Vitronectin receptor antibodies inhibit infection of HeLa and A549 cells by adenovirus type 12 but not by adenovirus type 2.

The penton base gene from adenovirus type 12 (Ad12) was sequenced and encodes a 497-residue polypeptide, 74 residues shorter than the penton base from Ad2. The Ad2 and Ad12 proteins are highly conserved at the amino- and carboxy-terminal ends but diverge radically in the central region, where 63 residues are missing from the Ad12 sequence. Conserved within this variable region is the sequence Arg-Gly-Asp (RGD), which, in the Ad2 penton base, binds to integrins in the target cell membrane, enhancing the rate or the efficiency of infection. The Ad12 penton base was expressed in Escherichia coli, and the purified refolded protein assembled in vitro with Ad2 fibers. In contrast to the Ad2 penton base, the Ad12 protein failed to cause the rounding of adherent cells or to promote attachment of HeLa S3 suspension cells; however, A549 cells did attach to surfaces coated with either protein and pretreatment of the cells with an integrin alpha v beta 5 monoclonal antibody reduced attachment to background levels. Treatment of HeLa and A549 cells with integrin alpha v beta 3 or alpha v beta 5 monoclonal antibodies or with an RGD-containing fragment of the Ad2 penton base protein inhibited infection by Ad12 but had no effect on and in some cases enhanced infection by Ad2. Purified Ad2 fiber protein reduced the binding of radiolabeled Ad2 and Ad12 virions to HeLa and A549 cells nearly to background levels, but the concentrations of fiber that strongly inhibited infection by Ad2 only weakly inhibited Ad12 infection. These data suggest that alpha v-containing integrins alone may be sufficient to support infection by Ad12 and that this pathway is not efficiently used by Ad2.     

Binding of adenovirus and its external proteins to Triton X-114. Dependence on pH

35S-Labeled adenovirus type 2 (Ad2) (10 ng/ml) was incubated with 1% Triton X-114 at various pH values varying from 3.0 to 8.0. The detergent phase was separated from the aqueous phase by centrifugation, and the amounts of Ad2 were determined in the two phases. At pH 7.0-8.0, less than 5% of Ad2 was associated with the detergent phase; at pH 5.0 or below, about 60% of Ad2 was associated with the detergent phase. When a mixture of 35S-labeled capsid proteins was used at pH 7.0, 60-70% of the total proteins were associated with the detergent at pH 5.0, but less than 5% of the proteins interacted with detergent at pH 7.0. Among the three major external proteins (hexon, penton base, and fiber), penton base had the highest association with Triton X-114 at pH 5.0. Both intact virus and the capsid proteins that were associated with Triton X-114 at pH 5.0 were released into the aqueous phase on subsequent incubation at pH 7.0. On the basis of these results, it is suggested that mildly acidic pH induces amphiphilic properties in adenovirus capsid proteins and may help Ad2 escape from acidic endocytic vesicles.     

Impact of human adenovirus type 3 dodecahedron on host cells and its potential role in viral infection

During human adenovirus type 3 (Ad3) infection, an excess of penton base and fiber proteins are produced which form dodecahedral particles composed of 12 pentamers of penton base and 12 trimers of fiber protein. No biological functions have yet been ascribed to Ad3 dodecahedra. Here, we show that dodecahedra compete with Ad3 virions for binding to the cell surface and trigger cell remodeling, giving new insights into possible biological functions of dodecahedra in the Ad3 infectious cycle.     

Comparative analysis of adenovirus fiber-cell interaction: adenovirus type 2 (Ad2) and Ad9 utilize the same cellular fiber receptor but use different binding strategies for attachment.

We have analyzed the binding of adenovirus (Ad) serotypes from subgroups B, C, and D through fiber-virus and fiber-fiber cross-competition experiments. Since viruses in these distinct subgroups display markedly different tropisms, it was unexpected that the subgroup C viruses Ad2 and 5 and the subgroup D virus Ad9 cross-competed for the same cellular fiber receptor. The subgroup B serotype Ad3 recognized a receptor distinct from the Ad2, 5, and 9 fiber receptor. However, despite sharing the same fiber receptor, Ad2 and Ad9 displayed markedly different binding characteristics that appeared to result from direct Ad9 binding to cells via alpha(v)-integrins. Unlike Ad2, Ad9 binding to many cell lines was not abrogated by competition with the fiber 9 knob (F9K). Ad9 binding to fiber receptor-deficient cells was blocked by a monoclonal antibody to alpha(v)-integrins. In contrast, Ad9 binding to alpha(v)-deficient cells that express fiber receptor was blocked by F9K. Transfection of an alpha(v)-integrin-deficient cell line with a plasmid that expresses alpha(v)beta5 resulted in Ad9 binding that was not significantly blocked by F9K but was blocked with a combination of F9K and penton base. These results imply that the shorter length of fiber 9 (11 nm) relative to fiber 2 (37 nm) permits fiber-independent binding of Ad9 penton base to alpha(v)-integrins. The difference in fiber length may explain the different binding characteristics and tissue tropisms of each virus despite both utilizing the same fiber and penton base receptors.     

Adenovirus-dependent release of choline from plasma membrane vesicles at an acidic pH is mediated by the penton base protein.

It has been suggested that during receptor-mediated endocytosis of human adenovirus (Ad) type 2 into cells, Ad disrupts the membrane of endocytic vesicles to enter the cytosol. To understand the mechanism of Ad-mediated disruption of the endocytic vesicles, I exposed Ad to plasma membrane vesicles derived from KB cells. Ad caused release of choline from the plasma membrane vesicles preloaded with [3H]choline. The efflux of choline was dependent on (i) the concentration of Ad, with a half-maximal effect at 0.5 microgram/ml; (ii) the pH of the buffer, with the optimum pH of the reaction ranging from 5.5 to 6.0; (iii) the length of the incubation, with a half-maximal release at 2 min; and (iv) the temperature of the incubation, with the optimum temperature being 37 degrees C. The Ad-dependent release of choline was inhibited by anti-penton base, while antihexon did not block the effect. These results suggest roles for a low-pH environment and the penton base protein in the Ad-dependent efflux of choline from plasma membrane vesicles.     

Two Types of Functionally Distinct Fiber Containing Structural Protein Complexes Are Produced during Infection of Adenovirus Serotype 5

Adenoviruses are common pathogens. The localization of their receptors coxsackievirus and adenovirus receptor, and desmoglein-2 in cell-cell junction complexes between polarized epithelial cells represents a major challenge for adenovirus infection from the apical surface. Structural proteins including hexon, penton base and fiber are excessively produced in serotype 5 adenovirus (Ad5)-infected cells. We have characterized the composition of structural protein complexes released from Ad5 infected cells and their capacity in remodeling cell-cell junction complexes. Using T84 cells as a model for polarized epithelium, we have studied the effect of Ad5 structural protein complexes in remodeling cell-cell junctions in polarized epithelium. The initial Ad5 infection in T84 cell culture was inefficient. However, progressive distortion of cell-cell junction in association with fiber release was evident during progression of Ad5 infection. Incubation of T84 cell cultures with virion-free supernatant from Ad5 infected culture resulted in distortion of cell-cell junctions and decreased infectivity of Ad5-GFP vector. We used gel filtration chromatography to fractionate fiber containing virion–free supernatant from Ad5 infected culture supernatant. Fiber containing fractions were further characterized for their capacity to inhibit the infection of Ad5-GFP vector, their composition in adenovirus structural proteins using western blot and LC-MS/MS and their capacity in remolding cell-cell junctions. Fiber molecules in complexes containing penton base and hexon, or mainly hexon were identified. Only the fiber complexes with relatively high content of penton base, but not the fiber-hexon complexes with low penton base, were able to penetrate into T84 cells and cause distortion of cell-cell junctions. Our findings suggest that these two types of fiber complexes may play different roles in adenoviral infection.     

Cryo-EM visualization of an exposed RGD epitope on adenovirus that escapes antibody neutralization.

Interaction of the adenovirus penton base protein with alpha v integrins promotes virus entry into host cells. The location of the integrin binding sequence Arg-Gly-Asp (RGD) on human type 2 adenovirus (Ad2) was visualized by cryo-electron microscopy (cryo-EM) and image reconstruction using a mAb (DAV-1) which recognizes a linear epitope, IRGDTFATR. The sites for DAV-1 binding corresponded to the weak density above each of the five 22 A protrusions on the adenovirus penton base protein. Modeling of a Fab fragment crystal structure into the adenovirus-Fab cryo-EM density indicated a large amplitude of motion for the Fab and the RGD epitope. An unexpected finding was that Fab fragments, but not IgG antibody molecules, inhibited adenovirus infection. Steric hindrance from the adenovirus fiber and a few bound IgG molecules, as well as epitope mobility, most likely prevent binding of IgG antibodies to all five RGD sites on the penton base protein within the intact virus. These studies indicate that the structure of the adenovirus particle facilitates interaction with cell integrins, whilst restricting binding of potentially neutralizing antibodies.     

Adenovirus interaction with distinct integrins mediates separate events in cell entry and gene delivery to hematopoietic cells.

A major impediment to the effective use of adenovirus vectors for gene therapy is a lack of knowledge of how these vectors interact with diverse cell types in vivo. Adenovirus attachment to most human cell types is mediated by the fiber protein, which binds to an as yet unidentified cell receptor. In contrast to this, we report that adenovirus type 2 (Ad2) attachment to hematopoietic cells is facilitated by interaction of the penton base protein with members of the beta2 integrin family. Adenovirus particles were capable of binding to human monocytic cells, which lack fiber receptors, and virus binding could be blocked by a soluble penton base or by a function-blocking monoclonal antibody to integrin alphaMbeta2. To confirm the role of alphaMbeta2 integrins in Ad2 binding to hematopoietic cells, we analyzed virus attachment and gene delivery to CHO cells expressing recombinant beta2 integrins. alphaMbeta2-expressing CHO cells supported 3- to 5-fold-higher levels of Ad2 binding and 5- to 10-fold-larger amounts of gene delivery than did nontransfected CHO cells, indicating that alphaMbeta2 facilitates adenovirus attachment to and infection of hematopoietic cells. While beta2 integrins promote Ad2 attachment to hematopoietic cells, further studies demonstrated that alphav integrins were required for the next step in infection, virus internalization into cell endosomes. These studies reveal a novel pathway of Ad2 infection of hematopoietic cells mediated by distinct integrins which facilitate separate events in virus entry. They also suggest a possible strategy for selective adenovirus-mediated gene delivery to hematopoietic cells.     

Integrin alpha3beta1 is an alternative cellular receptor for adenovirus serotype 5

Many adenovirus serotypes enter cells by high-affinity binding to the coxsackievirus-adenovirus receptor (CAR) and integrin-mediated internalization. In the present study, we analyzed the possible receptor function of α3β1 for adenovirus serotype 5 (Ad5). We found that penton base and integrin α3β1 could interact in vitro. In vivo, both Ad5-cell binding and virus-mediated transduction were inhibited in the presence of anti-α3 and anti-β1 function-blocking antibodies, and this occurred in both CAR-positive and CAR-negative cell lines. Peptide library screenings and data from binding experiments with wild-type and mutant penton base proteins suggest that the Arg-Gly-Asp (RGD) in the penton base protein, the best known integrin binding motif, is only part of the binding interface with α3β1, which involved multiple additional contact sites.     

Integrin alpha v beta 5 selectively promotes adenovirus mediated cell membrane permeabilization

Human adenovirus type 2 (Ad2) enters host cells by receptor-mediated endocytosis, an event mediated by the virus penton base binding to cell surface integrins alpha v beta 3 and alpha v beta 5. While both alpha v integrins promote virus internalization, alpha v beta 5 is involved in the subsequent event of membrane permeabilization. Cells transfected with the beta 5 or beta 3 subunit, expressing either alpha v beta 5 and alpha v beta 3, respectively, were capable of supporting Ad2 infection to varying degrees. In this case, cells expressing alpha v beta 5 were significantly more susceptible to Ad2-induced membrane permeabilization, as well as to Ad2 infection, than cells expressing alpha v beta 3. Adenovirus-mediated gene delivery was also more efficient in cells expressing alpha v beta 5. These results suggest that the interaction of alpha v beta 5 with Ad2 penton base facilitates the subsequent step of virus penetration into the cell. These studies provide evidence for the involvement of a cellular receptor in virus- mediated membrane permeabilization and suggest a novel biological role for integrin alpha v beta 5 in the infectious pathway of a human adenovirus.     

Desmoglein 2 is a receptor for adenovirus serotypes 3, 7, 11 and 14

We have identified desmoglein-2 (DSG-2) as the primary high-affinity receptor used by adenoviruses Ad3, Ad7, Ad11 and Ad14. These serotypes represent key human pathogens causing respiratory and urinary tract infections. In epithelial cells, adenovirus binding of DSG-2 triggers events reminiscent of epithelial-to-mesenchymal transition, leading to transient opening of intercellular junctions. This opening improves access to receptors, for example, CD46 and Her2/neu, that are trapped in intercellular junctions. In addition to complete virions, dodecahedral particles (PtDds), formed by excess amounts of viral capsid proteins, penton base and fiber during viral replication, can trigger DSG-2-mediated opening of intercellular junctions as shown by studies with recombinant Ad3 PtDds. Our findings shed light on adenovirus biology and pathogenesis and may have implications for cancer therapy.     

Novel fiber-dependent entry mechanism for adenovirus serotype 5 in lacrimal acini

The established mechanism for infection of most cells with adenovirus serotype 5 (Ad5) involves fiber capsid protein binding to coxsackievirus-adenovirus receptor (CAR) at the cell surface, followed by penton base capsid protein binding to alpha(v) integrins, which triggers clathrin-mediated endocytosis of the virus. Here we determined the identity of the capsid proteins responsible for mediating Ad5 entry into the acinar epithelial cells of the lacrimal gland. Ad5 transduction of primary rabbit lacrimal acinar cells was inhibited by excess Ad5 fiber or knob (terminal region of the fiber) but not excess penton base. Investigation of the interactions of recombinant Ad5 penton base, fiber, and knob with lacrimal acini revealed that the penton base capsid protein remained surface associated, while the knob domain of the fiber capsid protein was rapidly internalized. Introduction of rabbit CAR-specific small interfering RNA (siRNA) into lacrimal acini under conditions that reduced intracellular CAR mRNA significantly inhibited Ad5 transduction, in contrast to a control (nonspecific) siRNA. Preincubation of Ad5 with excess heparin or pretreatment of acini with a heparinase cocktail each inhibited Ad5 transduction by a separate and apparently additive mechanism. Functional and imaging studies revealed that Ad5, fiber, and knob, but not penton base, stimulated macropinocytosis in acini and that inhibition of macropinocytosis significantly reduced Ad5 transduction of acini. However, inhibition of macropinocytosis did not reduce Ad5 uptake. We propose that internalization of Ad5 into lacrimal acini is through a novel fiber-dependent mechanism that includes CAR and heparan sulfate glycosaminoglycans and that the subsequent intracellular trafficking of Ad5 is enhanced by fiber-induced macropinocytosis.     

A human cell line selected for resistance to adenovirus infection has reduced levels of the virus receptor.

To investigate determinants of host cell susceptibility to infection, cells partially resistant to infection were selected from the rare cells which remained adherent after infection of a culture of A549 cells with Ad2RAE, a mutant of adenovirus type 2 whose vertex capsomers lack an Arg-Gly-Asp (RGD) sequence which mediates binding of wild-type virus to integrins. Integrins promote the internalization of attached virions, whereas adsorption itself results from binding of the viral fibers to an unidentified cellular receptor. Following three rounds of selection, a persistently infected culture was established in which virus replication was detected in approximately 5% of the cells. Uninfected cells were readily cloned from the culture, indicating that at any particular time the majority of cells in the culture were uninfected. The resistance of one clone of uninfected cells to infection was correlated with a 10-fold reduction in the concentration of fiber receptors on these cells compared with the parental A549 cell line, indicating that efficiency of virus adsorption depends on the receptor concentration. Surprisingly, the rate at which host cells internalized RGD-negative virus also was strongly dependent on the fiber receptor concentration. While internalization of wild-type virus is promoted by the binding of integrins to the penton base RGD sequence, these results suggest that virus also can enter cells by an alternate pathway which requires binding of virions to multiple fiber receptors.     

Reduction of natural adenovirus tropism to mouse liver by fiber-shaft exchange in combination with both CAR- and alphav integrin-binding ablation

The primary receptor, the coxsackievirus and adenovirus receptor (CAR), and the secondary receptor, αv integrins, are the tropism determinants of adenovirus (Ad) type 5. Inhibition of the interaction of both the fiber with CAR and the penton base with the αv integrin appears to be crucial to the development of targeted Ad vectors, which specifically transduce a given cell population. In this study, we developed Ad vectors with ablation of both CAR and αv integrin binding by mutating the fiber knob and the RGD motif of the penton base. We also replaced the fiber shaft domain with that derived from Ad type 35. High transduction efficiency in the mouse liver was suppressed approximately 130- to 270-fold by intravenous administration of the double-mutant Ad vectors, which mutated two domains each of the fiber knob and shaft and the RGD motif of the penton base compared with those of conventional Ad vectors (type 5). Most significantly, the triple-mutant Ad vector containing the fiber knob with ablation of CAR binding ability, the fiber shaft of Ad type 35, and the penton base with a deletion of the RGD motif mediated a >30,000-fold lower level of mouse liver transduction than the conventional Ad vectors. This triple-mutant Ad vector also mediated reduced transduction in other organs (the spleen, kidney, heart, and lung). Viral DNA analysis showed that systemically delivered triple-mutant Ad vector was primarily taken up by liver nonparenchymal cells and that most viral DNAs were easily degraded, resulting in little gene expression in the liver. These results suggest that the fiber knob, fiber shaft, and RGD motif of the penton base each plays an important role in Ad vector-mediated transduction to the mouse liver and that the triple-mutant Ad vector exhibits little tropism to any organs and appears to be a fundamental vector for targeted Ad vectors.     

Interaction between HeLa cells and adenovirus type 2 virions neutralized by different antisera.

Three adenovirus type 2-specified immunogens elicited neutralizing antibodies when injected into rabbits; these were the fiber, the hexon, and the penton base. Adenovirus type 2 virions, neutralized by antihexon- or anti-penton base antisera, attached to HeLa cells to the same extent as untreated control virus, and after attachment, neutralized viruses also became sensitive to DNase treatment. A fraction of 75 to 80% of the attached antibody-treated virions penetrated the plasma membrane, which should be compared with an 84 to 88% penetration level in the control series. A majority of the antihexon-neutralized virions was found in intracellular vesicles, as revealed with an electron microscope, but in the case of anti-penton base neutralization, a maximum of 50% of the virions was retained within vesicles, and ca. 30% was free in the cytoplasmic compartment. A value greater than 45% was never obtained for neutralization with a monospecific anti-penton base antiserum, which could imply the existence of alternative pathways for virus penetration into HeLa cells--one of these being sensitive to treatment with anti-penton base antiserum. Antisera containing antifiber specificities efficiently aggregated virions, and the aggregation data mirrored the degree of neutralization. Antifiber-neutralized virions attached to cells to a three- to five times greater extent than untreated control virus, but the former virions had a reduced ability to become sensitive to DNase treatment. Around 15% of the attached antifiber-treated virions was found as large aggregates inside multivesicular bodies or lysosomes.     

Human adenovirus-host cell interactions: comparative study with members of subgroups B and C.

Host cell interactions of human adenovirus serotypes belonging to subgroups B (adenovirus type 3 [Ad3] and Ad7) and C (Ad2 and Ad5) were comparatively analyzed at three levels: (i) binding of virus particles with host cell receptors; (ii) cointernalization of macromolecules with adenovirions; and (iii) adenovirus-induced cytoskeletal alterations. The association constants with human cell receptors were found to be similar for Ad2 and Ad3 (8 x 10(9) to 9 x 10(9) M-1), and the number of receptor sites per cell ranged from 5,000 (Ad2) to 7,000 (Ad3). Affinity blottings, competition experiments, and immunofluorescence stainings suggested that the receptor sites for adenovirus were distinct for members of subgroups B and C. Adenovirions increased the permeability of cells to macromolecules. We showed that this global effect could be divided into two distinct events: (i) cointernalization of macromolecules and virions into endocytotic vesicles, a phenomenon that occurred in a serotype-independent way, and (ii) release of macromolecules into the cytoplasm upon adenovirus-induced lysis of endosomal membranes. The latter process was found to be type specific and to require unaltered and infectious virus particles of serotype 2 or 5. Perinuclear condensation of the vimentin filament network was observed at early stages of infection with Ad2 or Ad5 but not with Ad3, Ad7, and noninfectious particles of Ad2 or Ad5, obtained by heat inactivation of wild-type virions or with the H2 ts1 mutant. This phenomenon appeared to be a cytological marker for cytoplasmic transit of infectious virions within adenovirus-infected cells. It could be experimentally dissociated from vimentin proteolysis, which was found to be serotype dependent, occurring only with members of subgroup C, regardless of the infectivity of the input virus.     

Penton-Dodecahedral Particles Trigger Opening of Intercellular Junctions and Facilitate Viral Spread during Adenovirus Serotype 3 Infection of Epithelial Cells

Human adenovirus serotypes Ad3, Ad7, Ad11, and Ad14 use the epithelial junction protein desmoglein 2 (DSG2) as a receptor for infection. During Ad infection, the fiber and penton base capsid proteins are produced in vast excess and form hetero-oligomers, called pentons. It has been shown for Ad3 that pentons self-assemble into penton-dodecahedra (PtDd). Our previous studies with recombinant purified Ad3 PtDd (produced in insect cells) showed that PtDd bind to DSG2 and trigger intracellular signaling resulting in the transient opening of junctions between epithelial cells. So far, a definitive proof for a function of Ad3 PtDd in the viral life cycle is elusive. Based on the recently published 3D structure of recombinant Ad3 PtDd, we generated a penton base mutant Ad3 vector (mu-Ad3GFP). mu-Ad3GFP is identical to its wild-type counterpart (wt-Ad3GFP) in the efficiency of progeny virus production; however, it is disabled in the production of PtDd. For infection studies we used polarized epithelial cancer cells or cell spheroids. We showed that in wt-Ad3GFP infected cultures, PtDd were released from cells before viral cytolysis and triggered the restructuring of epithelial junctions. This in turn facilitated lateral viral spread of de novo produced virions. These events were nearly absent in mu-Ad3GFP infected cultures. Our in vitro findings were consolidated in mice carrying xenograft tumors derived from human epithelial cancer cells. Furthermore, we provide first evidence that PtDd are also formed by another DSG2-interacting Ad serotype, the newly emerged, highly pathogenic Ad14 strain (Ad14p1). The central finding of this study is that a subgroup of Ads has evolved to generate PtDd as a strategy to achieve penetration into and dissemination in epithelial tissues. Our findings are relevant for basic and applied virology, specifically for cancer virotherapy.     

Adenovirus types 11p and 35p show high binding efficiencies for committed hematopoietic cell lines and are infective to these cell lines

Hematopoietic cells are attractive targets for gene therapy. However, no satisfactory vectors are currently available. A major problem with the most commonly used adenovirus vectors, based on adenovirus type 2 (Ad2) or Ad5, is their low binding efficiency for hematopoietic cells. In this study we identify two adenovirus serotypes with high affinity for hematopoietic cells. The binding efficiency of prototype serotypes Ad4p, Ad11p, and Ad35p for different committed hematopoietic cell lines representing T cells (Jurkat), B cells (DG75), monocytes (U937-2), myeloblasts (K562), and granulocytes (HL-60) was evaluated and compared to that of Ad5v, the commonly used adenovirus vector, using flow cytometry. In contrast to Ad5v, which bound to less than 10% of the cells in all experiments, Ad11p and Ad35p showed high binding efficiency for all of the different hematopoietic cell lines. Ad4p bound to the lymphocytic cell lines to some extent but less well to the myelomonocytic cell lines. The abilities of the different serotypes to infect, replicate, and form complete infectious particles in the hematopoietic cell lines were also investigated by immunostaining, (35)S labeling of viral proteins, and titrations of cell lysates. Ad11p and Ad35p infected the highest proportion of cells, and Ad11p infected all of the cell lines investigated. The Ad11p hexon was expressed equally well in K562 and A549 cells. Jurkat cells also showed high levels of expression of Ad11p hexons, but the production of infectious particles was low. The binding properties of virions were correlated to their ability to infect and be expressed.     

Adenovirus 3 penton dodecahedron exhibits structural changes of the base on fibre binding.

It was recently shown that co-expression of adenovirus type 3 (Ad3) penton base and fibre in the baculovirus system produces dodecahedral particles, as does the expression of the penton base alone. The structure of both of these dodecahedral particles, with and without fibre, has been determined by cryoelectron microscopy and 3-dimensional reconstruction techniques to a resolution of 25 and 20 A, respectively. The general form of the penton base resembles that of the base protein in the recent reconstruction of adenovirus type 2. There is a remarkable difference in the penton base structure with and without the fibre. The five small protuberances on the outer surface of each base move away from the 5-fold axis by approximately 15 A when the fibre is present. These protuberances are of relatively low density and most probably represent a flexible loop possibly containing the RGD site involved in integrin binding. The fibre is apparently bound to the outer surface of the penton base, rather than inserted into it. The fibre is flexible and the shaft contains two distinct globular regions 26 A in diameter. The volume of the inner cavity of the dodecahedron is 350 +/- 100 nm3. This small volume precludes the use of the inner cavity to house genetic information for gene therapy; however, the possibility remains of linking the gene to the dodecahedron surface in the hope that it will be internalized with the dodecahedron.