A post-assembly conformational change makes the SARS-CoV-2 polymerase elongation-competent

Area of Science:

• Virology
• Molecular Biology
• Structural Biology

Background:

• Coronaviruses (CoV) utilize sixteen non-structural proteins (nsps) to form the replication-transcription complex (RTC).
• The RTC core, comprising nsp12 RNA-dependent RNA polymerase (RdRp), nsp7, and nsp8, is essential for viral RNA synthesis.
• The precise assembly mechanism and activation of the core RTC into a processive polymerase remain incompletely understood.

Purpose of the Study:

• To elucidate the assembly mechanism of the core RTC.
• To investigate the role of RNA in RTC assembly and activation.
• To identify potential therapeutic targets within the RTC activation pathway.

Main Methods:

• Investigated the assembly kinetics of the core RTC in the presence and absence of RNA templates.
• Utilized biochemical assays to monitor protein-ligand interactions and conformational changes.

Main Results:

• The core RTC preferentially assembles with RNA, with nsp12 binding first, followed by nsp7 and nsp8.
• RNA-dependent assembly and subsequent conformational changes are required for processive elongation.
• The apo-RTC (without RNA) requires significantly longer to achieve an elongation-competent state.

Conclusions:

• RNA binding is a critical initial step in core RTC assembly and activation.
• A time-dependent conformational change, facilitated by RNA, is necessary for processive viral RNA synthesis.
• The obligatory activation step presents a potential target for antiviral therapeutic strategies.