a. Naked viruses
Naked viruses are predominantly released by host cell lysis. While some viruses are cytolytic and lyse the host cell more or less directly, in many cases it is the body's immune defenses that lyse the infected cell.
For lytic viruses (most non-enveloped viruses), release is a simple process - the cell breaks open and releases the virus.
b. Enveloped viruses
With enveloped viruses, the host cell may or may not be lysed. The viruses obtain their envelopes from host cell membranes by budding. As mentioned above, prior to budding, viral proteins and glycoproteins are incorporated into the host cell's membranes. During budding the host cell membrane with incorporated viral proteins and glycoproteins evaginates and pinches off to form the viral envelope. Budding occurs either at the outer cytoplasmic membrane, the nuclear membrane, or at the membranes of the Golgi apparatus
Enveloped viruses acquire the lipid membrane as the virus buds out through the cell membrane. Virion envelope proteins are picked up during this process as the virus is extruded. Budding may or may not kill the cell, but is controlled by the virus - the physical interaction of the capsid proteins on the inner surface of the cell membrane forces the particle out through the membrane.
1. Viruses obtaining their envelope from the cytoplasmic membrane are released during the budding process
Release of an Enveloped Virus by Budding, step 1.
With enveloped viruses, the host cell may or may not be lysed. The viruses obtain their envelopes from host cell membranes by budding. As mentioned above, prior to budding, viral proteins and glycoproteins are incorporated into the host cell's membranes. During budding the host cell membrane with incorporated viral proteins and glycoproteins evaginates and pinches off to form the viral envelope. Budding occurs either at the outer cytoplasmic membrane, the nuclear membrane, or at the membranes of the Golgi apparatus
Enveloped viruses acquire the lipid membrane as the virus buds out through the cell membrane. Virion envelope proteins are picked up during this process as the virus is extruded. Budding may or may not kill the cell, but is controlled by the virus - the physical interaction of the capsid proteins on the inner surface of the cell membrane forces the particle out through the membrane.
1. Viruses obtaining their envelope from the cytoplasmic membrane are released during the budding process
Release of an Enveloped Virus by Budding, step 1.
The virus obtains its envelope from the host cell's cytoplasmic membrane as it buds from the cell surface.
Release of an Enveloped Virus by Budding, step 2.
The virus obtains its envelope from the host cell's cytoplasmic membrane as it buds from the cell surface.
Transmission Electron Micrograph of Rubella Viruses Budding from a Host Cell
Image provided by Dr. Fred Murphy and Sylvia Whitfield.
Courtesy of the Centers for Disease Control and Prevention.
The rubella virus causes German measles. The virus gets its envelope by budding from the host cell's cytoplasmic membrane.
2. Viruses obtaining their envelopes from the membranes of the nucleus, the endoplasmic reticulum, or the Golgi apparatus are then released by exocytosis via transport vesicles.
Release of an Enveloped Virus by Fusion of a Transport Vesicle with the Host Cell's Cytoplasmic Membrane, step-1
The assembled virus is placed in a transport vesicle.
Release of an Enveloped Virus by Fusion of a Transport Vesicle with the Host Cell's Cytoplasmic Membrane, step-2.
The transport vesicle fuses with the host cell's cytoplasmic membrane releasing the vitrus from the cell.
Some viruses, capable of causing cell fusion, may be transported from one cell to adjacent cells without being released, that is, they are transmitted by cell-to-cell contact whereby an infected cell fuses with an uninfected cell.
The assembled virus is placed in a transport vesicle. Then, the transport vesicle fuses with the host cell's cytoplasmic membrane, releasing the virus from the cell.
The uninfected cells and infected cell then fuse together forming a multinucleated giant cell or syncytium.
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