The gp64 protein is an envelope glycoprotein found on the surface of certain baculoviruses, such as Autographa californica multiple nucleopolyhedrovirus (AcMNPV). It is a major component of the budded virus (BV) form, responsible for spreading infection within an insect host. This protein facilitates the virus’s entry into host cells. Gp64 is important for understanding viral biology and for its potential in biotechnological applications.
Structure and Role in Baculovirus Life Cycle
The gp64 protein is an integral membrane protein appearing as a homotrimer on the viral envelope and infected cell surfaces. It is unusual among viral fusion proteins for having an intermolecular disulfide bond linking its monomers. The protein has multiple domains: a fusion domain for membrane merger, a transmembrane domain for anchoring, and a multimerization domain for trimer formation.
Gp64 is essential for the initial steps of infection. It enables the budded virus to attach to host cells and facilitates entry by interacting with host cell receptors. This protein is expressed during both early and late phases of viral infection. Beyond its role in attachment and entry, gp64 is also necessary for the efficient budding of new virions from infected cells.
Mediating Viral Entry
Gp64 facilitates viral entry into host cells through endocytosis and pH-dependent membrane fusion. Baculoviruses are internalized via receptor-mediated endocytosis, often involving clathrin-mediated pathways. The virus is engulfed by the cell membrane, forming an endosomal vesicle inside the cell.
Inside the endosome, the environment becomes acidic (pH 5.0-5.5). This pH change triggers a conformational alteration in gp64. The low pH causes the gp64 trimers to refold, exposing a fusion peptide. This peptide then inserts into the endosomal membrane, a step for membrane fusion.
The fusion peptide’s insertion initiates the merger of the viral envelope with the endosomal membrane. This membrane fusion event is crucial, allowing the viral nucleocapsid, containing the viral genetic material, to be released from the endosome directly into the host cell cytoplasm. The release of the nucleocapsid enables the virus to proceed with its replication cycle. Low pH-activated gp64 trimers may form a “fusion machine” that precedes membrane merger.
Applications in Biotechnology and Research
The gp64 protein’s properties have led to its widespread use in various biotechnological and research applications, particularly within baculovirus expression systems. Baculoviruses, engineered to express foreign genes, are commonly used to produce recombinant proteins in insect cells. Gp64’s ability to promote efficient viral entry and budding makes it an effective component in these systems. Researchers can leverage gp64 to display foreign proteins on the surface of baculovirus virions, which is beneficial for vaccine development and antibody production.
This display technology allows the generation of antibodies without needing to purify the target antigens beforehand, as the whole virus displaying the antigen can be used for immunization. For instance, baculovirus particles displaying gp64-fusion proteins have been used to produce monoclonal antibodies against human nuclear receptors. Furthermore, gp64-pseudotyped lentivirus vectors have demonstrated efficient gene transduction in a wide variety of mammalian cells, expanding their utility beyond insect systems.
Beyond expression systems, gp64 is also explored for its potential in gene delivery and as a target for antiviral therapies. Its capacity to facilitate entry into both insect and mammalian cells makes baculoviruses, and specifically gp64, attractive candidates for gene therapy vectors. Understanding the detailed mechanism of gp64-mediated entry can inform the design of more efficient and specific gene delivery tools. Conversely, targeting gp64’s fusion mechanism could lead to the development of novel antiviral compounds, as inhibiting its function would prevent viral entry and subsequent infection.