Envelope-Fusion-Syncytium Formation in Microplitis bicoloratus bracovirus Maturation

Viruses. 2022 Oct 2;14(10):2183. doi: 10.3390/v14102183.

Abstract

The viral envelope is essential for virus maturation. Virus-mediated syncytium formations are induced by viral envelope proteins that cause membrane fusion of the infected cells. Polydnaviridae (Polydnavirus) are enveloped viruses with multiple nucleocapsids, and virions mature in symbiotic parasitoid wasp ovaries. However, the mechanism governing the envelope packaging of multiple nucleocapsids remains unclear. In this study, we used transmission electron microscopy to examine the process whereby multiple nucleocapsids of Microplitis bicoloratus bracovirus are packaged into an envelope and observed envelope-fusion-syncytium formation in symbiotic wasp calyx cells during virus maturation. The virus maturation process in calyx cells comprised four stages: pre-virogenic stroma, virogenic stroma, assembly, and fusion. Each virus contained a single envelope with one nucleocapsid in the assembly stage; multiple envelopes then fused to form a viral envelope with multiple nucleocapsids (i.e., the envelope-fusion-syncytium) around the envelope fusion core in the fusion stage. The envelope-fusion-syncytium then stabilized the virions that were released into the lumen of the ovary across the calyx epithelial layer. The phagocytic calyx epithelial cells on the border of the calyx and ovary lumen cleared the majority of non-enveloped nucleocapsids. In contrast, non-phagocytic calyx epithelial cells with microvilli and a cuticular line between the ovary wall and the lumen remained intact in the ovary lumen. These results indicate that envelope-fusion-syncytium formation is important for packaging multiple nucleocapsids in bracovirus maturation.

Keywords: Microplitis bicoloratus bracovirus; electron microscopy; fused envelope; phagocytic calyx epithelial cells; syncytium formation; viral maturation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Female
  • Giant Cells
  • Nucleocapsid
  • Polydnaviridae* / genetics
  • Viral Envelope Proteins
  • Wasps*

Substances

  • Viral Envelope Proteins

Grants and funding

This work was funded by the Science and Technology Planning Project in Key Areas of Yunnan Province [grant number 202001BB050002], National Natural Science Foundation of China [grant numbers 32160662, 31772225, 31471823, 31260448, and 31060251], NSFC-NRF [grant number 31411140238 to K.L.], and Yunnan Department of Science and Technology [grant number 2013FA003 to K.L.]. K.L. was supported by the Donglu Scholar Program of Yunnan University; M.D. was supported by National College student innovation and entrepreneur practice Training Program (2021), and the College student innovation and entrepreneur practice Training Program of YNU (2020).