Viruses – Differences between enveloped and
non-enveloped viruses Despite being exposed to billions of viruses
per day, only some viruses actually cause infection, indicating that viruses affect
humans in different ways. In this series of Chalk Talk facts, we’ll be looking at how
the characteristics of viruses correlate with their structure. Let’s start with the virus
particle, also known as the virion, which is the extracellular form of a virus used
to spread from one cell or organism to another. In contrast to bacteria or fungi, viruses
are complex molecular structures; however, they don’t count as living organisms as
they don’t have their own metabolism. Viruses come in many shapes and sizes such as rods,
bullets, or an icosahedron or sphere. A virion consists of a viral genome, which
can be either DNA or RNA, and is enclosed in a protein capsid that provides protection.
These viruses are referred to as non-enveloped viruses. This is in contrast to viruses surrounded
by a biological membrane, known as an envelope, which contains lipids and proteins. As its
name suggests, these viruses are also called enveloped viruses.
A virion is only infectious if fully assembled. If the envelope of the virus is destroyed,
it is no longer infectious. Viral genomes don’t encode the full set
of proteins required for independent metabolism. However, they only encode certain proteins
that link the virus to the metabolic pathways of the cell they infect, such as energy metabolism
or translational machinery. Therefore, viruses are considered obligate intracellular parasites
and the cells they infect are referred to as host cells. After entry into the cell,
the viral genome is released from its protective shell and, depending on whether it has a DNA
or RNA genome, interferes with host transcription and translation processes. Transfer of the virus genome and its release into the cell requires the viral capsid to
be sufficiently stable to protect the genome while being labile enough to be released or
uncoated inside the host cell. Structures with such opposite properties are called metastable.
Let’s take a look at capsid energy during its formation to gain a better understanding:
A capsid consists of identical subunits made up of structural proteins connected to each
other in a process termed self-assembly. As the individual proteins assemble to form a
large capsid, their surface charge and polarity is minimized while their contact region is
maximized. This leads to a decrease in the energy of the capsid system and provides the
driving force for capsid self-assembly. At the same time, the energy stored in the order
of the system increases, which can act as a driving force for viral uncoating.
As capsid subunits are held together by weak non-covalent bonds, dissociation can be initiated
by exposing the capsid to thermal energy. Viral uncoating can also be initiated by mechanical
traction or a pH change. However, the trigger releasing the viral genome depends on the
virus and the host cell. Contrary to what one would expect, an envelope
increases viral sensitivity to physical influencing factors as biological membranes are relatively
fragile structures. Consequently, enveloped viruses can’t survive the extreme acid of
the stomach and don’t usually enter through the gastrointestinal tract in the host. Enveloped
viruses are also more sensitive to heat, dryness, and disinfectants such as ethanol or propanol,
making them an easy target of the hygienic measures.
So what’s the advantage of a viral envelope? The biological membrane that forms the envelope
is derived from the host cell, originating from compartments such as the endoplasmic
reticulum or Golgi apparatus, or from the plasma membrane. This provides a shield, protecting
the virus particle more efficiently against the attack of the host immune system, although
the viral envelope is embedded with viral proteins.
Enveloped viruses can also exit the host cell without disrupting the host cell membrane.
In contrast, non-enveloped viruses exit the cell by lysis, which is a highly immunogenic
event. As a rule of thumb, enveloped viruses are usually less immunogenic than non-enveloped
viruses. In other words, the envelope helps the virus to evade the host immune response.
In the upcoming Chalk Talk facts episodes, we’ll be taking a detailed look at the viral
genomes and the viral life cycle to obtain a better understanding of viral infections.