Freeze-fracture study of adenovirus-induced KB cell surface alterations

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.

Crystallization, enzymatic cleavage, and the polarity of the adenovirus type 2 fiber

Crystals of the fiber protein of adenovirus type 2 have been grown. Analysis of these crystals (type I crystals) showed that they were composed of fiber polypeptide with a lower apparent molecular weight (60 kDa) than that of the soluble or virion-incorporated fiber (62 kDa). N-terminal sequencing revealed that the fiber polypeptide chain of 60 kDa was cleaved at tyrosine17 from the N-end. The C-terminus remained intact. Assays with protease inhibitors suggested that the spontaneous cleavage of the fiber occurring upon its crystallization was due to a cellular, calcium-dependent, chymotrypsin-like protease co-purifying with the fiber and activated during hydroxyapatite chromatography. Crystallization of fiber purified in the presence of chymostatin provided crystals of a different structure under the electron microscope (crystals of type II), composed of 62-kDa fiber polypeptide units. The 62-kDa fiber from the type II crystals, as well as the 62-kDa fiber isolated from infected cell extracts, were able to associate with the penton base in vitro to form a penton capsomer. The 60-kDa fiber has lost this capacity. The accessibility of the N- and C-termini of the fiber inside the penton structure was probed by anti-peptide sera after limited proteolysis. The results are consistent with a polarity of the fiber in which its N-terminus is oriented toward the penton base, the C-terminal domain corresponding to the distal knob.

Processing of vimentin occurs during the early stages of adenovirus infection

Evidence is presented that a fraction of vimentin, a component of cytoskeleton recently found to be associated with intracytoplasmic, migrating adenovirus type 2 (Ad2), is processed into smaller polypeptides at early times after infection. The extent of vimentin cleavage appears to depend upon both the multiplicity of infection and the adenovirus serotype. Ad2, Ad5, Ad4, and Ad9 induced similar vimentin cleavage in infected cells, whereas Ad3, Ad7, and Ad12, for which most infecting particles are found sequestered within phagosomes, induced very little, if any, vimentin breakdown. This suggests that vimentin processing is in some way related to the number of virus particles migrating through the cytoplasm. Experiments performed in vitro and in vivo with adenovirus temperature-sensitive mutants H2 ts1 and H2 ts112 and UV-inactivated wild-type Ad2 indicated that vimentin processing is due to a nonvirion, cytoskeleton-associated, proteolytic enzyme activated by adenovirus and sharing characteristics with the protease described by Nelson and Traub (W.J. Nelson and P. Traub, J. Cell Sci. 57:25-49, 1982). The activity of this protease appears to be required for productive infection by adenovirus serotypes 2 and 5 (subgroup C), 4 (subgroup E), and 9 (subgroup D) but not by the oncogenic serotypes 3 and 7 (subgroup B) and 12 (subgroup A).

Early interactions between animal viruses and the host cell: relevance to viral vaccines

Viral recognition of specific receptors in the host cell plasma membrane is the first step in virus infection. Attachment is followed by a redistribution or capping of virus particles on the cell surface which may play a role in the uptake process. Certain viruses penetrate the plasma membrane directly but many, both enveloped and non-enveloped viruses, are endocytosed at coated pits and subsequently pass into endosomes. The low pH environment of the endosome facilitates passage of the viral genome into the cytoplasm. For some viruses the mechanism of membrane penetration is now known to be linked to a pH-mediated conformational change in external virion proteins. As a consequence of infection there are alterations in the permeability of the plasma membrane which may contribute to cellular damage. Recent advances in the understanding of these processes are reviewed and their relevance to the development of new strategies for vaccines emphasised.

Virus-receptor interaction in the adenovirus system: characterization of the positive cooperative binding of virions on HeLa cells

The established positive cooperativity of adenovirus 2 binding to HeLa cells revealed a strong temperature dependence. The degree of cooperativity, quantified by means of Hill coefficients, progressively increased from 10 degrees C to reach a maximum level, which was maintained between 20 and 37 degrees C. On the other hand, negative cooperativity of virion attachment was apparent at 3.0 degrees C and on glutaraldehyde-stabilized cells. The corresponding monovalent ligand of the system, the fiber antigen, demonstrated only weak-positive cooperativity of the binding at 37.0 degrees C, which was absent at 3.0 degrees C. Dithiothreitol and dansylcadaverine, reagents inhibiting clustering of ligand-receptor complexes in the plasma membrane, markedly reduced the degree of positive cooperative binding at 37.0 degrees C. Evidently, the positive cooperative binding of adenovirus to HeLa cells at 37.0 degrees C is a consequence of both the multivalency of virus attachment proteins, i.e., fibers, on the virion and of the capacity of the receptor sites to migrate in the plane of the plasma membrane, forming local aggregates of virus-receptor site complexes.

Virus receptor interaction in the adenovirus system. II. Capping and cooperative binding of virions on HeLa cells

Adenovirus type 2 attachment to HeLa cells was analyzed under controlled conditions. The temperature-dependent attachment kinetics revealed an inflection point at around 20 degrees C, and above this temperature the increase of the rate was doubled. In multiplicity dependence experiments, the attachment exhibited positive cooperative binding at 37 degrees C. This binding pattern was inhibited by low temperatures and prefixation of cells with 0.015% glutaraldehyde. Attachment of rhodamine-labeled virions revealed capping of the particles on 15% of the cells at 37 degrees C. Capping was inhibited by low temperatures, glutaraldehyde fixation of cells, and treatment with cytochalasin B, azide, and 2-deoxyglucose. Consequently, we propose that the adenovirus type 2 attachment to cells leads to rearrangements in the plasma membrane, resulting in cooperative binding and capping of the virus particles.

Lipid domain structures in biological membranes

Phase separation represents a possibility for segregation of lipidic membrane components into structurally distinct domains. Freeze-fracture electronmicroscopy is a useful method for detection of lipid domains. Indications of a possible domain-nature of structures are a regular pattern within a separated area, a regular outline of such an area and a local modulation of curvature (evagination or invagination). Candidates for domain structures in biological membranes are smooth particle-free areas and arrays of regularly arranged particles. The interpretation of the particle-free areas is more reliable than that of the arrays with regularly arranged particles. Phase separation in biological membranes can be induced experimentally by lowering the temperature, but physiologically the isothermically induced domains are more important. Factors in control of isothermic domain formation are divalent cations, proteins, cholesterol etc. Suggestions on the biological relevance of domain formation concern mainly their role in the mechanism of membrane fusion, but domains in form of transient or stable membrane structures seem to occur also otherwise and disturbances in domain formation or artificially induced domains can be suitable for pathological alterations.