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.