Applying 3D correlative structured illumination microscopy and X-ray tomography to characterise herpes simplex virus-1 morphogenesis

Numerous viral genes are involved in assembly of herpes simplex virus-1 (HSV-1), but their relative importance and function remain poorly characterised. Transmission electron microscopy has been used to study viral protein function in cells infected with HSV-1 mutants; however, these studies were usually conducted without correlative light microscopy to identify specific viral components. In this study, fluorescent capsid (eYFP-VP26) and envelope (gM-mCherry) proteins were imaged by structured illumination microscopy under cryogenic conditions (cryoSIM) and cellular ultrastructure was captured from the same infected cells using cryo-soft-X-ray tomography (cryoSXT). Nine fluorescent HSV-1 mutants, each lacking a different viral protein, were compared to assess the importance of viral proteins in different stages of HSV-1 morphogenesis. The relative importance of five viral proteins to nuclear egress were ranked (pUL34 > pUL21 > VP16 > pUL16 > pUS3) according to the levels of attenuation observed for each virus. Correlative imaging also revealed the roles of five viral proteins in cytoplasmic envelopment. VP16 was found to be important in capsid delivery to envelopment compartments, while cytoplasmic clusters of virus particles plus features of stalled envelopment not previously described were observed in the absence of pUL11, pUL51, gK, and gE. Finally, this 3D imaging approach was used to capture different assembly stages during cytoplasmic envelopment and to determine that envelopment occurs by particle budding rather than wrapping. The findings demonstrate that tomographic 3D correlative imaging is an emerging technology that sheds new light on viral protein functions and virion morphogenesis. ImportanceTo date, the characterisation of HSV-1 mutants in the study of virus assembly has been limited to transmission electron microscopy (TEM) without the addition of correlative light microscopy to identify fluorescently labelled viral proteins. In addition, only a small number of mutants are typically used in each study. Herein, a comparative analysis of nine HSV-1 mutants lacking specific structural proteins was performed using correlative fluorescence microscopy and X-ray tomography for the first time, revealing the relative roles of each viral protein in virus assembly. pUL11, pUL51, gK, and gE were shown to have important roles in cytoplasmic envelopment, with the loss of their functions leading to various stalled cytoplasmic envelopment phenotypes involving polarised arrays of capsids at one side of cytoplasmic vesicles that to our knowledge have never been seen with TEM. This correlative imaging approach enabled the study of cytoplasmic envelopment in 3D, revealing an envelopment mechanism driven by capsid budding rather than membrane wrapping. Further cryoSIM experiments revealed that pUL16 and pUL21 are important in nuclear egress of HSV-1 and that VP16 promotes nuclear egress and delivery of capsids to cytoplasmic envelopment compartments. By providing novel and comparative insights into the roles of different viral proteins in various stages of HSV-1 assembly, these findings highlight the utility of correlative cryo-fluorescence plus cryo-soft-X-ray tomography for probing trajectories of intracellular pathogen assembly.