Hexon Capsomers Research Articles

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.

Adenovirus assembly: self-assembly of partially digested hexons

Papainic hydrolysis of adenovirus type 2 hexon capsomers was apparently limited and induced self-aggregation of hexons in a preferred oligomeric species, hexon trimers, sedimenting at about 19X, and retaining their immunological properties. These hexon trimers were composed of two classes of trimer molecules with different electric charges. The relevance of this trimer aggregation phenomenon to the adenovirus assembly process is discussed.

Adenovirus binds to rat brain microtubules in vitro.

We have found by negative staining electron microscopy that when similar concentrations of adenovirus and reovirus (viruses of about the same diameter, 75 to 80 nm, and density, 1.34 to 1.36 g/cm3) were incubated with a carbon support film containing microtubules, 72% of adenovirus on the grid, but only 32% (equivalent to random association) of reovirus, were associated with microtubules. Similar concentrations of both larger and smaller particles, such as polystyrene latex spheres and coliphage f2, also exhibited a low degree of interaction, viz., 17 to 37%, with microtubules. Moreover, 90% of microtubule-associated adenovirus binds to within +/- 4 nm of the edge of microtubules, but lower fractions (again equivalent to a random association) of the other particles bind to the edge of the microtubules. The mechanism behind this phenomenon, which we denote as "edge binding," is presently obscure; however, it provides us with a second, albeit empirical, method to distinguish between the microtubular association of adenovirus and other particles. We found that edge binding of adenovirus also occurred when adenovirus was initially placed on the carbon support film and then incubated with microtubules and when adenovirus and microtubules were mixed prior to placement on the support. In contrast, reovirus or the other particles prepared by similar techniques exhibited a random amount of edge binding. The binding of adenovirus appears to involve the hexon capsomers of the virion since (i) high resolution electron micrographs showed that the edge of the virus was in contact with the edge of the microtubules, and (ii) adenovirions briefly treated with formamide to remove pentons and fibers bind as efficiently as intact virions. Core structures, which were obtained by further formamide degradation of the virion, do not associate with microtubules. These observations support the hypothesis of Dales and Chardonnet (1973) that the transport of adenovirions within infected cells is mediated by interaction with microtubules.

Adenovirus assembly. Cross-linking of adenovirus type-2 hexons in vitro.

Cross‐linking of adenovirus hexon capsomers by glutaraldehyde in vitro has been studied as a chemical probe for quaternary structure of hexon protein and virus capsid assembly. Glutaraldehyde cross‐linking of hexons gave rise to three main oligomeric species, trimer, hexamer and nonamer, as analyzed on dodecylsulfate‐acrylamide gels, which corresponded in terms of complete hexon capsomers, to single hexon capsomer, capsomer dimer and trimer. The major species was found to be the trimer of the hexon polypeptide unit, viz. the intramolecularly cross‐linked hexon capsomer. This result indicated that intraoligomeric cross‐linking took place more readily than the interoligomeric reactions. As shown by electron microscopy the spatial arrangement of the cross‐linked hexon capsomer trimer was reminiscent of the structure of groups of three hexons clustered with 3‐fold symmetry to form the groups of nine hexons isolated from disrupted adenovirus capsid.

Antibody to the type 5 adenovirus hexon polypeptide: detection of nascent polypeptides in the cytoplasm of infected KB cells.

Antibody to type 5 adenovirus hexon polypeptides reacted with the isolated hexon polypeptide chanis (monomers) but not with the native multimeric hexon capsomers (trimers). Indirect immunofluorescence detected hexon polypeptides mainly in the cytoplasm of KB cells infected with types 2 and 5 adenoviruses. Cells infected with adenovirus mutants H5ts116 or H5ts115, which are defective in the production of immunologically detectable hexons, synthesized immunologically detectable hexon polypeptides at the nonpermissive temperature.

Thermosensitive events in the replication of adenovirus type 2 at 42°

The production of infectious adenovirus type 2 is reduced more than 99% when infected KB suspension cultures are maintained at 42°. The abortive event(s) at 42° occur late after infection, for cultures held at 42° for 12 hr and then shifted to 37° yield normal amounts of infectious virus. Although 31 S viral DNA and virion polypeptides are synthesized at 42°, no 800 S viral particles are formed. The failure to produce virus at the elevated temperature can be attributed to lack of capsomere assembly. Although infected cultures incubated at 42° accumulate large amounts of structural viral polypeptides, formation of hexon capsomeres is reduced 99% and penton and fiber assembly are diminished more than 90%.

The polypeptides of adenovirus: II. Soluble proteins, cores, top components and the structure of the virion

The adenovirion has been shown to have a much more complex structure than was previously assumed. Eight different types of polypeptides have been assigned a place in the particle. Three are present in an outer capsid, three in an inner core, and two others in association with hexons. The hexon capsomere is composed of about three molecules of a single type of peptide of molecular weight (MW) 120,000 which comprises about 50% of the total virion protein. Groups of such capsomeres, released from the virion by 5 M urea, are found to be associated with two minor polypeptides, each of MW 13,000. The penton-base is composed of a single type of peptide of MW 70,000, and the fiber is composed of another peptide of MW 62,000. The internal core released by treatment of the virion with 5 M urea, contains the viral DNA in association with three arginine-rich peptides of MW 44,000, 24,000, and 24,000, comprising some 20% of the total virion protein. The same three peptides are relatively lacking in the “empty” capsids found as the “top components” of cesium chloride gradients of crude virus preparations.

A proposed terminology for the adenovirus antigens and virion morphological subunits

Recombinant hexons from subgroup C adenoviruses (Ad2 and Ad5) and from a member of subgroup B (Ad3) adenoviruses have been expressed in insect cells. When expressed alone, all three hexons were found to be insoluble and accumulated as inclusion bodies in the cytoplasm. However, co-expression of recombinant Ad2, Ad5 or Ad3 hexon with Ad2 L4-100K protein resulted in the formation of soluble trimeric hexons. EM analysis of hexons revealed that they were indistinguishable from native hexon capsomers isolated from Ad2-infected human cells, or released from partially disrupted adenovirions. This suggests that 100K acts as a chaperone for hexon folding and self-assembly into capsomer in insect cells. Since 100K protein assists in the trimerization of subgroup C hexon, and of subgroup B hexon protein, it implies that it functions in a manner that is both homo- and heterotypic. During the course of recombinant protein expression, the 100K protein was found in association with hexon monomers and trimers within the cytoplasm. In the nucleus, however, 100K was found in complexes with hexon trimers exclusively. EM observation of purified 100K protein samples showed a dumb-bell-shaped molecule compatible with a monomeric protein. EM analysis of hexon-100K protein complexes showed that interaction of hexon with the 100K protein occurred via one of the globular domains of the 100K protein molecule. Our data confirm the role of the 100K protein as a scaffold protein for hexon, and provide evidence suggesting its function in hexon nuclear import in insect cells.

Tuberculin-testing of a School Population (Musselburgh, 1953)

ABSTRACT The 14.4-kDa hexon-associated protein IX (pIX) acts as a cement in the capsids of primate adenoviruses and confers a thermostable phenotype. Here we show that deletion of amino acids 100 to 114 of adenovirus type 5 pIX, which eliminates the conserved coiled-coil domain, impairs its capacity to self-associate. However, pIXΔ100-114 is efficiently incorporated into the viral capsid, and the resulting virions are thermostable. Deletion of the central alanine-rich domain, as in pIXΔ60-72, does not impair self-association, incorporation into the capsid, or the thermostable phenotype. These data demonstrate, first, that the self-association of pIX is dispensable for its incorporation into the capsid and generation of the thermostability phenotype and, second, that the increased thermostability results from pIX monomers binding to different hexon capsomers rather than capsid stabilization by pIX multimers.