Membrane proteins organize a symmetrical virus.

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Alphaviruses are enveloped viruses with icosahedral symmetry, maturing via budding at the plasma membrane.
• A prevailing model suggests viral maturation is driven by interactions between membrane spike proteins and a preformed nucleocapsid (NC).
• This model posits that the nucleocapsid dictates the T=4 geometry through specific interactions with capsid proteins (CPs).

Purpose of the Study:

• To challenge the prevailing model of alphavirus maturation.
• To investigate the role of capsid proteins (CPs) and membrane proteins in determining viral geometry.
• To elucidate the mechanisms underlying the organization of the icosahedral conformation in alphaviruses.

Main Methods:

• Utilized a Semliki Forest virus mutant with defective capsid protein (CP) assembly.
• Analyzed viral particle formation in the absence of preformed nucleocapsids.
• Examined the contribution of spike-spike and spike-CP interactions to viral structure.

Main Results:

• The capsid gene mutant, unable to form nucleocapsids, still produced particles with correct T=4 geometry and protein stoichiometry.
• This demonstrates that intact nucleocapsids are not essential for achieving the characteristic icosahedral conformation.
• Viral membrane proteins, through spike-spike and spike-CP interactions, were shown to directly organize the viral structure.

Conclusions:

• The findings challenge the model where nucleocapsids solely dictate alphavirus T=4 geometry.
• Membrane proteins play a direct and crucial role in organizing the icosahedral conformation of alphaviruses.
• This highlights a novel mechanism in viral assembly and maturation involving direct protein-protein interactions at the membrane interface.