Characterization of a Primordial Major Capsid-Scaffolding Protein Complex in Icosahedral Virus Shell Assembly

Area of Science:

• Structural biology
• Virology
• Biophysics

Background:

• Complex double-stranded DNA (dsDNA) viruses utilize major capsid proteins (MCPs) that self-assemble into icosahedral shells, a process typically guided by scaffolding proteins.
• The rapid co-polymerization of MCPs and scaffolding proteins hinders the study of early assembly stages, leading to aberrant structures when scaffolds are absent.

Purpose of the Study:

• To investigate the structure and biophysical properties of an assembly-deficient phage lambda major capsid protein (MCP(W308A)).
• To elucidate the role of protein conformation in the initial steps of viral capsid assembly.

Main Methods:

• X-ray crystallography to determine the 2.7 Å structure of MCP(W308A).
• Biophysical assays to assess protein solubility, folding, and binding interactions with scaffolding protein.
• Computational analyses to understand the protein's conformational state and its implications for assembly.

Main Results:

• The MCP(W308A) mutant is folded, soluble, and binds scaffolding protein but fails to assemble beyond an initial complex.
• The crystal structure reveals MCP(W308A) adopts a 'pre-assembly' conformation with folded N-arm and E-loops.
• Analyses suggest MCP(W308A) is thermodynamically locked in this conformation, preventing necessary self-association for shell formation.

Conclusions:

• Dynamic interactions between MCPs are crucial for high-fidelity viral shell assembly.
• The study provides a structural and biophysical understanding of a key intermediate in scaffold-chaperoned MCP polymerization, relevant to large dsDNA viruses.
• Findings offer broad biological significance for understanding fundamental virus assembly mechanisms.