hypothalamus, the spinal trigeminal nuclei, and the thalamic nuclei supports a role for NPFF in pain modulation. Among the receptors with the highest amino acid sequence homology to NPFF1 and NPFF2 are members of the orexin, NPY, and cholecystokinin families, which have been implicated in feeding. These similarities together with the finding that BIBP3226, an anorexigenic Y1 receptor ligand, also binds to NPFF1 suggest a potential role for NPFF1 in feeding. The identification of NPFF1 and NPFF2 will help delineate their roles in these and other physiological functions.

In vitro selection and characterization of HIV-1 with reduced susceptibility to PMPA

9-(2-phosphonomethoxypropyl)adenine (PMPA) has demonstrated remarkable anti-simian immunodeficiency virus (SIV) activity in macaque models of SIV infection and transmission prevention. Recently, PMPA and its oral prodrug, bis-POC PMPA, have also shown potent anti-human immunodeficiency virus type 1 (HIV-1) activity in Phase I clinical studies. In vitro experiments were performed to address the resistance properties of PMPA. After eight passages in increasing concentrations of PMPA, HIV-1IIIB was able to grow in the presence of 2 microM PMPA, fivefold above the IC50 of PMPA for wild-type parental virus. Sequence analysis of the reverse transcriptase (RT) genes from four of 15 RT clones demonstrated the presence of a K65R substitution in RT and recombinant HIV expressing the K65R RT mutation showed a threefold to fourfold increase in IC50 value for PMPA as compared to wild-type. Additional experiments demonstrated that viruses expressing other nucleoside-associated RT resistance mutations all showed wild-type or < threefold reduced susceptibility to PMPA in vitro. Interestingly, lamivudine-resistant viruses expressing the M184V RT mutation showed wild-type to slightly increased susceptibility to PMPA in vitro and addition of the M184V mutation to HIV with the K65R mutation resulted in reversion to wild-type susceptibility for PMPA. In agreement with the cell culture findings, Escherichia coli-expressed K65R RT showed fivefold reduced susceptibility to PMPA diphosphate, the active moiety of PMPA. Furthermore, in combination experiments, PMPA with hydroxyurea showed synergistic inhibition of HIV replication in vitro. The potent antiretroviral activity and favourable resistance profile of PMPA and bis-POC PMPA are being further investigated in ongoing clinical trials.

[Anterior segment reconstruction and secondary posterior chamber intraocular lens implantation supported by fibromembrane]

OBJECTIVE

To evaluate the methods and effects of anterior segment reconstruction and secondary posterior chamber intraocular lens (IOL) implantation supported by fibromembrane in pupillary area.

METHODS

Anterior segment was reconstructed in 86 eyes with different anterior segment disorders. Anterior segment reconstruction includes: partial penetrating keratoplasty, loosing front adhesion, suturing detached peripheral iris, loosing rear adhesion, reshaping pupil, perforating fibromembrane in the pupillary area. The IOL was secondarily implanted into the posterior chamber, and the patients were followed up in 3 - 32 months.

RESULTS

The surgery was successful in all patients. The postoperative visual acuities in 71 eyes were >or= 0.5 (82.6%), including >or= 0.9 (39.5%) in 34 eyes and <or= 0.2 in 15 eyes (17.4%). After IOL implantation, the IOL correctly centered was in 68 eyes (79.1%), and deviated in 18 eyes (20.9%). Anterior chamber hemorrhage occurred in 5 eyes, 4 of them were recovered by conservative treatment and the blood membrane in another eye was sucked out through its primary incision. All patients had slight postoperative inflammation and without severe long term complications.

CONCLUSION

With the support of enough fibromembrane in pupillary area, anterior segment reconstruction and secondary posterior chamber IOL implantation can be performed, thus IOL suture fixation is not necessary, and its associated complications can be avoided.

Prevalence of HIV-1 resistant to antiretroviral drugs in 81 individuals newly infected by sexual contact or injecting drug use. Investigators of the Quebec Primary Infection Study

OBJECTIVE

Prolonged treatment with antiretroviral drugs results in the selection of HIV-1 variants with mutations conferring resistance to nucleoside and non-nucleoside reverse transcriptase inhibitors (NRTI and NNRTI) or to protease inhibitors (PI). There is serious concern about transmission of resistant viruses to newly infected persons. This study monitored the prevalence of resistant viruses in individuals undergoing primary HIV infection.

DESIGN

Resistance testing was performed on 81 individuals infected between 1997 and 1999 by injecting drug use (n =21), sexual (n = 56), or unknown (n = 4) transmission.

METHODS

Automated sequencing was used to genotype the reverse transcriptase (RT) and protease regions of virus isolated from patients' plasma. The phenotypic susceptibility of stimulated peripheral blood mononuclear cells to antiretroviral drugs was assayed. Line probe assays detected quasispecies variations in wild-type and mutated RT codons.

RESULTS

A high prevalence of PI and RT genotypic variants, associated with high-level resistance to antiretroviral drugs, was observed in individuals newly infected by injecting drug use (PI = 24%, RT = 24%) or sexual transmission (PI = 12%, RT = 22%). The PI mutations, L101, V82A, and L90M, were found in 10.5, 3 and 4% of cases, respectively; whereas for RT, primary mutations at positions T215Y (zidovudine), M184V (lamivudine), T69D/A (zalcitabine), and K103N (multi-NNRTI) were present in 8, 5, 4, and 4% of subjects, respectively. Resistance to NRTI was demonstrated by phenotypic, genotypic, and line probe analyses. Transmission of multidrug (NRTI/NNRTI/PI) resistance in eight subjects (9.9%) was confirmed by showing that source partners possessed viruses of similar genotype.

CONCLUSIONS

The transmission of drug-resistant HIV is a serious problem that merits further attention by public health officials as well as virologists and clinicians.

Reverse transcriptase inhibitors can selectively block the synthesis of differently sized viral DNA transcripts in cells acutely infected with human immunodeficiency virus type 1

We have recently reported that the in vitro inhibition of human immunodeficiency virus type 1 (HIV-1) reverse transcription by inhibitors of reverse transcriptase (RT) occurred most efficiently when the expected DNA products of RT reactions were long (Quan et al. , Nucleic Acids Res. 26:5692-5698, 1998). Here, we have used a quantitative PCR to analyze HIV-1 reverse transcription within acutely infected cells treated with RT inhibitors. We found that levels of minus-strand strong-stop DNA [(-)ssDNA] formed in acutely infected MT2 cells were only slightly reduced if cells were infected with viruses that had been generated in the presence of either azidothymidine or nevirapine (5 microM) and maintained in the presence of this drug throughout the viral adsorption period and thereafter. Control experiments in which virus inoculation of cells was performed at 4 degrees C, followed directly by cell extraction, showed that less than 1% of total (-)ssDNA within acutely infected cells was attributable to its presence within adsorbed virions. In contrast, synthesis of intermediate-length reverse-transcribed DNA products decreased gradually as viral DNA strand elongation took place in the presence of either of these inhibitors. This establishes that nucleoside and nonnucleoside RT inhibitors can exert similar temporal impacts in regard to inhibition of viral DNA synthesis. Generation of full-length viral DNA, as expected, was almost completely blocked in the presence of these antiviral drugs. These results provide insight into the fact that high concentrations of drugs are often needed to yield inhibitory effects in cell-free RT assays performed with short templates, whereas relatively low drug concentrations are often strongly inhibitory in cellular systems.

Mutations within four distinct gag proteins are required to restore replication of human immunodeficiency virus type 1 after deletion mutagenesis within the dimerization initiation site

Human immunodeficiency virus type 1 (HIV-1) genomic RNA segments at nucleotide (nt) positions +240 to +274 are thought to form a stem-loop secondary structure, termed SL1, that serves as a dimerization initiation site for viral genomic RNA. We have generated two distinct deletion mutations within this region, termed BH10-LD3 and BH10-LD4, involving nt positions +238 to +253 and +261 to +274, respectively, and have shown that each of these resulted in significant diminutions in levels of viral infectiousness. However, long-term culture of each of these viruses in MT-2 cells resulted in a restoration of infectiousness, due to a series of compensatory point mutations within four distinct proteins that are normally cleaved from the Gag precursor. In the case of BH10-LD3, these four mutations were MA1, CA1, MP2, and MNC, and they involved changes of amino acid Val-35 to Ile within the matrix protein (MA), Ile-91 to Thr within the capsid (CA), Thr-12 to Ile within p2, and Thr-24 to Ile within the nucleocapsid (NC). The order in which these mutations were acquired by the mutated BH10-LD3 was MNC > CA1 > MP2 > MA1. The results of site-directed mutagenesis studies confirmed that each of these four substitutions contributed to the increased viability of the mutated BH10-LD3 viruses and that the MNC substitution, which was acquired first, played the most important role in this regard. Three point mutations, MP2, MNC, and MA2, were also shown to be sequentially acquired by viruses that had emerged in culture from the BH10-LD4 deletion. The first two of these were identical to those described above, while the last involved a change of Val-35 to Leu. All three of these substitutions were necessary to restore the infectiousness of mutated BH10-LD4 viruses to wild-type levels, although the MP2 mutation alone, but neither of the other two substitutions, was able to confer some viability on BH10-LD4 viruses. Studies of viral RNA packaging showed that the BH10-LD4 deletion only marginally impaired encapsidation while the BH10-LD3 deletion caused a severe deficit in this regard.

Enhanced impairment of chain elongation by inhibitors of HIV reverse transcriptase in cell-free reactions yielding longer DNA products

We have studied the relationship between the length of HIV-1 reverse transcriptase (RT)-mediated nucleotide polymerization and inhibitors of these reactions in cell-free RT assays performed in the presence of either of two dideoxynucleoside triphosphates (ddNTPs), i.e. AZTTP or 3TCTP, or nevirapine, a non-nucleoside RT inhibitor. These reactions employed a heterologous RNA template and three DNA oligonucleotide primers, i.e. pAR, dPR and PA, that yielded distinct full-length products of 65, 192 and 376 nt, respectively, in the absence of inhibitor. We now show that the extent of inhibition of RT activity was greatest with use of the PA primer, which normally yielded the longest reaction product, and that lesser degrees of inhibition were noted in the reactions that generated shorter products. For example, at a concentration of 5 microM AZTTP, the extent of inhibition was 75% with the PA primer but only 40% and <10% when reactions were primed by the dPR and pAR primers, respectively. Similar results were obtained when either a mutated form of HIV RT (i.e. M184V), associated with resistance to 3TC, was tested in the presence of 3TCTP or when RT derived from Moloney murine leukemia virus (M-MuLV) was tested in the presence of AZTTP.

Characterization of human immunodeficiency virus type-1 (HIV-1) particles that express protease-reverse transcriptase fusion proteins

We have selectively mutagenized specific residues at the junction between the protease (PR) and reverse transcriptase (RT) genes of human immunodeficiency virus type 1 (HIV-1) to study the effects of PR-RT fusion proteins in the context of a full-length, infectious proviral construct. Mutant viruses derived from COS-7 cells transfected with this construct were analyzed in regard to each of viral replication, maturation, and infectivity. Immunoblot analysis revealed that the mutation prevented cleavage between the PR and RT proteins and that both existed as a PR-RT fusion protein in each of cellular and viral lysates. Interestingly, intracellular PR that existed within the PR-RT fusion protein remained functionally active, whereby HIV-1 precursor proteins were processed efficiently. Furthermore, the RT component of the fusion protein also retained its enzymatic activity as shown in RT assays. Electron microscopy revealed that the mutant viruses containing the PR-RT fusion protein possessed wild-type morphology. These viruses also displayed wild-type sensitivities to inhibitors of each of the HIV-1 PR and RT activities. However, viruses containing the PR-RT fusion protein were 20 times less infectious than wild-type viruses. This defect was further pronounced when mutated Gag-Pol proteins were overexpressed as a consequence of an additional mutation that interfered with frameshifting. Thus, unlike cleavage site mutations at the N terminus of PR, a cleavage site mutation between PR and RT did not affect the enzymatic activities of either PR or RT and viruses containing PR-RT fusion proteins were viable.

Permeability characteristics of tetragastrins across intestinal membranes using the Caco-2 monolayer system: comparison between acylation and application of protease inhibitors

PURPOSE

Three types of acyl tetragastrin (TG), acetyl-TG (C2-TG), butyryl-TG (C4-TG) and caproyl-TG (C6-TG) were synthesized and their in vitro intestinal permeability characteristics were examined using Caco-2 monolayers.

METHODS

The disappearance of acyl-TGs from the apical side of Caco-2 monolayers was estimated by analyzing degradation and permeation processes in terms of clearance.

RESULTS

The amount of native TG transported to the basolateral side was very low due to its large degradation clearance (CLd) on the apical side. Degradation of TG was reduced by chemical modification with fatty acids, which resulted in an increase in the transport of TG across Caco-2 monolayers. In addition, the permeation clearance (CLp) value of carboxyfluorescein (CF), a paracellular transport and undegradable marker, was increased in the presence of acyl-TGs. Furthermore, we investigated the effects of the protease inhibitors bacitracin and gabexate on the transport of TG across Caco-2 monolayers. In the presence of a low concentration (0.1 mM) of protease inhibitor, the CLd value of TG was reduced, but they did not affect its CLp value. However, a higher concentration (1.0 mM) of bacitracin significantly reduced TG degradation on the apical side, and further increased its CLp value.

CONCLUSIONS

We demonstrated that acylation of TG made it resistant to intestinal proteases and caused it to enhance absorption of drugs, including itself, across Caco-2 monolayers. Further, bacitracin acted as both a protease inhibitor and an absorption enhancer.

Dominance of the E89G substitution in HIV-1 reverse transcriptase in regard to increased polymerase processivity and patterns of pausing

The substitution of a glycine for glutamic acid at position 89 in human immunodeficiency virus-1 (HIV-1) reverse transcriptase (RT) (E89G) confers resistance to several nucleoside and non-nucleoside inhibitors of RT. As residue 89 contacts the template strand, it has been suggested that this mutation may modulate the conformation of the RT.template/primer complex. In addition, certain mutations in RT that confer resistance to nucleoside analogs, such as M184V, are located near the polymerase active site. To characterize further these substitutions, we performed processivity assays alongside an analysis of pausing profiles with wild-type (wt) RT and recombinant RTs containing substitutions at E89G, M184V, or both. We now show that E89G RT has higher processivity than wt enzyme as well as a different pattern of pausing sites. Similar findings were obtained with the doubly mutated RT, although enzyme containing only the M184V mutation had lower processivity than wt. Consistent with these observations, and from a mechanistic standpoint, both E89G-containing as well as doubly mutated RT had decreased dissociation constants from a complex consisting of RT and template-primer, in comparison with either wt RT or M184V-containing RT. No significant differences were observed among the various enzymes in regard to Km values for the heteropolymeric RNA template used in these studies. Viruses containing the E89G mutation synthesized longer strand DNA products than either wt viruses or viruses containing only the M184V mutation in endogenous RT assays. Thus, the E89G substitution is a dominant determinant in regard to each of the koff values from an RT.template/primer complex, RT processivity, and specific patterns of pausing during DNA polymerization.

Mechanistic studies of early pausing events during initiation of HIV-1 reverse transcription

We have investigated the role of sequences that surround the primer binding site (PBS) in the reverse transcriptase-mediated initiation of (-) strand DNA synthesis in human immunodeficiency virus type 1. In comparisons of reverse transcription initiated from either the cognate primer tRNALys.3 or a DNA primer D-Lys.3, bound to PBS sequences, we observed that a +3 pausing site occurred in both circumstances. However, the initiation reaction with tRNALys.3 was also characterized by a pausing event after incorporation of the first nucleotide. Alteration of sequences at the 5'-end instead of the 3'-end of the PBS resulted in elimination of the +3 pausing site, suggesting that this site was template sequence-dependent. In contrast, the pausing event at the +1 nucleotide position was still present in experiments that employed either of these mutated RNA templates. The mutations at the 5'-end of the PBS also caused a severely diminished rate of initiation and the strong arrest of reactions at the +1 stage when tRNALys.3 was used as primer. Therefore, we propose that the +1 pausing event is an initiation-specific event in regard to reactions primed by tRNALys.3 and that sequences at the 5'-end of the PBS may facilitate the release of reverse transcription from initiation to elongation.

Endogenous reverse transcriptase assays reveal synergy between combinations of the M184V and other drug resistance-conferring mutations in interactions with nucleoside analog triphosphates

Resistance of HIV-1 reverse transcriptase (RT) to nucleoside analogs (e.g. AZT, ddC and 3TC) is conferred by various amino acid substitutions or combinations thereof on the RT molecule. The M184V mutation, that confers high and low-level resistance to 3TC and ddC, respectively, can restore sensitivity to AZT when introduced into RT against a background of AZT-resistance. The K65R mutation, that confers low level resistance to both 3TC and ddC, can also restore sensitivity to AZT. This information is of potential utility in choosing combinations of anti-viral drugs for clinical use. To explore this subject further, we have used an endogenous RT reaction to study mutated viruses containing M184V alone or M184V combined with each of the K65R, E89G or both the M41L and T215Y substitutions. Endogenous assays possess the advantage of utilizing genomic RNA as template in a reaction mixture that includes each of tRNALys.3 and viral nucleocapsid protein, necessary for specific initiation of reverse transcription, as well as all other viral proteins that might impact on this process. We now show that viruses containing both M184V and K65R displayed synergistic resistance to 3TC triphosphate (3TCTP), while the same combination yielded the same level of resistance to ddC triphosphate (ddCTP) as that manifested by K65R alone. The combination of M184V and E89G displayed synergistic resistance against ddCTP but not 3TCTP, while viruses containing only E89G were highly resistant to 3TCTP and displayed low-level resistance to ddCTP. The results show that endogenous RT assays can reveal variable synergistic, antagonistic, or neutral effects in regard to drug sensitivity, depending on the presence of specific amino acid substitutions in RT itself.

Sequence elements downstream of the human immunodeficiency virus type 1 long terminal repeat are required for efficient viral gene transcription

We investigated the role of a 54-nucleotide region (+200 to +253) located downstream of the HIV-1 long terminal repeat (LTR) on virus gene expression and found, using RT-PCR and p24 CA analysis, that deletion of this region inhibited synthesis of both viral RNA and protein. CAT assays showed that these results were attributable to decreased transcription efficiency of the HIV-1 LTR and not to the stability of the RNA transcripts produced. Further deletional analysis and transfection studies showed that the most important sequences with regard to proviral DNA expression were located between nucleotide positions +218 and +237. Furthermore, substitutional mutational analysis showed that a CTCTCTC sequence at positions +227 to +233, homologous to the pyrimidine-rich initiator (Inr) region found in several promoters, was required for efficient production of both viral RNA and protein. Deletion of the sequence +200 to +217, homologous to the interferon-stimulated response element (ISRE), resulted in impaired LTR promoter activity and decreased synthesis of viral RNA and protein. However, when the latter region was replaced by homologous ISRE sequences from an interferon-stimulated gene (ISG-54), an even more severe effect on HIV gene expression and replication was observed, suggesting that ISRE-like sequences in HIV act differently from homologous sequences in interferon-responsive cellular genes.

The alpha chain of laminin-1 is independently secreted and drives secretion of its beta- and gamma-chain partners

A mammalian recombinant strategy was established to dissect rules of basement membrane laminin assembly and secretion. The alpha-, beta-, and gamma-chain subunits of laminin-1 were expressed in all combinations, transiently and/or stably, in a near-null background. In the absence of its normal partners, the alpha chain was secreted as intact protein and protein that had been cleaved in the coiled-coil domain. In contrast, the beta and gamma chains, expressed separately or together, remained intracellular with formation of betabeta or betagamma, but not gammagamma, disulfide-linked dimers. Secretion of the beta and gamma chains required simultaneous expression of all three chains and their assembly into alphabetagamma heterotrimers. Epitope-tagged recombinant alpha subunit and recombinant laminin were affinity-purified from the conditioned medium of alphagamma and alphabetagamma clones. Rotary-shadow electron microscopy revealed that the free alpha subunit is a linear structure containing N-terminal and included globules with a foreshortened long arm, while the trimeric species has the typical four-arm morphology of native laminin. We conclude that the alpha chain can be delivered to the extracellular environment as a single subunit, whereas the beta and gamma chains cannot, and that the alpha chain drives the secretion of the trimeric molecule. Such an alpha-chain-dependent mechanism could allow for the regulation of laminin export into a nascent basement membrane, and might serve an important role in controlling basement membrane formation.

Identification of sequences downstream of the primer binding site that are important for efficient replication of human immunodeficiency virus type 1

Reverse transcription of retroviruses is initiated from an 18-nucleotide (nt) primer binding site (PBS), located within the 5' region of viral genomic RNA, to which the host cell-derived tRNA primer is annealed and also involves viral genomic sequences outside the PBS. We constructed proviral DNA clones of human immunodeficiency virus (HIV) that had selective deletions of either a 7-nt segment found immediately downstream of the PBS or an extended nontranslated 54-nt stretch located immediately downstream of the PBS and containing the aforementioned 7-nt segment. Synthesis of minus-strand strong-stop DNA was assessed with MT-4 cells infected with viruses derived from COS-7 cells that had been transfected with these various constructs. We found that similar levels of minus-strand strong-stop DNA as well as DNA produced after template switching were expressed in MT-4 cells infected with COS-7-derived wild-type viruses or with viruses that had the 7-nt segment deleted. In contrast, significantly lower levels of viral DNA were detected in MT-4 cells after infection with viruses that had deletions of the 54-nt stretch. Furthermore, the molecular clone containing the 7-nt deletion was able to replicate with wild-type kinetics, while that containing the 54-nt deletion displayed a significantly diminished capacity in this regard. Further deletion analysis showed that a 16-nt segment at the 3' end of this 54-nt segment was largely responsible for these effects. We also conducted studies to determine levels of viral mRNA in COS-7 cells that had been transfected with equivalent amounts of DNA derived from either a wild-type HIV construct or our various deletion mutants. In the case of transfections performed with the 7-nt deletion mutant and wild-type HIV DNA, high levels of viral mRNA transcripts were detected, which was not the case for the 54 nt-deletion mutant. However, these various mRNAs possessed similar stabilities, as shown through studies in which transcript formation was arrested by treatment of cells with actinomycin D. Thus, the 54-nt segment of 5' nontranslated RNA, located downstream of the PBS, is involved in efficient expression of each of viral DNA, mRNA, and infectious virus.