Visualizing influenza A virus assembly by in situ cryo-electron tomography

Area of Science:

• Virology
• Cell Biology
• Structural Biology

Background:

• Influenza A virus (IAV) assembly involves packaging eight ribonucleoprotein complexes (vRNPs) into pleomorphic particles.
• vRNPs are sorted via Rab11a-mediated trafficking to the plasma membrane for assembly.
• Matrix protein 1 (M1) orchestrates virion assembly beneath the viral envelope.

Purpose of the Study:

• To elucidate the molecular mechanisms of vRNP distribution and cytosolic trafficking during IAV assembly.
• To understand the coordination of IAV assembly by M1 protein.
• To reveal the structural organization of M1 protein during intracellular assembly stages.

Main Methods:

• In situ cryo-electron tomography (cryo-ET) was employed to visualize IAV assembly intermediates.
• Analysis focused on vRNP clustering, membrane association, and M1 protein structures.
• Rab11a-dependent and HA-independent vRNP trafficking mechanisms were investigated.

Main Results:

• HA-containing membranes act as Rab11a-dependent platforms for vRNP clustering, reducing inter-vRNP distances.
• vRNPs can cluster on NA-containing membranes, demonstrating HA-independent assembly.
• The characteristic 7+1 vRNP bundle forms during budding, coordinated by M1 layer assembly.
• Intracellular M1 forms unique multilayered helical assemblies, distinct from the M1 layer in mature virions.
• These M1 assemblies are compact in the nucleus and partially dissociate in the cytoplasm.

Conclusions:

• vRNP clustering and trafficking during IAV assembly are membrane-assisted but HA-independent.
• M1 protein plays a crucial role in coordinating IAV assembly through distinct intracellular structures.
• The findings provide novel molecular insights into the intricate process of influenza virus assembly.