What Are Paramyxoviruses? Types, Transmission & Biological Impact
Paramyxovirus is any virus belonging to the Paramyxoviridae family. Paramyxoviruses have enveloped virions (or virus particles) with diameters ranging from 150 to 200 nm (1 nm = 109 metres). The nucleocapsid that consists of a protein shell (or the capsid) and has the viral nucleic acids has helical symmetry.
Paramyxovirus Virus Genome and Paramyxovirus Causes
The paramyxovirus virus genome consists of a single strand of negative-sense non-segmented RNA (ribonucleic acid). An endogenous RNA polymerase is present and is necessary for the transcription of the negative-sense strand as well into the positive-sense strand, thereby enabling the proteins to be encoded from RNA. The lipoprotein envelope has two glycoprotein spikes designated from the fusion factor (F) and hemagglutinin-neuraminidase (HN).
Subfamilies
Paramyxoviridae has 2 subfamilies called Pneumovirinae and Paramyxovirinae, each of which has multiple genera. Rubulavirus, which includes several types of mumps viruses and human parainfluenza viruses, is an example of Paramyxoviridae genera; Avulavirus, which contains the agents that cause distemper in dogs and cats, measles in humans, and rinderpest in cattle; and Morbillivirus, which contains the agents that cause distemper in dogs and cats, measles in humans, and rinderpest in cattle. The subfamily Pneumovirinae is made up of Pneumovirus species that cause serious respiratory syncytial virus disease in human infants.
Structure
Virions are enveloped and may be pleomorphic or spherical and capable of producing filamentous virions. The diameter is up to 150 nm. Genomes are linear and around 15kb in length. Fusion proteins and the attachment proteins appear as spikes on the virion surface. Matrix proteins present inside the envelope stabilize the structure of the virus. The nucleocapsid core is composed of nucleocapsid proteins, genomic RNA, polymerase, and phosphoproteins proteins.
Genome
The genome is negative-sense RNA, non-segmented, 15–19 kilobases in length, and contains 6 - 10 genes. Extracistronic (noncoding) regions are:
A 5’ trailer sequence, which is 50–161 nucleotides long
A 3’ leader sequence, which is 50 nucleotides in length that acts as a transcriptional promoter.
Intergenomic regions between every gene, which are 3 nucleotides long for morbilliviruses, henipaviruses, and respiro viruses, and variable-length (one-56 nucleotides) for rubella viruses.
Each gene has transcription start or stops signals at the beginning and end, which are transcribed as part of the gene.
Gene sequence within the genome is conserved across the paramyxovirus family because of a phenomenon called transcriptional polarity, where the genes closest to the 3’ end of the genome are transcribed in a greater abundance compared to those towards the 5’ end. This is a result of the genome structure. After every gene is transcribed, the RNA-dependent RNA polymerase pauses to release new mRNA when it encounters the intergenic sequence.
A chance exists that it will dissociate from the RNA genome when the RNA polymerase is paused. It must re-enter the genome at the leader sequence if it dissociates, rather than continuing to transcribe the genome’s length. The result is, the further downstream genes are from the leader sequence, the less they will be transcribed by the RNA polymerase.
Evidence for the single promoter model was verified when viruses were exposed to UV light. UV radiation may cause dimerization of RNA that prevents the transcription by RNA polymerase. If the viral genome follows the multiple promoter model, the level of inhibition of the transcription should correlate with the RNA gene length. However, the genome was best described by the single promoter model. The degree of transcription inhibition was proportional to the distance from the leader sequence when the paramyxovirus genome was exposed to UV light. It means, the further the gene is from the sequence of leaders, the greater the chance of RNA dimerization, inhibiting the RNA polymerase.
The virus takes advantage of the single promoter model by storing its genes in the order in which proteins are necessary for paramyxovirus infection. For example, nucleocapsid protein (N), which is one of the paramyxovirus examples is required in greater amounts than RNA polymerase (L).
Proteins
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The above figure is the illustration of Paramyxoviridae virus virion.
N – the nucleocapsid protein binds to genomic RNA (one molecule per hexamer) and prevents it from being digested by nucleases.
P – the phosphoprotein binds to both L and N proteins and forms the part of the RNA polymerase complex.
M – the matrix protein assembles between the nucleocapsid and envelope core; it organizes and maintains the structure of the virion.
F – the fusion protein projects as a trimer from the envelope surface and mediates the entry of the cell by inducing fusion between the cell membrane and the viral envelope by class I fusion. The defining characteristics of the members of the family Paramyxoviridae are the need for a neutral pH for fusogenic activity.
H/HN/G – The cell attachment proteins extend from the spike's surface and span viral envelopes.
L – the large protein is given as the catalytic subunit of RNA-dependent RNA polymerase (RDRP).
Accessory Proteins – it is a mechanism called RNA editing that allows multiple proteins to be produced from the P-gene.
FAQs on Paramyxovirus Explained: Structure, Genome, and Role in Disease
1. What is a Paramyxovirus and what are its key structural features?
A Paramyxovirus is a type of medium-sized, enveloped virus belonging to the Paramyxoviridae family. Its key structural feature is its genetic material, which is a single-stranded, negative-sense RNA genome. This means the viral RNA cannot be directly translated into proteins and must first be transcribed into a complementary positive-sense strand. The virus particle (virion) is typically spherical and its envelope is studded with proteins like hemagglutinin-neuraminidase (HN) for attachment and fusion (F) protein for entering host cells.
2. What are the main diseases in humans caused by Paramyxoviruses?
Paramyxoviruses are responsible for several common and significant human diseases, particularly affecting the respiratory system. The most well-known examples include:
- Measles: Caused by the measles virus, leading to fever, cough, and a characteristic rash.
- Mumps: Caused by the mumps virus, resulting in painful swelling of the salivary glands.
- Respiratory Syncytial Virus (RSV): A major cause of lower respiratory tract infections like bronchiolitis and pneumonia, especially in infants and young children.
- Parainfluenza: Human parainfluenza viruses (HPIVs) are a common cause of croup and other upper and lower respiratory illnesses.
3. How is a Paramyxovirus transmitted from one person to another?
Paramyxoviruses are highly contagious and primarily spread through respiratory droplets. When an infected person coughs, sneezes, or talks, they release tiny droplets containing the virus into the air. Transmission occurs when a healthy individual inhales these droplets or touches a surface contaminated with the droplets and then touches their own eyes, nose, or mouth. This mode of transmission makes them spread easily in crowded places like schools and daycare centres.
4. How does a Paramyxovirus replicate inside a host cell?
The replication of a Paramyxovirus is a unique process that occurs entirely in the cytoplasm of the host cell, not the nucleus. The steps are:
- Attachment and Entry: The virus attaches to the host cell surface using its HN protein and then fuses its envelope with the cell membrane using its F protein, releasing its RNA genome into the cytoplasm.
- Transcription: The virus uses its own enzyme, RNA-dependent RNA polymerase, to create positive-sense mRNA strands from its negative-sense RNA genome.
- Translation: The host cell's ribosomes translate the viral mRNA into new viral proteins.
- Replication and Assembly: The polymerase also creates full-length positive-sense RNA templates to mass-produce new negative-sense RNA genomes. These new genomes and proteins assemble into new virus particles near the cell membrane.
- Release: The newly formed viruses exit the host cell by budding off from the cell membrane, acquiring their envelope in the process.
5. What is the main difference between Paramyxovirus and Orthomyxovirus?
While both are RNA viruses causing respiratory illnesses, the key difference lies in their genetic structure. Orthomyxoviruses, like the influenza virus, have a segmented RNA genome (multiple separate pieces of RNA). In contrast, Paramyxoviruses, like the measles virus, have a non-segmented genome (a single, continuous strand of RNA). This structural difference means Orthomyxoviruses can undergo antigenic shift (reassortment of segments), leading to pandemics, which is not a feature of Paramyxoviruses.
6. Why are there effective vaccines for diseases like measles and mumps?
Effective vaccines, like the MMR (Measles, Mumps, Rubella) vaccine, exist for these Paramyxovirus diseases primarily because the viruses are antigenically stable. Unlike influenza viruses, their surface proteins do not change rapidly from year to year. This stability allows the immune system, once trained by the vaccine, to consistently recognise and neutralise the virus for a long time, often providing lifelong immunity. The vaccine uses a live, attenuated (weakened) form of the virus to trigger a robust and lasting immune response without causing the actual disease.
7. What are the general treatment options for Paramyxovirus infections?
For most Paramyxovirus infections, there is no specific antiviral cure. Treatment is primarily supportive and symptomatic, aimed at managing the symptoms and preventing complications while the body's immune system fights off the infection. This includes:
- Rest and adequate fluid intake.
- Fever reducers like paracetamol.
- Medications to relieve symptoms like cough or congestion.
