SRPK2 Mediates HBV Core Protein Phosphorylation and Capsid Assembly via Docking Interaction

Area of Science:

• Virology
• Molecular Biology
• Structural Biology

Background:

• Serine-arginine protein kinase (SRPK) family members, SRPK1 and SRPK2, are known to phosphorylate hepatitis B core protein (Cp).
• Cp phosphorylation is essential for pregenomic RNA encapsidation during hepatitis B virus (HBV) nucleocapsid assembly.
• SRPK2 demonstrates higher kinase activity towards Cp compared to SRPK1.

Purpose of the Study:

• To identify specific sites on Cp phosphorylated by SRPK2.
• To elucidate the mechanism by which SRPK2 interacts with and regulates Cp phosphorylation.
• To investigate the role of SRPK2-Cp interaction in controlling HBV capsid assembly and packaging.

Main Methods:

• Identification of Cp phosphorylation sites using biochemical assays.
• Determination of the interaction mechanism between SRPK2 and Cp via structural studies (cryo-electron microscopy).
• In vitro assembly assays to assess the effect of SRPK2-Cp interaction and phosphorylation on capsid formation.
• SRPK2-knockout experiments in HepG2 cells to evaluate its role in the HBV life cycle.

Main Results:

• Specific Cp phosphorylation sites targeted by SRPK2 were identified.
• SRPK2 utilizes a specific docking groove to interact with Cp's C-terminal arginine-rich domain.
• SRPK2 binding to unphosphorylated Cp inhibited premature capsid assembly in vitro; phosphorylation reactivated assembly.
• Cryo-electron microscopy revealed SRPK2 decorating the HBV capsid surface via C-terminal Cp interaction.
• SRPK2 knockout in HepG2 cells reduced Cp phosphorylation, confirming SRPK2's importance in the HBV life cycle.

Conclusions:

• SRPK2 plays a critical role in regulating HBV capsid assembly through phosphorylation of Cp.
• The interaction between SRPK2's docking groove and Cp is crucial for controlling the timing of capsid assembly and viral packaging.
• SRPK2 is an essential cellular kinase for the HBV life cycle.