Virus Capsid Symmetry

Capsid Symmetry

• The protein shell enclosing the genome can have two geometric configurations: helical (rod-shaped or coiled) or icosahedral (spherical or symmetric)
• Helical symmetry: found in paramyxoviridae, consists of repeated units of a single polypeptide species that self-assemble into a helical cylinder with the viral nucleic acid
• Icosahedral symmetry: more complex, consists of several different polypeptides grouped into structural subassemblies called capsomers, which are hydrogen-bonded to each other to form an icosahedron

Viral Envelope

• An important structural feature used in defining a viral family is the presence or absence of a lipid-containing membrane surrounding the nucleocapsid
• Enveloped viruses: have a flexible and coiled nucleocapsid within the envelope, derived from host cell membranes, and appear roughly spherical
• Naked viruses: do not have an envelope, and the nucleocapsid is the outermost structure

Steps in the Replication Cycle of Viruses

• Attachment: virus particle attaches to a host cell, involving specific molecular structures on the virion surface and receptor molecules in the host cell membrane
• Penetration: enveloped viruses involve penetration of the virus into the host cell, whereas naked viruses release their genetic material directly into the host cell
• Uncoating: loss of one or more structural components of the virion, leading to a loss of the ability to infect other cells
• Gene expression and replication: viral genes are expressed, and the viral genome is replicated using the cellular machinery of the host cell
• Assembly and release: nucleocapsids are assembled, and progeny viruses are released from the host cell

Attachment

• Attachment sites on the viral surface: specialized structures such as glycoprotein spikes or unique folding of capsid proteins
• Host cell receptor molecules: specific for each virus family, and can be molecular structures that usually carry out normal cell functions

Replication of DNA Viruses

• Each virus family differs in significant ways from all others in terms of the details of the macromolecular events comprising the replication cycle
• Smaller viral genomes: depend on the host cell to provide the functions needed for viral replication
• Larger viral genomes: provide virtually all enzymatic and regulatory molecules needed for a complete replication cycle

Replication of RNA Viruses

• Viruses with RNA genomes must overcome two specific problems: replicating the viral genome and producing several viral proteins in eukaryotic host cells
• No host cell RNA polymerase can use the viral parental RNA as a template for synthesis of complementary RNA strands
• Translation of eukaryotic mRNAs begins at only a single initiation site, and they are translated into only a single polypeptide
• RNA viruses must express the genetic information for at least two proteins: an RNA-dependent RNA polymerase and a minimum of one type of capsid protein

Mechanisms of RNA Virus Genome Replication

• Four broad patterns or “types” of replication:
  • Type I: RNA viruses with a single-stranded genome (ssRNA) of (+) polarity that replicates via a complementary (–) strand intermediate
  • Type II: RNA viruses with a single-stranded genome (ssRNA) of (–) polarity that replicates via a complementary (+) strand intermediate
  • Type III: RNA viruses with a double-stranded genome (dsRNA) that replicates via a semi-conservative mechanism
  • Type IV: RNA viruses with a single-stranded genome (ssRNA) of (+) polarity that replicates via a semi-conservative mechanism

Assembly and Release of Progeny Viruses

• Assembly of nucleocapsids generally takes place in the host cell compartment where the viral nucleic acid replication occurs
• Naked viruses: the virion is complete at this point, and release of progeny is usually a passive event resulting from the disintegration of the dying cell
• Enveloped viruses: virus-specific glycoproteins are synthesized and transported to the host cell membrane, and the nucleocapsid is enveloped by a process of “budding”