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Published on: January 9, 2019
Evolutionarily related small viral fusogens hijack distinct but modular actin nucleation pathways to drive cell-cell
Ka Man Carmen Chan1,2,3, Ashley L Arthur3,4, Johannes Morstein3,5
1University of California Berkeley/University of California San Francisco Graduate Group in Bioengineering, Berkeley, CA 94720.
Abstract:
Fusion-associated small transmembrane (FAST) proteins are a diverse family of nonstructural viral proteins. Once expressed on the plasma membrane of infected cells, they drive fusion with neighboring cells, increasing viral spread and pathogenicity. Unlike viral fusogens with tall ectodomains that pull two membranes together through conformational changes, FAST proteins have short fusogenic ectodomains that cannot bridge the intermembrane gap between neighboring cells. One orthoreovirus FAST protein, p14, has been shown to hijack the actin cytoskeleton to drive cell-cell fusion, but the actin adaptor-binding motif identified in p14 is not found in any other FAST protein. Here, we report that an evolutionarily divergent FAST protein, p22 from aquareovirus, also hijacks the actin cytoskeleton but does so through different adaptor proteins, Intersectin-1 and Cdc42, that trigger N-WASP-mediated branched actin assembly. We show that despite using different pathways, the cytoplasmic tail of p22 can replace that of p14 to create a potent chimeric fusogen, suggesting they are modular and play similar functional roles. When we directly couple p22 with the parallel filament nucleator formin instead of the branched actin nucleation promoting factor N-WASP, its ability to drive fusion is maintained, suggesting that localized mechanical pressure on the plasma membrane coupled to a membrane-disruptive ectodomain is sufficient to drive cell-cell fusion. This work points to a common biophysical strategy used by FAST proteins to push rather than pull membranes together to drive fusion, one that may be harnessed by other short fusogens responsible for physiological cell-cell fusion.
Insights
Fusion-associated small transmembrane (FAST) proteins use a "pushing" mechanism to fuse cells. This study reveals how different FAST proteins hijack the actin cytoskeleton via distinct pathways to achieve cell-cell fusion.
Area of Science:
- Virology
- Cell Biology
- Biophysics
Background:
- Fusion-associated small transmembrane (FAST) proteins are viral proteins that mediate cell-cell fusion to spread infection.
- Unlike typical viral fusogens, FAST proteins possess short ectodomains, necessitating alternative mechanisms for membrane fusion.
- Previous studies showed orthoreovirus p14 FAST protein utilizes the actin cytoskeleton via a specific motif.
Purpose of the Study:
- To investigate the mechanism of cell-cell fusion mediated by the aquareovirus FAST protein, p22.
- To determine if p22 utilizes a similar actin-hijacking strategy as p14, despite lacking the identified motif.
- To explore the modularity of FAST proteins and the biophysical principles underlying their fusion activity.
Main Methods:
- Comparative analysis of FAST protein p22 and p14.
- Genetic manipulation to create chimeric FAST proteins.
- Investigating interactions with actin adaptors (Intersectin-1, Cdc42, N-WASP) and formin.
- Assessing cell-cell fusion efficiency.
Main Results:
- Aquareovirus p22 FAST protein hijacks the actin cytoskeleton using Intersectin-1 and Cdc42 to activate N-WASP and branched actin assembly.
- The cytoplasmic tail of p22 can functionally replace that of p14, indicating modularity and conserved roles.
- p22-mediated fusion is maintained when coupled with formin (parallel filament nucleator), suggesting mechanical pressure is key.
Conclusions:
- FAST proteins employ a common biophysical strategy of 'pushing' membranes together via localized mechanical force, rather than 'pulling'.
- This mechanism, involving actin cytoskeleton manipulation, is conserved across divergent FAST proteins despite different adaptor usage.
- The findings may extend to other short fusogens involved in physiological processes.
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