Human Metapneumovirus HMPV:  Classification, Morphology and Molecular Organization

To fight effectively against hMPV scientists are continuously working on it to understand its morphology (i.e. shape, size and structures) and molecular composition. The researchers also working on the phenomenon of how it interacts with human body cells and completes its life cycle in human cells. Detailed knowledge about its morphology and molecular composition

 helps to grasp how the hMPV virus causes disease and how researchers approach it to develop effective treatments and vaccines against this virus. In this article, we will discuss the available details about its morphology, molecular composition and its interaction with human body cells.

Introduction

Human Metapneumovirus (hMPV) is a negative sense-single-stranded  non segmented RNA virus(-ssRNA) that belongs to the Pneumoviridae family of viruses within the order Mononegavirales. Since its discovery in 2001, hMPV has gathered significant attention due to its role in respiratory infections. It affects individuals of all age groups, particularly infants,

 young children, elderly persons, and immunocompromised individuals. Recently, it has been in the limelight all over the world due to its outbreak in China as an epidemic. The cases of hMPV patients are also reported in some other parts of the world including India, a thickly populated country.

Diagrammatic Structure of Human Metapneumovirus(HMPV)

Classification of HMPV Virus

Realm	Riboviria
Kingdom	Orthornavirae
Phylum	Negarnaviricota
Class	Monjiviricetes
Order	Mononegavirales
Family	Pneumoviridae
Genus	Metapneumovirus
Species	Metapneumovirus hominis

Morphology of Human Metapneumovirus (HMPV)

Human Metapneumovirus is an enveloped virus having a pleomorphic shape. It may be spherical or filamentous having spikes all over the surface. However, it shares structural similarities with viruses of the Paramyxovirdae family but also exhibits its unique characteristics.

Morphology of Human Metapneumovirus (HMPV)

Size

The size of the virus varies between 150 to 600 nanometers in diameter.

External Structure

The envelope of the virus consists of the lipid bilayer derived from the host cell membrane. The outer lipid layer consists of three membrane surface glycoproteins in the form of spikes of 13 to 17 nm which facilitate its entry in the host cell. The fusion (F) proteins facilitate the fusion of the viral envelope with the host cell membrane and allow the viral genome to enter the host cell. The Glycoprotein (G) gets attached to host cell receptors to initiate the infection process. The small hydrophobic (SH) proteins are also reported on envelope surface with unknown functions.

Internal Structure

Nucleocapsid: 

It is helical in shape consisting of the viral RNA genome and associated proteins. The HMPV genome is a negative sense-single-stranded RNA (-ssRNA). The associated proteins of nucleocapsid ensure the protection and proper packaging of the viral genome.

Matrix (M) Protein: 

The inner side of the lipid bilayer consists of a protein known as matrix (M) protein. It acts as a bridge between the envelope and nucleocapsid (viral RNA) and plays a crucial role in virus assembling and stability.

Molecular Composition of HMPV

Lipid

The lipids that are present in the form of a layer constitute the envelope of the hMPV virus. It is derived from the host cell membrane and contains host-derived lipids. It protects the inner viral components and facilitates the entry of the virus into the host cell with the help of embedded spike proteins.

Proteins

F- Protein (Fusion Protein): 

It is present on the outer surface of the envelope of the virus in the form of spikes that fuse with the host cell to facilitate the entry of genetic material i.e. RNA genome into the host cell. The fusion of virus with host cell (respiratory tract epithelial cells) is the first step of infection cycle. This protein is considered as the most conserved protein among all protein associated with hMPV

G- Protein (Glycoprotein): 

It is also present on the outer surface of the envelope of the virus in the form of spikes and is involved in the attachment of the virus with the host cell to initiate infection. This is the protein that helps the virus to recognize the correct or target cell in the respiratory tract. It binds to the glycosaminoglycans to facilitate the attachment of viral particle to the host cell. It is supposed to contribute to the entry of neutrophils in the airways of respiratory tract.  

SH-Protein (Small Hydrophobic Protein): 

It is a small hydrophobic protein present on the outer surface of the virus envelope. The function associated with it is not understood completely. Recent researches indicates that it belongs to the viroporin class of proteins, which alter membrane permeability. By altering the host cell membrane permeability, it blocks the signaling of interferons which in turn suppresses the immune response of the body toward viral infection. This immune suppressing role of SH protein may help to explain why hMPV is a leading cause of hospitalization in children and older adults.

M- Protein (Matrix Protein): 

It is present on the inner surface of the bilipid layer. It is essential for virus assembly and release. It also maintains the connection between the envelope and the nucleocapsid. This protein consists of the high-affinity binding site for Ca⁺ ions. Interestingly it is secreted by infected cells in soluble form

N- Protein (Nucleoprotein): 

It is an associated protein of the nucleocapsid and is involved in the proper packaging and protection of the viral RNA genome. It bind with the negative-sense-single-stranded RNA.

P- Protein (Phosphoprotein): 

It is also an associated protein of the nucleocapsid and functions as a cofactor of the viral polymerase enzyme protein (L protein) in the replication and transcription of the viral RNA. This complex is required for stabilization and the synthesis of new genetic material

L-Protein (Polymerase Protein): 

It is also a part of nucleocapsid and attached with RNA-dependent RNA polymerase and facilitates genome replication and mRNA synthesis. It is a large polymerase protein that has binding sites for zinc and exhibits multifunctional catalytic activity. 

M2-1- Protein (Putative Transcription Factor): 

It is involved in the reverse transcription of genomic RNA to DNA and transcription of this DNA into new RNA genome for new viruses. The M2-1 protein play a role in modulating the processivity of the RNA polymerase involved in the regulation of transcription of RNA. The M2-1 protein is also responsible for the modulation of the immune response produced by the host cells. An epitope from the M2-1 protein stimulates a protective CTL (Cytotoxic T lymphocyte) response.

M2-2 -Protein (RNA Synthesis Regulatory Factor): 

It regulates the synthesis of RNA in the infected cell of the host i.e. multiplication of viral genome. The M2-2 protein also  play a role in modulating the processivity of the RNA polymerase involved in the regulation of transcription of RNA like M2-1. The M2-2 protein is also responsible for the modulation of the immune response produced by the host cells. M2- 2 inhibits the TLR (Toll-like receptors) driven responses of host cells and is considered as one of the main molecular sensors targeted by hMPV to avoid its detection by host immune cells.

Conclusion

hMPV is a complex enveloped virus consisting of its self-synthesized components i.e. RNA genome and specific proteins and envelope from the host cell membrane. Its unique structure enables it to infect and spread within the human respiratory tract. The deep knowledge about its morphology, structure and molecular composition helps us to understand its ability to interact with various body organs and is key to understanding how it causes disease. This helps the researchers to develop better treatment strategies, vaccines, and preventative measures. Hence, the foundational knowledge regarding its structure is critical for combating hMPV and reducing its impact on public health, especially in vulnerable populations.

Frequently Asked Questions (FAQ)

Is Human Metapneumovirus RNA virus

Yes, Human Metapneumovirus (hMPV) is an RNA virus. It is a negative sense single- stranded enveloped non segmented RNA virus.

What is the family of Human Metapneumovirus(hMPV)?

The Human Metapneumovirus (hMPV) virus belongs to the Pneumoviridae family of viruses. Its scientific name is Metapneumovirus hominis.

What is the size of HMPV virus?

Human Metapneumovirus (hMPV) is an enveloped virus and its size varies between 150 to 600 nanometers in diameter. It has a pleomorphic shape. It may be spherical or filamentous having spikes all over the surface.

How big is Human Metapneumovirus HMPV?

Human Metapneumovirus (hMPV) is about 150 to 600 nm in diameter. It is an enveloped virus. The envelope developed from the host cell membrane which consists of glycoproteins in the form of spikes synthesized by the viral genome. These spikes facilitate the attachment and entry of the virus into the host cell.