Importance of an arginine-glycine-aspartic acid (RGD) motif in the VP7 protein of African horse sickness virus to mediate insect cell binding

Abstract

African horse sickness, a rapidly fatal disease of equines, is caused by African horse sickness virus (AHSV) which is transmitted to its vertebrate hosts through the bites of midges belonging to the Culicoides genus. The outer capsid of the virus surrounds an inner core composed of the structural proteins VP3 and VP7. The outermost core protein VP7 has an arginine-glycine-aspartate (RGD) tripeptide motif, which is one of the ligand sites recognized by the integrin family of cell surface receptors. To evaluate the ability of the AHSV VP7 protein to bind to insect cells and the functional significance of the VP7 RGD motif in this interaction, core-like particles (CLPs) were produced with the baculovirus expression system by co-expression of VP3 and sVP7, which encodes a soluble version of the AHSV-4 VP7 protein. Insect cell binding assays indicated that the CLPs were capable of binding to Culicoides sonorensis-derived cells, indicating a role for VP7 in this process. Next, recombinant baculoviruses expressing mutant sVP7 proteins in which the RGD motif was either deleted or mutated were synthesized. Except for deletion of the RGD motif, the RGD-mutated sVP7 proteins were able to form trimers and, when co-expressed with VP3, formed CLPs that retained the ability to bind to insect cells. These results therefore indicate that VP7 facilitates binding of CLPs to insect cells and that this interaction occurs in an RGD-independent manner.