Reassessing the mechanism of genome packaging in plant viruses with lessons from ATPase fold

Abstract

Plant viruses cause various disease in crops and are responsible for huge economic losses. Understanding their mode of infection and spread is crucial for developing control strategies. Genome packaging is an important step in the process of viral maturation. Three systems of viral genome packaging are known till date. The type I packaging system, found in most of the small plant viruses, involves co-condensation of nucleic acid with viral capsid proteins (CPs) leading to the assembly of virion particles without utilizing ATP. The type II and III packaging systems, present in phages and nucleo-cytoplasmic large DNA viruses, exhibit genome translocation inside the prohead by utilizing ATP. Discovery of the ATPase fold in the CPs of many small plant viruses and the requirement of other ATPases during genome encapsidation have changed the perception about genome packaging in the type I system. Based on recent studies, it seems that the genome packaging mechanisms of plant viruses such as potexvirus have nothing in common with those of other well-characterized passive, energy-independent type I systems. Moreover, these studies have suggested that in these plant viruses, the genome encapsidation process is more intricately coupled with energy utilization than in other systems. In this comprehensive review, we present a novel, expanded sub-classification system for the type I system on the basis of ATP employed.