A novel role of the potyviral helper component proteinase contributes to enhance the yield of viral particles

J Virol. 2014; 88 (17): 9808-9818.

Valli A, Gallo A, Calvo M, Pérez JD, García JA.

J Virol. 2014; 88 (17): 9808-9818The helper component proteinase (HCPro) is an indispensable, multifunctional protein from members of the genus Potyvirus and other viruses of the family Potyviridae. This viral factor is directly involved in diverse steps of the viral infection, such as aphid transmission, polyprotein processing, and suppression of host antiviral RNA silencing.

In this paper, we show that although a chimeric virus based on the potyvirus Plum pox virus lacking HCPro, which was replaced by an heterologous silencing suppressor, caused an efficient infection in Nicotiana benthamiana plants, its viral progeny had a very reduced infectivity. Making use of different approaches, here we provide direct evidences of a previously unknown function of HCPro, by which this viral factor enhances the stability of its cognate capsid protein (CP), positively affecting the yield of virions, and consequently improving the infectivity of the viral progeny.

Site-directed mutagenesis revealed that the ability of HCPro to stabilize CP and enhance the yield of infectious viral particles is unlinked to any of its previously known activities, and helped us to delimit the region of HCPro involved in this function in the central region of the protein. Moreover, this function is highly specific and cannot be fulfilled by the HCPro of a heterologous potyvirus. The importance of this novel requirement in regulating the sorting of the viral genome to be subjected to replication, translation and encapsidation, thus contributing to the synchronization of these viral processes, is discussed.

IMPORTANCE: Potyviruses form one of the most numerous groups of plant viruses, being a major cause of crop loss worldwide. It is well known that these pathogens make use of virus-derived multitasking proteins, as well as dedicated host factors, to successfully infect their hosts. Here, we describe a novel requirement for the proper yield and infectivity of a potyviral progeny. In this case, such a function is accomplished by the extensively studied viral factor HCPro, which seems to use an unknown mechanism that is not linked to its previously described activities. To our knowledge, it is the first time that a factor different from CP is directly involved in the yield of potyviral particles. With the data presented here, we hypothesize that this capacity of HCPro might be involved in the coordination of mutually exclusive activities of the viral genome by controlling correct assembly of CP in stable virions.

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