Hierarchical multi-step organization during viral capsid assembly.

Area of Science:

• Virology
• Biophysics
• Structural Biology

Background:

• The formation of the human immunodeficiency virus type 1 (HIV-1) core is essential for viral replication.
• The precise mechanism of capsid protein self-assembly remains incompletely understood.
• Insights into early assembly stages could reveal novel therapeutic targets.

Purpose of the Study:

• To elucidate the kinetics and structural mechanisms of HIV-1 capsid protein assembly.
• To investigate the autoaccelerating and cooperative nature of capsid assembly.
• To characterize the structural evolution during different assembly phases.

Main Methods:

• Spectroscopic analysis (opacity) was employed to monitor the kinetics of capsid protein assembly.
• Cryogenic transmission electron microscopy (cryo-TEM) was utilized to visualize the structural intermediates.
• Kinetic experiments involved varying the concentration of pre-incubated capsid proteins.

Main Results:

• Capsid assembly proceeds through three distinct kinetic phases: lag, linear increase, and plateau.
• Pre-incubation of capsid proteins shortened the lag phase and accelerated the linear assembly rate, indicating autoacceleration.
• Cryo-TEM revealed short and long tubular assemblies during the lag phase, transitioning to bundled arrays at later stages.

Conclusions:

• HIV-1 capsid assembly is a multi-step process initiated by rapid nucleation and elongation.
• Subsequent packing of tubular structures into bundles characterizes the later stages of assembly.
• This detailed understanding of viral core formation provides a foundation for developing new antiviral strategies.