Ac23, an Envelope Fusion Protein Homolog in the Baculovirus Autographa californica Multicapsid Nucleopolyhedrovirus, Is a Viral Pathogenicity Factor

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Abstract
Viral envelope fusion proteins are important structural proteins that mediate viral entry and may affect or determine the host range of a virus. The acquisition, exchange, and evolution of such envelope proteins may dramatically affect the success and evolutionary divergence of viruses. In the family Baculoviridae, two very different envelope fusion proteins have been identified. Budded virions of group I nucleopolyhedroviruses (NPVs) such as the Autographa californica multicapsid nucleopolyhedrovirus (Ac M NPV), contain the essential GP64 envelope fusion protein. In contrast group II NPVs and granuloviruses have no gp64 gene but instead encode a different envelope protein called F. F proteins from group II NPVs can functionally substitute for GP64 in gp64 null Ac M NPV viruses, indicating that GP64 and these F proteins serve a similar functional role. Interestingly, Ac M NPV (and other gp64 -containing group I NPVs) also contain an F gene homolog ( Ac23 ) but the Ac M NPV F homolog cannot compensate for the loss of gp64 . In the present study, we show that Ac23 is expressed and is found in budded virions. To examine the function of F protein homologs from the gp64 -containing baculoviruses, we generated an Ac23 null Ac M NPV genome by homologous recombination in E. coli. We found that Ac23 was not required for viral replication or pathogenesis in cell culture or infected animals. However, Ac23 accelerated the mortality of infected insect hosts by approximately 28% or 26 h. Thus, Ac23 represents an important viral pathogenicity factor in larvae infected with Ac M NPV.