LECTURE 15:
Exit of virions from cells
Waqas Nasir Chaudhry
Viro100:
Virology
3 Credit hours
NUST Centre of Virology & Immunology
Formation of virion membranes
• Enveloped virions acquire their membrane
envelopes by one of two mechanisms;
1. Either they modify a host cell membrane and
then nucleocapsids bud through it
2. The virus directs synthesis of new membrane,
which forms around the nucleocapsids
Budding through cell membranes
• Most enveloped viruses acquire their envelopes
by budding through a membrane of the host cell
• Regions of membrane through which budding will
occur, become modified by the insertion of one
or more species of virus protein, the vast majority
of which are glycoproteins
• Cell proteins may not be totally excluded from
these regions and may become incorporated into
virus envelopes; for example, HIV envelopes
contain major histocompatibility complex class II
proteins
• Budding of virions involves interaction
between the cytoplasmic tail of a virus
glycoprotein in the membrane and another
virus protein.
• M (membrane, matrix) protein
• M proteins have an affinity for membranes,
and bind to nucleocapsids as well as to the
virus glycoproteins, ‘stitching’ the two
together during budding.
• The vital role played by the M protein in the
budding process has been demonstrated using
mutants of measles virus (a paramyxovirus)
and rabies virus (arhabdovirus).
• Roles similar to that of the M protein are
played by the M1 protein of influenza A virus
and the MA (matrix) domain of the retrovirus
Gag protein
• Viruses that bud from the cell do so from
particular regions of the plasma membrane,
and if the cell is polarized then budding may
take place primarily from one surface.
Membrane trafficking pathways within the host cell
De novo synthesis of viral membranes
• A minority of viruses direct the synthesis of
lipid membrane late in the replication cycle.
• In some cases the membrane forms a virion
envelope (e.g. poxviruses);
• In other cases the membrane forms a layer
below the surface of the capsid (e.g.
iridoviruses)
• Baculoviruses produce two types of enveloped
virion during their replication.
• One type of virion has the function of
spreading the infection to other cells within
the host, and this virion acquires its envelope
by budding from the plasma membrane.
• The other type of virion has the function of
infecting new host individuals.
• Its envelope is laid down around
nucleocapsids within the nucleus and the
resulting virions become incorporated into
occlusion bodies
Virion exit from the infected
cell
• The virions of many viruses are released from the
infected cell when it bursts (lyses), a process that
may be initiated by the virus.
• Many phages produce enzymes (lysins, such as
lysozymes) that break bonds in the peptidoglycan
of the host bacterial cell walls.
• Other phages synthesize proteins that inhibit host
enzymes with roles in cell wall synthesis; this
leads to weakening of the cell wall and ultimately
to lysis
Phage Lysins as Novel Alternatives to Antibiotics; Prof. Dr.
Vincet Fischetti
• Because of their modes of transmission, most
plant viruses leave their host cells in ways that
differ from those of animal and bacterial
viruses.
• Plant cells are separated from each other by
thick cell walls, but in many of them there are
channels, called plasmodesmata, through
which the plant transports materials.
• Viruses are able to spread within the host by
passing from cell to cell through
plasmodesmata.
• For spread to new
hosts many plant
viruses leave the
host cell as a
component of the
meal of a vector
(e.g. an aphid or a
nematode) that
feeds by ingesting
the contents of
cells
Spread of plant viruses through plasmodesmeta
Reviridae Genome Replication; Conservative mode
They are dsRNA viruses which replicate in the cytoplasm
(indicating they have everything they need for
replication and do not utilize the cells replication
enzymes) and they do not fully uncoat during the
process of replication.
The reason for their failure to fully uncoat is that the
coat is resistant to protease digestion, which prevents
them from being completely destroyed by the infected
cell.
The mRNA used in translation is synthesized from the
negative strand of the dsRNA.
Each negative strand produces many positive strands.
The process utilizes particle-associated transcriptase.
Neither of the strands of dsRNA appear among the
transcription products, and both strands remain in the
uncoated core particle.