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

Abstract

African horse sickness, caused by African horse sickness virus (AHSV) that is transmitted by midges of the Culicoides genus, leads to rapid mortality among horses. Proteases in the saliva of Culicoides midges cleave the VP2 outer capsid protein, resulting in infectious sub-virus particles that have increased infectivity for the Culicoides vector insect and Culicoides-derived cells (KC cells). The AHSV VP7 protein has an arginine-glycine-aspartate (RGD) motif, but the functional relevance of this protein and motif in facilitating binding to insect cells is unknown. To investigate, core-like particles (CLPs) were produced using the baculovirus expression system through the co-expression of VP3 and sVP7, which is a soluble version of the AHSV-4 VP7 protein. Insect cell binding assays indicated that the CLPs bind to KC cells, suggesting a role for VP7 in this interaction. Subsequently, recombinant baculoviruses expressing mutant sVP7 proteins were synthesized, in which the RGD motif was either deleted or mutated. All RGD-mutated sVP7 proteins, except for the deletion of the RGD motif, formed trimers and, when co-expressed with VP3, assembled into CLPs that retained the ability to bind to insect cells. These findings indicate that VP7 facilitates the binding of CLPs to insect cells through an RGD-independent mechanism.

HIGHLIGHTS • VP7 is the outermost protein of AHSV core particles and possesses an RGD motif. • AHSV CLPs, formed by VP3 and VP7 proteins, bind to Culicoides-derived insect cells. • RGD-mutant CLPs retain the ability to bind to insect cells. • VP7 acts as an attachment protein for insect cells, independent of the RGD motif. • The findings provide new insights into early AHSV–insect vector interactions.