The VP7 protein of the African horse sickness virus core particle facilitates binding to Culicoides sonorensis cells in an RGD-independent manner.

Research Overview

• This study investigates how the African horse sickness virus (AHSV) protein VP7 assists the virus in binding to insect cells of the transmitting vector, Culicoides midges.
• It was found that VP7 promotes binding in a manner that does not depend on the previously suspected RGD (arginine-glycine-aspartate) motif.

Background and Significance

• African Horse Sickness Virus (AHSV): AHSV causes a severe and often fatal disease in horses, transmitted by biting midges from the genus Culicoides.
• Virus Transmission: The virus is passed to horses through Culicoides midges, which themselves become infected through feeding.
• Virus Structure: AHSV has several structural proteins, including VP2 (outer capsid) and VP7 (core protein) that potentially play roles in infection and transmission.
• Role of Protease Cleavage: Proteases from Culicoides saliva cleave the VP2 protein, producing sub-virus particles that infect insect vector cells more effectively.
• VP7 and RGD Motif: VP7 contains an RGD motif, which in other viruses often mediates cell binding by interacting with integrins.

Research Objectives

• To determine the function of the VP7 protein—particularly whether the RGD motif facilitates binding to insect vector cells (Culicoides sonorensis KC cells).
• To investigate if core-like particles (CLPs) containing VP7 can bind to insect cells in an RGD-dependent or -independent manner.

Methodology

• Production of Core-Like Particles (CLPs): Researchers used a baculovirus expression system to co-express VP3 with a soluble form of VP7 (sVP7) to generate CLPs mimicking the virus core.
• Binding Assays: These CLPs were tested in vitro for their ability to bind to the KC insect cells.
• Mutational Analysis: Recombinant baculoviruses were engineered to express mutant versions of sVP7 where the RGD motif was either deleted entirely or mutated to disrupt its function.
• Assessment of Assembly and Binding: The assembly of CLPs with these mutant VP7 proteins was checked along with their binding capacity to insect cells.

Key Findings

• CLPs containing wild-type VP7 were capable of binding to Culicoides KC cells, indicating VP7’s involvement in this interaction.
• All mutant sVP7 proteins except for the one with the deleted RGD motif could still form trimers and assemble into CLPs with VP3, showing structural integrity was largely maintained despite mutation.
• Importantly, CLPs containing mutant VP7 proteins (with altered RGD motifs) still bound to KC insect cells efficiently.
• This demonstrated that the binding function of VP7 to insect cells does not rely on the RGD motif, suggesting an RGD-independent mechanism facilitates attachment.

Conclusions and Implications

• VP7 plays a significant role in mediating the binding of AHSV core particles to the insect vector cells, promoting transmission.
• The commonly assumed integrin-binding RGD motif in VP7 is not required for this binding, which shifts the focus to other potential binding mechanisms or domains within VP7.
• This insight improves understanding of how AHSV interacts with its insect vector and may help guide future strategies for controlling virus transmission or developing vaccines targeting viral entry mechanisms.