Assembly and exit of virions from cells

LECTURE 14:

Viro100: Virology3 Credit hoursNUST Centre of Virology & Immunology

Waqas Nasir Chaudhry

2

VIRAL LIFE CYCLE

ATTACHMENT

PENETRATION HOSTFUNCTIONS

ASSEMBLY(MATURATION)

Transcription

REPLICATION

RELEASE

UNCOATING

Translation

MULTIPLICATION

• Once threshold quantities of progeny virus genomes and structural proteins have accumulated in the infected cell, assembly of virions can commence.

• These components are assembled into nucleocapsids.– Helical viruses– Icosahedral viruses

• If the virion also contains lipid then the assembly process also includes the acquisition of this component, either as an internal membrane or as an envelope

The assembly of virions and nucleocapsids of ssRNAviruses with helical symmetry involves coating the genome with multiple copies of a protein.

A few copies of a protein species bind to a helical ssRNA molecule, then more copies bind until the RNA is completely coated

• For virus with icosahedral symmetry, first step is the construction of an empty protein shell, known as a procapsid

• The procapsid is filled with a copy of the virus genome

• During or after this process (packaging) it may undergo modification to form the mature capsid

• Modification of the procapsid may result in a change from a spherical to an icosahedral shape.

• Modification happens with cleavage of one or more of the structural proteins by proteases

• The prohead of many tailed phages also has a portal through which a copy of the virus genome enters.

• A further function of the portal is as a ‘connector’ to join the tail to the head

• Each of these three components is symmetrical, but in a different way• The connector has 12 fold rotational symmetry• The icosahedron vertex attached to one end of the connector has

five fold rotational symmetry• The tail attached to the other end of the connector has six fold

rotational symmetry

Genome packaging

• How are virus genomes selected from all the cell and virus nucleic acids?

• Achieved through the recognition by a virus protein of a specific virus genome sequence, known as a packaging signal

• Virus genomes are packaged into small volumes, which means that repulsion between the negative charges on their phosphate groups must be overcome.

• This may be aided by packaging basic proteins, which are positively charged, along with the genome.

• Papillomaviruses and polyomaviruses coat their DNA with host cell histones, the basic proteins associated with the DNA in eukaryotic cells.

• Some viruses package other positively charged materials, such as polyamines and cations.

• The dsDNA of large icosahedral viruses such as herpesviruses and the tailed phages is packed so tightly that the pressure within the capsid is about ten times greater than the pressure inside a champagne bottle.

Portal inside view for genome packaging