Genetic analysis of adenovirus type 2. I. Isolation and genetic characterization of temperature-sensitive mutants

Functional genetic and biophysical analyses of membrane disruption by human adenovirus

The identification of the adenovirus (AdV) protein that mediates endosome penetration during infection has remained elusive. Several lines of evidence from previous studies suggest that the membrane lytic factor of AdV is the internal capsid protein VI. While these earlier results imply a role for protein VI in endosome disruption, direct evidence during cell entry has not been demonstrated. To acquire more definitive proof, we engineered random mutations in a critical N-terminal amphipathic α-helix of VI in an attempt to generate AdV mutants that lack efficient membrane penetration and infection. Random mutagenesis within the context of the AdV genome was achieved via the development of a novel technique that incorporates both error-prone PCR and recombineering. Using this system, we identified a single mutation, L40Q, that significantly reduced infectivity and selectively impaired endosome penetration. Furthermore, we obtained biophysical data showing that the lack of efficient endosomalysis is associated with reduced insertion of the L40Q mutation in protein VI (VI-L40Q) into membranes. Our studies indicate that protein VI is the critical membrane lytic factor of AdV during cellular entry and reveal the biochemical basis for its membrane interactions.

Genetic Analysis of Adenovirus Type 2 II. Preliminary Phenotypic Characterization of Temperature-Sensitive Mutants

The properties of temperature-sensitive mutants of adenovirus type 2 representing 12 complementation groups were studied. All mutants were normal with respect to adsorption as measured by viral inclusion formation and viral DNA synthesis as shown by velocity sedimentation in alkaline sucrose gradients. One mutant, however, formed viral inclusions of altered morphology at the nonpermissive temperature. The synthesis of the major capsid proteins was examined by immunodiffusion. On this basis, the complementation groups could be arranged as follows: (i) one group was negative for all three proteins; (ii) three groups failed to synthesize penton bases; (iii) eight groups were positive for hexons, pentons, and fibers. The assembly of virus particles at 39 C was examined by equilibrium sedimentation in CsCl; three groups were found defective, whereas two of the penton-negative groups were positive for virion production. Tests of the thermolability of virions at 50 C revealed eight groups labile whereas the remainder were insensitive to heat inactivation. None of five mutants inoculated in newborn rats induced tumors, although three of them were capable of in vitro transformation.

Adenovirus protein VI mediates membrane disruption following capsid disassembly

In contrast to enveloped viruses, the mechanisms involved in membrane penetration by nonenveloped viruses are not as well understood. In these studies, we determined the relationship between adenovirus (Ad) capsid disassembly and the development of membrane lytic activity. Exposure to low pH or heating induced conformational changes in wild-type Ad but not in temperature-sensitive Ad (ts1) particles that fail to escape the early endosome. Wild-type Ad but not ts1 particles permeabilized model membranes (liposomes) and facilitated the cytosolic delivery of a ribotoxin. Alterations in wild-type Ad capsids were associated with the exposure of a pH-independent membrane lytic factor. Unexpectedly, this factor was identified as protein VI, a 22-kDa cement protein located beneath the peripentonal hexons in the viral capsid. Recombinant protein VI and preprotein VI, but not a deletion mutant lacking an N-terminal amphipathic alpha-helix, possessed membrane lytic activity similar to partially disassembled virions. A new model of Ad entry is proposed based on our present observations of capsid disassembly and membrane penetration.

Switch from capsid protein import to adenovirus assembly by cleavage of nuclear transport signals

Replication and assembly of adenovirus occurs in the nucleus of infected cells, requiring the nuclear import of all viral structural proteins. In this report we show that nuclear import of the major capsid protein, hexon, is mediated by protein VI, a structural protein located underneath the 12 vertices of the adenoviral capsid. Our data indicate that protein VI shuttles between the nucleus and the cytoplasm and that it links hexon to the nuclear import machinery via an importin alpha/beta-dependent mechanism. Key nuclear import and export signals of protein VI are located in a short C-terminal segment, which is proteolytically removed during virus maturation. The removal of these C-terminal transport signals appears to trigger a functional transition in protein VI, from a role in supporting hexon nuclear import to a structural role in virus assembly.

Isolation and characterization of temperature-sensitive mutants of adenovirus type 7

Fifty temperature-sensitive mutants, which replicate at 32 degrees C but not at 39.5 degrees C, were isolated after mutagenesis of the vaccine strain of adenovirus type 7 with hydroxylamine (mutation frequency of 9.0%) or nitrous acid (mutation frequency of 3.8%). Intratypic complementation analyses separated 46 of these mutants into seven groups. Intertypic complementation tests with temperature-sensitive mutants of adenovirus type 5 showed that the mutant in complementation group A failed to complement H5ts125 (a DNA-binding protein mutant), that mutants in group B and C did not complement adenovirus type 5 hexon mutants, and that none of the mutants was defective in fiber production. Further phenotypic characterization showed that at the nonpermissive temperature the mutant in group A failed to make immunologically reactive DNA-binding protein, mutants in groups B and C were defective in transport of trimeric hexons to the nucleus, mutants in groups D, E, and F assembled empty capsids, and mutants in group G assembled DNA-containing capsids as well as empty capsids. The mutants of the complementation groups were physically mapped by marker rescue, and the mutations were localized between the following map coordinates: groups B and C between 50.4 and 60.2 map units (m.u.), groups D and E between 29.6 and 36.7 m.u., and group G between 36.7 and 42.0 m.u. or 44.0 and 47.0 m.u. The mutant in group A proved to be a double mutant.

Comparison of the interactions of the adenovirus type 2 major core protein and its precursor with DNA

The interactions of the major core protein of adenovirus type 2 (Ad2) protein VII, and its precursor, protein pre-VII, with viral DNA, were studied using UV light induced crosslinking of 32P-labelled oligonucleotides to the proteins. Proteolytic fragments of these two proteins that contain DNA-binding domains were identified by virtue of their covalently attached, alkali-resistant 32P-radioactivity. The overall efficiency of crosslinking of protein pre-VII to DNA, in H2ts1 virions assembled at 39 degrees C, was comparable to that of the crosslinking of protein VII to DNA in Ad2 virions. However, a protease V8 fragment comprising the N-terminal half of protein pre-VII crosslinked to DNA at least ten times more efficiently than the corresponding N-terminal fragment of protein VII, which is truncated by the removal of 23 amino acids from the N-terminus of protein pre-VII during virion maturation.

Recombination in adenovirus: DNA sequence analysis of crossover sites in intertypic recombinants

The nucleotide sequence of the adenovirus type 5 genome has been determined for a 620-bp region that spans the C terminus of the pVI gene and the N terminus of the hexon gene, and compared to the adenovirus type 2 DNA sequence: 25 base changes have been identified, most of which do not lead to alterations in the amino acid sequence and regulatory signals in the region. Crossover sites in three intertypic recombinants have been previously located in this region of the genome by fine restriction mapping. A sequence determination for the three recombinants, and the four ts mutants used in generating the ts+ recombinants, was carried out. The crossovers were in each case located in a small region of complete sequence homology (from 45 to 156 nucleotides long) flanked on either side by sequences derived from each parent. These structures are compatible with a reciprocal crossing over model of generalised recombination, where a recombinant joint has resolved in a region of high DNA homology. For the recombinants considered here, this region abutts onto a neighbouring region of much lower sequence homology, and it is possible that the position of the crossover is determined at least in part by the termination of branch migration at a heterologous boundary.

Physical mapping of adenovirus type 2 temperature-sensitive mutations by restriction endonuclease analysis of interserotypic recombinants

The genome structures of about 100 interserotypic ts recombinants produced in crosses between human adenovirus type 2 (H2) and 5 (H5) temperature-sensitive mutants were analyzed by cleavage with restriction endonucleases to determine the map coordinates of the following temperature-sensitive mutants: penton base plus fiber-defective H2 ts103, -104, and -136, assembly-defective H2 ts112, fiber-defective H2 ts125, hexon-defective H2 ts118 and -121, and DNA-negative H2 ts111. H5 ts1 (100 K defective), H5 ts36 (DNA negative), H5 ts125 (mutated in the early 72,000-dalton protein), H5 ts22 (fiber defective), H5 ts58 (IIIa defective), and H5 ts18 and -19 were used as one of the parents. The physical locations of the H2 temperature-sensitive mutations thus defined are discussed in relation to the genetic map, the biological function altered, and the positions of the structural genes on the genome.

Processing of the adenovirus terminal protein

The termini of nascent adenovirus DNA molecules synthesized in vivo are covalently bound to a protein with an apparent molecular weight of 80,000. This protein represents a precursor to the 55,000-dalton protein known to be bound to the 5' termini of mature adenovirus genomes. Processing of the 80-kilodalton precursor to the 55-kilodalton terminal protein is not required for continued adenovirus DNA replication and is probably accomplished during a late stage of virion maturation.

Morphogenesis of human adenovirus type 2 studied with fiber- and fiber and penton base-defective temperature-sensitive mutants

The nature, polypeptide composition, and antigenic composition of the particles formed by six human adenovirus type 2 temperature-sensitive (ts) mutants were studied. ts115, ts116, and ts125 were phenotypically fiber-defective mutants, and ts103, ts104, and ts136 failed to synthesize detectable amounts of fiber plus penton base at 39.5 degrees C. The mutants belonged to five complementation groups, one group including ts116 and ts125. Except for ts103 and ts136, the other mutants were capable of producing particles at 39.5 degrees C. ts116 and ts125 accumulated light assembly intermediate particles (or top components) at nonpermissive temperatures, with few virus particles. The sodium dodecyl sulfate polypeptide pattern of ts116- or ts125-infected cells, intermediate particles, and virus particles showed that polypeptide IV (fiber) was smaller by a molecular weight of 2,000 than that in the wild-type virion and was glycosylated. In fiber plus penton base-defective ts104-infected cells, equivalent quantities of top components and viruses with a buoyant density (rho) of 1.345 g/ml (rho = 1.345 particles) were produced at 39.5 degrees C. These rho = 1.345 particles corresponded to young virions, as evidenced by the presence of uncleaved precursors to proteins VI, VIII, and VII. These young virions matured upon a shift down. Virus capsid vertex antigenic components underwent a phase of eclipse during their incorporation into mature virus particles. No antigenic penton base or IIa was detected in intermediate particles of all the ts mutants tested. Only hexon and traces of fiber antigens were found in ts104 young virions. Penton base and IIIa appeared as fully antigenically expressed capsid subunits in mature wild-type virions or ts104 virions after a shift down. The ts104 lesion is postulated to affect a regulatory function related in some way to penton base and fiber overproduction and the maturation processing of precursors PVI, PVII, and PVII.

Uncoating of adenovirus type 2

The uncoating of adenovirus type 2 and a temperature-sensitive mutant, tsl, was studied. HEp-2 cells were infected with 32P- OR 125I-labeled purified virions for various lengths of time, and the nuclear and cytoplasmic fractions were analyzed by sucrose gradient velocity sedimentation and sodium dodecyl sulfate-polyacryl-amide gel electrophoresis. Within 1 h of infection, virions were converted into three subviral structures: (1) subviral structures in the cytoplasm with a density greater than virions but which qualitatively still contained all virus polypeptides; (ii) corelike structures associated with both the nuclear and cytoplasmic fractions and composed of viral DNA and polypeptides VIa2, V and PVII; and (iii) putative DNA-terminal protein complexes in the nuclei. The kinetic and compartmentalization studies suggested that the DNA-terminal protein complex is the end product of uncoating. The virions which were synthesized by tsl at the nonpermissive temperature and contained the precursor polypeptides PVI and PVII were found to be blocked in uncoating at the corelike stage. This block in uncoating provides the explanation for the lack of infectivity of these virions. A model for the uncoating of adenovirus is proposed.

Genetic analysis of adenovirus type 2. VIII. Physical locations of temperature-sensitive mutations

Temperature-sensitive mutations of human adenoviruses can be physically located on the viral genome by determining the DNA structures of recombinants formed in genetic crosses between different members of the same subgroup. We have analyzed the DNA structures of many interserotypic recombinants formed in crosses between temperature-sensitive (ts) mutants of adenovirus type 2 and adenovirus type 5 with the restriction endonucleases BamHI, EcoRI, HindIII, and and Sma I. In this way, we have mapped the physical coordinates of adenovirus type 2 (Ad2) ts1, Ad2 ts3, Ad2 ts4, and Ad2 ts48, and refined the mapping of Ad5 ts1.

Characterization of a temperature-sensitive, hexon transport mutant of type 5 adenovirus

Infection of KB cells at 39.5 degrees C with H5ts147, a temperature-sensitive (ts) mutant of type 5 adenovirus, resulted in the cytoplasmic accumulation of hexon antigen; all other virion proteins measured, however, were normally transported into the nucleus. Immunofluorescence techniques were used to study the intracellular location of viral proteins. Genetic studies revealed that H5ts147 was the single member of a nonoverlapping complementation group and occupied a unique locus on the adenovirus genetic map, distinct from mutants that failed to produce immunologically reactive hexons at 39.5 degrees C ("hexon-minus" mutants). Sedimentation studies of extracts of H5ts147-infected cells cultured and labeled at 39.5 degrees C revealed the production of 12S hexon capsomers (the native, trimeric structures), which were immunoprecipitable to the same extent as hexons synthesized in wild type (WT)-infected cells. In contrast, only 3.4S polypeptide chains were found in extracts of cells infected with the class of mutants unable to produce immunologically reactive hexon protein at 39.5 degrees C. Hexons synthesized in H5ts147-infected cells at 39.5 degrees C were capable of being assembled into virions, to the same extent as hexons synthesized in WT-infected cells, when the temperature was shifted down to the permissive temperature, 32 degrees C. Infectious virus production was initiated within 2 to 6 h after shift-down to 32 degrees C; de novo protein synthesis was required to allow this increase in viral titer. If ts147-infected cells were shifted up to 39.5 degrees C late in the viral multiplication cycle, viral production was arrested within 1 to 2 h. The kinetics of shutoff was similar to that of a WT-infected culture treated with cycloheximide at the time of shift-up. The P-VI nonvirion polypeptide, the precursor to virion protein VI, was unstable at 39.5 degrees C, whereas the hexon polypeptide was not degraded during the chase. It appears that there is a structural requirement for the transport of hexons into the nucleus more stringent than the acquisition of immunological reactivity and folding into the 12S form.

Isolation and characterization of temperature-sensitive mutants of adenovirus type2

Fourteen temperature-sensitive mutants of human adenovirus type2, which differed in their plaquing efficiencies at at the permissive and nonpermissive temperatures by 4 to 5 orders of magnitude, were isolated. These mutants, which could be assigned to seven complementation groups, were tested for their capacity to synthesize adenovirus DNA at the nonpermissive temperature. Three mutants in three different complementation groups proved deficient in viral DNA synthesis. The DNA-negative mutant H2ts206 complemented the DNA-negative mutants H5ts36 and H5ts125, whereas mutant H2ts201 complemented H5ts36 only. Among the DNA-negative mutants, H2ts206 synthesized the smallest amount of viral DNA at the nonpermissive temperature (39.5 C). Data obtained in temperature shift experiments indicated that a very early function was involved in temperature sensitivity. In keeping with this observation, early virus-specific mRNA was not detected in cells infected with H2ts206 and maintained at 39.5 C. Prolonged (52 h) incubation of cells infected with H2ts206 at the nonpermissive temperature led to the synthesis of a high-molecular-weight form of viral DNA.

Genetic analysis of adenovirus type 2 III. Temperature sensitivity of processing viral proteins

Using sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis of [35S]methionine-labeled adenovirus type 2-infected KB cell extracts, a total of 23 virus-induced polypeptides was detected. This technique was applied to the analysis of the temperature-sensitive mutant, ts 1, which has previously been shown to be defective in a late function. By means of pulse-chase experiments, ts 1 was shown to be defective in the processing of the precursor polypeptide (Pre VII) to the major core protein VII. Two other putative precursor polypeptides, Va (27K) and Vb (24K), were also not processed. Thus, the ts 1 mutation blocked the appearance of six post-translational clevage products, i. e., polypeptides VI, VII, VIII, X, XI, and XII. All of these polypeptides are virion components. Processing was temperature sensitive in a shift-up experiment, whereas it was normal in a shift-down experiment. The kinetics of the temperature-shift experiments suggested that infectious virus could be recovered if enough time is provided for processing to take place. Processing was not inhibited by cycloheximide. The analysis of purified virus particles and empty shells (TCs) revealed the presence of the precursor and putative precursor polypeptides Pre-VII, Va and Vb, instead of their cleavage products, in both types of particles. Based on these results we propose that the ts 1 gene codes for or regulates an endoprotease which is responsible for the completion of the last step in virus maturation, that is, the conversion of "young virions" into mature infectious virions by a series of maturation cleavages.