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Morphology and morphogenesis of Sindbis virus as seen with freeze-etching techniques
Abstract:
Freeze-etch electron microscope studies of the morphogenesis and morphology of Sindbis virus confirmed results obtained by other workers employing thin-sectioning techniques. The 68-nm virion was found to have a nucleocapsid 36 nm in diameter surrounded by a double-layered, unit membrane. The membranous envelope is acquired as the capsid buds through the plasma membrane of the infected cell. The freeze-etch technique also provided the following new information. (i) At any one time, budding occurs in patches rather than evenly over the cell surface. (ii) The nucleocapsid is composed of capsomers 7 nm in diameter. (iii) The capsid interacts strongly with the membrane, both prior to budding and after maturation. (iv) The 7- to 10-nm particles characteristic of the internal faces of plasma membranes, which presumably represent host membrane proteins, are present in early stages of budding but disappear as morphogenesis progresses. (v) Fusion of the cell membrane at the base of the budding virion is a two-step process; the inner leaflet fuses into a sphere before the outer one. (vi) The outer surface of the viral envelope is covered with 4-nm subunits with a center-to-center spacing of 6 nm.
Insights
Freeze-etch electron microscopy revealed Sindbis virus (SV) morphology and budding. New insights detail viral envelope acquisition, capsomer structure, and membrane interactions during SV morphogenesis.
Area of Science:
- Virology
- Cell Biology
- Microscopy
Background:
- Sindbis virus (SV) is an enveloped RNA virus.
- Previous studies used thin-sectioning electron microscopy to examine SV morphology.
- Understanding viral morphogenesis is crucial for antiviral development.
Purpose of the Study:
- To investigate Sindbis virus morphogenesis and morphology using freeze-etch electron microscopy.
- To confirm previous findings and provide novel insights into the viral budding process.
Main Methods:
- Freeze-etch electron microscopy was employed to study Sindbis virus.
- Comparative analysis with thin-sectioning techniques was performed.
Main Results:
- Confirmed the 68-nm virion size, 36-nm nucleocapsid, and double-layered membrane envelope.
- Identified budding occurring in patches, not uniformly over the cell surface.
- Revealed the nucleocapsid is composed of 7-nm capsomers and interacts strongly with the host membrane.
- Described a two-step fusion process of the cell membrane during budding.
- Characterized the 4-nm subunits on the viral envelope surface.
Conclusions:
- Freeze-etch electron microscopy offers detailed insights into Sindbis virus structure and replication.
- The study elucidated specific stages of viral envelope acquisition and membrane fusion.
- Findings contribute to a deeper understanding of enveloped virus morphogenesis.