Info

Disease

Time (days)

State of Virus

Virus disappears after

State of Virus

Virus disappears after

■ Targeting of the virion to the site where it will reproduce

■ Uncoating of the virion—separation of protein coat from nucleic acid

■ Replication of nucleic acid and protein

■ Maturation of the viral particles

■ Spreading of the virus within the host

■ Shedding of the infectious virions outside the host

■ Transmission to the next host, thereby repeating the infection cycle ■ lytic phage replication, p. 327

Step 1: Attachment

The process of attachment (adsorption) is basically the same in all virus-cell interactions, except that the process is more complex in animal viruses than in phages. Animal viruses usually do not contain a single specific attachment appendage, a tail with fibers, as does phage T4, for instance. Rather, surface projections containing attachment proteins or spikes protrude all over the surface of a virion (see figure 13.2). Frequently, there are several different attachment proteins. The receptors to which the viral attachment proteins bind are usually glycoproteins located on the plasma membrane, and often more than one receptor is required for effective attachment. For example, the HIV virus must bind to two key molecules on the cell surface before it can enter the cell. The receptors number in the tens to hundreds of thousands per host cell. As with bacterial phage receptors, the normal function of these glycoproteins is completely unrelated to their role in virus attachment. For example, many receptors are immunoglobulins; other viruses use hormone receptors and per-meases. ■ immunoglobulins, p. 398 ■ glycoproteins, p. 29

Different viruses may use the same receptor, and related viruses may use different receptors. Certain viruses can bind to more than one type of receptor and thus be able to invade different kinds of cells. The binding of the attachment proteins to their receptors often changes the shape of viral proteins concerned with entry of the virion and facilitates their entry. Because a virion must bind to specific receptors, frequently a particular virus can infect only a single or a limited number of cell types within a host species, and most viruses can infect only a single species. This may account for the resistance that some animals have to certain diseases. For example, dogs do not contract measles from humans, and humans do not contract distemper from cats. Some viruses however—for example, those that cause zoonoses—can infect unrelated animals such as horses and humans with serious consequences.

Step 2: Entry

The mechanism of entry of animal viruses into host cells depends on whether the virion is enveloped or naked. In the case of enveloped viruses, two mechanisms exist. In one mechanism, the envelope of the virion fuses with the plasma membrane of the host after attachment to a host cell receptor (figure 14.8a). This fusion is promoted by a specific fusion protein on the surface of the virion. In some viruses, such as measles, mumps, influenza, and HIV, the protein, which recognizes a target protein on the cell, changes its shape when it con-

14.3 Interactions of Animal Viruses with Their Hosts 349

tacts the host cell. Following fusion, the nucleocapsid is released directly into the cytoplasm, where the nucleic acid separates from the protein coat.

In another mechanism, enveloped viruses adsorb to the host cell with their protein spikes, and the virions are taken into the cell in a process termed endocytosis (figure 14.8b). In this process, the host cell plasma membrane surrounds the whole virion and forms a vesicle. Then, the envelope of the virion fuses with the plasma membrane of the vesicle. The nucleocapsid is then released into the host's cytoplasm. ■ endocytosis, p. 72

In the case of naked virions, the virion also enters by endocytosis. Since the virus has no envelope, however, it cannot fuse with the plasma membrane. Rather, after being engulfed, the virus dissolves the vesicle, resulting in release of the nucleo-capsid into the cytoplasm.

Entry by animal viruses differs from phage penetration in two ways. First, the envelope of the virion and the plasma membrane of the host may fuse. Such fusion is not possible when the outside covering of the host has a rigid cell wall. Second, the entire virion is taken into the cell, whereas in the case of phages, the protein coat remains on the outside of the bacterium.

Step 3: Targeting to the Site of Viral Replication

Following penetration, the virion must be targeted to the site where it will multiply. Most DNA viruses multiply in the nucleus, but how the virion gets to the nucleus is not known.

Step 4: Uncoating

In all viruses, the nucleic acid separates from its protein coat prior to the start of replication. This process is termed uncoating.

Step 5: Replication of Nucleic Acid and Proteins

The first step in replication is transcription of the nucleic acid of the virion. Diverse strategies are followed by viruses of different families for the synthesis of mRNA. In large part, the transcription strategy depends on whether the virus is RNA or DNA and whether the nucleic acid is single- or double-stranded (figure 14.9). For example, in the case of positive single-stranded RNA viruses, the RNA itself functions as a messenger, whereas in the case of negative single-stranded RNA viruses, the RNA must be transcribed into a positive strand. In this case, the RNA-dependent RNA polymerase required for transcription enters the cell as part of the virion since the uninfected cell does not have such an enzyme. Replication of RNA molecules is unique to viruses. ■ nomenclature for nucleic acid strandedness, p. 173

In all cases, the patterns of transcription of the viral genome follow the same patterns as phages having the same type of genome follow. If a viral enzyme is required for transcription, it may be carried into the cell in the virion or the viral genome may encode the enzyme early in replication. In the case of phages, enzymes do not enter the cell; thus, if host cell enzymes are not available, the enzymes are encoded by the entering viral genome.

In all virus systems, whether phage, animal, or plant, the nucleic acids and proteins replicate independently of one another. The replication of viral nucleic acid depends to varying

350 Chapter 14 Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants

Envelope^ „cmo^ ^Capsid protein - Nucleic acid

Protein spikes

Protein spikes

Host cell— plasma membrane

Receptors..

Envelope

Receptors..

Host cell— plasma membrane

Adsorption— spikes of virion attach to specific host cell receptors

Nucleic acid separates from capsid coat protein

Nucleocapsid released into cytoplasm—viral envelope remains part of plasma membrane

Nucleic acid

Nucleic acid separates from capsid coat protein

Nucleic acid

Protein spikes

Membrane fusion— envelope of virion fuses with plasma membrane

Nucleocapsid released into cytoplasm—viral envelope remains part of plasma membrane

Capsid protein Nucleic acid

Host cell— plasma membrane

Adsorption of virion to host cell; endocytosis begins

Endocytosis continues

Plasma membrane surrounds the virion— vesicle forms

Envelope

Protein spikes

Capsid protein Nucleic acid

Adsorption of virion to host cell; endocytosis begins

Endocytosis continues

Plasma membrane surrounds the virion— vesicle forms

Envelope of virion fuses with plasma membrane of host; nucleocapsid released from vesicle

Nucleocapsid is free within host's cytoplasm

Separation of nucleic acid from capsid protein

Nucleic acid

Envelope of virion fuses with plasma membrane of host; nucleocapsid released from vesicle

Nucleocapsid is free within host's cytoplasm

Separation of nucleic acid from capsid protein

Nucleic acid

Figure 14.8 Entry of Enveloped Animal Viruses into Host Cells (a) Entry following membrane fusion and (b) entry by endocytosis.

Figure 14.9 Strategies of Transcription Employed by Different Viruses Viruses that have the same genome structure follow the same strategy for the synthesis of (+) mRNA, which is then translated into protein.

Picornaviruses

Togaviruses

Coronaviruses

14.3 Interactions of Animal Viruses with Their Hosts 351

Most bacteriophages Papovaviruses Adenoviruses Herpesviruses

Phage M13 Phage 0X174 Parvoviruses

Retroviruses

Orthomyxoviruses

Paramyxoviruses

Rhabdoviruses

Key:

(± ) = double-stranded (+) = positive strand (-) = negative strand

Double-stranded (+) RNA

Reoviruses degrees on enzymes present in the host cell prior to infection. Whether enzymes of the host or phage-induced enzymes are used varies with the virus. In some cases, viral enzymes concerned with virus replication which are not present in the host enter the host cells along with the virion. As a general rule, the larger the viral genome, the fewer host cell enzymes are involved in replication. This is not surprising since enough DNA is present in large viral genomes to encode most enzymes of nucleic acid synthesis. For example, the largest of the DNA animal viruses, the poxviruses, like T4 phage, are totally independent of host cell enzymes for the replication of their nucleic acid. On the other hand, the very small parvoviruses depend so completely on the biosynthetic machinery of the host cell that they require that the host cell actually be synthesizing its own DNA at the time of infection so that viral DNA can also be synthesized. Most animal viruses are between these two extremes. For example, small DNA viruses such as the papovaviruses code for a protein that is involved in initiating DNA synthesis, but once started, the rest of the process is carried out by host cell enzymes. This replication is similar to that of bacteriophage 0X174, a single-stranded DNA virus that codes for proteins necessary for the initiation of phage DNA replication. Once replication has started, however, the viral DNA is synthesized by the same enzymes, such as DNA polymerase, that replicate the host cell E. coli DNA. The replication of the genome of many RNA viruses requires enzymes that are not found in the uninfected cell. Obviously, these must be either encoded by the virus or enter the cell with the virion. The varying degrees of dependency of different animal DNA viruses on host cell enzymes for DNA replication are summarized in table 14.5. However, all viruses require that host cells supply the machinery for the generation of energy and the biosynthesis of macromolecules. ■ 0X174, p. 327

Step 6: Maturation

The final assembly of the nucleic acid with its coat protein, the process of maturation, is preceded by formation of the protein

Table 14.5 Relationship of Virus Size to Dependency on Host Cell Enzymes for DNA Replication

Virus*

Increasing Size of Virus

Dependence on Host Cell DNA Synthetic Enzymes

Parvovirus ss DNA

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Responses

  • MICHAEL
    Which step associated with the separation of the viral nuclic acid from protein coat?
    3 years ago

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