Capsid-E2 Interactions Rescue Core Assembly in Viruses That Cannot Form Cytoplasmic Nucleocapsid Cores

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Alphavirus capsid proteins (CPs) possess two domains: the N-terminal domain (NTD) for RNA interaction and the C-terminal domain (CTD) for CP-CP and E2 spike protein interactions.
• Understanding the interplay between these domains is crucial for deciphering viral assembly mechanisms.

Purpose of the Study:

• To investigate how alterations in the CP NTD influence the interactions of the CP CTD with the E2 spike protein's cytoplasmic domain (cdE2).
• To elucidate the roles of NTD length and charge in alphavirus core formation and virion assembly.

Main Methods:

• Mutagenesis of the Ross River virus CP NTD to alter its length and charge.
• Analysis of CP behavior during cytoplasmic core formation and virion assembly.
• Assessment of core assembly competence after disassembly and reassembly.

Main Results:

• Changing the charge of the NTD significantly impacts core and virion assembly more than altering its length.
• NTD insertion mutants fail to form cytoplasmic cores but can assemble cores or core-like structures within virions.
• E2-CP interactions appear to rescue core formation, even in the absence of cytoplasmic cores.
• Some mutant virions yield disassembly-competent cores, indicating distinct roles for CP domains.

Conclusions:

• The NTD and CTD of alphavirus CPs have distinct yet coordinated roles in viral assembly.
• E2-CP interactions play a critical role in rescuing and maturing core formation during virion assembly.
• Alphaviruses exhibit multiple strategies to ensure genome encapsulation within a capsid core for successful replication.