A dual laser scanning confocal and transmission electron microscopy analysis of the intracellular localization, aggregation and particle formation of African horse sickness virus major core protein VP7

Abstract

The bulk of African horse sickness virus (AHSV) major core protein VP7 self-assembles into flat, hexagonal crystalline particles in a process appearing unrelated to viral replication. Why this unique characteristic of AHSV VP7 is genetically conserved, and whether VP7 aggregation and particle formation have an effect on cellular biology or the viral life cycle, is unknown. Here we investigated how different small peptide and enhanced green fluorescent protein (eGFP) insertions into the VP7 top domain affected VP7 localisation, aggregation and particle formation. This was done using a dual laser scanning confocal and transmission electron microscopy approach in conjunction with analyses of the solubility, aggregation and fluorescence profiles of the proteins. VP7 top domain modifications did not prevent trimerisation, or intracellular trafficking to one or two discrete sites in the cell. However, modifications that resulted in a misfolded and insoluble VP7-eGFP component blocked trafficking, and precluded protein accumulation at a single cellular site, perhaps by interfering with normal trimer-trimer interactions. Furthermore, the modifications disrupted the stable layering of the trimers into characteristic AHSV VP7 crystalline particles. It was concluded that VP7 trafficking is driven by a balance between VP7 solubility, trimer forming ability and trimer-trimer interactions.