Compartmentation of the nucleolar processing proteins in the granular component is a CK2-driven process

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The nucleolus is a dynamic nuclear subcompartment essential for ribosome biogenesis.
• Understanding the mechanisms of nucleolar protein targeting and complex formation is crucial for deciphering cellular processes.
• Nucleolar protein compartmentation influences rRNA transcription and processing.

Purpose of the Study:

• To investigate the mechanisms controlling the targeting of nucleolar processing proteins to rRNA transcription sites.
• To analyze the role of protein phosphorylation and CK2 kinase in nucleolar protein complex organization.
• To understand the dynamics of nucleolar reformation and its dependence on cellular conditions.

Main Methods:

• Digitonin-permeabilized cell assays were used to study nucleolar reorganization in vitro.
• Assays were performed under conditions modulating ATP/GTP levels, temperature, and kinase activity.
• Site-directed mutagenesis was employed to investigate the role of specific CK2 phosphorylation sites on protein B23.

Main Results:

• Nucleolar reformation dynamics were found to be ATP/GTP-dependent, temperature-sensitive, and driven by CK2 kinase.
• The protein B23, a key rRNA-processing factor, was identified as a substrate for CK2.
• Mutation of the major CK2 phosphorylation site on B23 disrupted nucleolar component organization, leading to separation of transcripts and processing proteins.

Conclusions:

• Phosphorylation, particularly by CK2, plays a critical role in controlling the compartmentation of rRNA-processing proteins within the nucleolus.
• CK2 kinase activity is essential for maintaining the functional organization of nucleolar protein complexes at rRNA transcription sites.
• These findings provide insights into the regulation of ribosome biogenesis and nucleolar structure.