What Is the Structure of the Marburg Virus?
Learn how the Marburg virus is constructed, from its genetic blueprint to the assembled proteins that form its distinct shape and enable it to infect cells.
The Marburg virus is a member of the Filoviridae family, a group of viruses that also includes the Ebola virus. First identified in 1967 following outbreaks in Germany and Serbia, this pathogen is responsible for Marburg virus disease, a severe and often fatal hemorrhagic fever. The virus is zoonotic, with the Egyptian fruit bat considered a primary natural reservoir, and it can spread between humans through direct contact with bodily fluids. Understanding the physical and genetic architecture of the Marburg virus is important for public health, as detailed knowledge of its structure informs the development of diagnostic tools, targeted antiviral therapies, and effective vaccines.
Macroscopic Features and Dimensions
The Marburg virus particle, or virion, is known for its distinctive filamentous, or thread-like, shape. However, it is also pleomorphic, meaning it can appear in various other forms, including U-shapes, 6-shapes, circles, or branched structures. These different shapes are observed in samples from infected hosts.
While their shape can vary, Marburg virions have a uniform diameter of approximately 80 nanometers. In contrast, their length is highly variable, with an average length of about 790 to 828 nanometers, though some particles have been measured to be as long as 14,000 nanometers. The virion is enveloped, meaning its core is wrapped in a lipid membrane derived from the host cell. Studding the surface of this envelope are small spikes, about 5 to 10 nanometers long, composed of viral proteins that play a role in infection.
The Marburg Genome
The genetic blueprint of the Marburg virus is a single molecule of ribonucleic acid (RNA). It is a linear, non-segmented, single-stranded RNA genome of negative polarity. This means the genetic sequence cannot be directly translated into proteins by the host cell’s machinery; it must first be transcribed into a complementary positive-sense strand. The genome is approximately 19.1 kilobases in length.
This RNA genome contains seven distinct genes arranged in a specific, linear order. Each gene codes for a structural or functional protein: the nucleoprotein (NP), polymerase cofactors (VP35, VP30), major and minor matrix proteins (VP40, VP24), the surface glycoprotein (GP), and the RNA-dependent RNA polymerase (L). The functions of these proteins are detailed in the following sections.
Core Components: Nucleocapsid and Matrix
The internal structure of the Marburg virion is centered around the nucleocapsid. The viral genome is tightly encased by the nucleoprotein (NP) to form a helical complex. This protein shell protects the RNA from degradation and provides the structural framework for the replication machinery. The major structural protein of this complex is NP, which polymerizes along the RNA strand.
Associated with the NP-RNA complex are three other proteins: VP35, VP30, and the L protein. The L protein is the large RNA-dependent RNA polymerase, the engine of viral replication and transcription. It works with VP35, which acts as a polymerase cofactor, and VP30, a minor nucleoprotein that helps activate the transcription process. Together, these four proteins form the functional ribonucleoprotein (RNP) complex.
Between the helical nucleocapsid and the outer viral envelope lies the matrix space, filled primarily by the matrix proteins VP40 and VP24. VP40 is the most abundant protein in the virion and forms a lattice-like layer that provides structural integrity, linking the inner nucleocapsid to the outer lipid envelope. VP40 is also a primary driver of the budding process, where new virus particles emerge from an infected cell. VP24 is the minor matrix protein, and its functions contribute to virion assembly and help the virus evade the host’s immune response.
The Viral Envelope and Surface Glycoprotein
The outermost layer of the Marburg virion is the viral envelope, a lipid bilayer membrane that the virus acquires from its host cell. As a new virus particle assembles inside an infected cell, it pushes against the cell’s plasma membrane and buds off, wrapping itself in a section of that membrane. This host-derived envelope is studded with the only viral protein exposed on the exterior: the glycoprotein (GP).
These glycoproteins form the spikes, or peplomers, that are visible on the surface of the virion. The GP protein is synthesized and processed within the host cell before being inserted into the membrane. During its maturation, it is cleaved by a host enzyme called furin into two subunits, GP1 and GP2, which remain linked. These subunits assemble into trimers to form the functional spikes that coat the viral surface.
The GP spikes are responsible for the virus’s ability to infect new cells. The GP1 subunit attaches to specific receptors on the surface of a host cell. Following attachment, the GP2 subunit mediates the fusion of the viral envelope with the host cell’s membrane. This fusion event creates an opening through which the viral nucleocapsid is released into the cytoplasm, allowing the replication cycle to begin.