Lecture-3: Viral Infectious Cycle

Department of Biotechnology

Southern Taiwan University of Science and Technology

Viral Infectious Cycle

• Attachment (adsorption) – of the virus to a susceptible

host cell

• Entry (penetration) – of viral nucleic acid

• Uncoating of capsid • Synthesis (replication)

– of viral nucleic acid and protein

• Assembly (maturation) – of capsids and packaging of

viral genomes into new viruses

• Release of viral particles from host cell

From: Fundamentals of Molecular Virology. N.H. Acheson

Attachment of Virus to Host Cell

• Recognize a susceptible cell

– Contains receptors for the virus (can be protein, carbohydrates, lipids, lipidproteins)

– A susceptible cell is capable of supporting viral replication (i.e., permissive cell)

• Needs viral attachment protein

• Most viral receptors are glycoproteins

Receptors in Cell Surface

• Cell surface glycoproteins used as virus receptors

From: Fundamentals of Molecular Virology. N.H. Acheson

Attachment

• Influenza viruses attach epithelial cell of the upper respiratory tract

From: Principles of Virology. 3rd ed, S.J. Flint et al.

Entry

• Enveloped and non-enveloped viruses have distinct entry strategies

• Entry for enveloped viruses

– Endocytosis

– Fusion

• Entry for non-enveloped viruses

– Endocytosis

– Nucleic acid translocation

Endocytosis

From: Principles of Virology. 3rd ed, S.J. Flint et al.

Entry

• Exit and entry of enveloped viruses occurs by vesicle transport

From: Fundamentals of Molecular Virology. N.H. Acheson

Entry (penetration)

• Entry of enveloped virus

– Fusion with plasma membrane

– Entry via endosomes

Entry

• Entry of enveloped viruses by endocytosis

From: Fundamentals of Molecular Virology. N.H. Acheson

Entry

• Passage from endosomes to the cytosol is often triggered by low PH (6.5-5.3)

From: Principles of Virology. 3rd ed, S.J. Flint et al.

Entry

• Entry of non-enveloped viruses

– By endocytosis

Entry

• Entry of non-enveloped viruses

– By neucleic acid translocation

– e.g. picornavirus

From: Fundamentals of Molecular Virology. N.H. Acheson

Cytosol is Crowded

From: Principles of Virology. 3rd ed, S.J. Flint et al.

Entry and Intracelluar Transport of Viruses

• Intracellular transport

– via endosome

– via microtubule (faster!)

From: Fundamentals of Molecular Virology. N.H. Acheson

Virus Entry and Movement in Cells

From: Principles of Virology. 3rd ed, S.J. Flint et al.

Uncoating

• To release viral genome within the host cell

• Uncoating may occur

– at the first step of entry

– during transportation

– arrive at nucleus membrane

– after entering host nucleus

From: Fundamentals of Molecular Virology. N.H. Acheson

Synthesis

• Synthesis of Viral Nucleic Acid

– Once a host has been infected, new copies of the viral nucleic acid and viral specific protein must be made in order to replicate

– Nucleic acid may be made in nucleus or cytoplasm

– DNA and RNA viruses replicate in different manner

Synthesis

• DNA and RNA viruses replicate in different manner

• DNA virus replication

– DNA mRNA

– Need DNA dependent RNA polymerase (from host cell)

Synthesis

• Synthesis of viral proteins (ALWAYS in cytoplasm) – Early protein

• Synthesized soon after infection

• Essential for replication of viral nucleic acid

• Typically act catalytically

• Synthesized in smaller amounts

– Late protein • Synthesized later

• Viral capsids

• Typically structural components

• Synthesized in larger amounts

Release

• Budding--enveloped • Lysis of cell--noneveloped

From: Fundamentals of Molecular Virology. N.H. Acheson

Release

• Block the release of viruses from cell

• Anti-influenza: Tamilfu (克流感)

From: http://www.pharmasquare.org/flash/Tamiflu.html