HDAC5-encoded Microprotein NISM Mediates Nucleolar Formation and Ribosomal RNA Synthesis

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Ribosome biogenesis is crucial for cell function, occurring within the nucleolus, a liquid condensate.
• Disordered proteins play roles in organizing cellular compartments like the nucleolus.

Purpose of the Study:

• To identify and characterize novel regulators of nucleolar integrity and ribosome biogenesis.
• To elucidate the mechanism by which a newly discovered microprotein influences nucleolar function and cellular stress responses.

Main Methods:

• Identification of a microprotein (NISM) encoded within the HDAC5 5'-UTR.
• Analysis of NISM's effects on rDNA transcription, nucleolar structure, and cell proliferation via overexpression and knockout.
• Biochemical assays and computational modeling to investigate NISM's interaction with DHX9 and its role in liquid-liquid phase separation.

Main Results:

• NISM overexpression impairs rDNA transcription, induces nucleolar stress, activates p53, and suppresses proliferation.
• NISM knockout disrupts nucleolar structure and activates p53.
• NISM interacts with DHX9, regulating pre-ribosomal RNA synthesis by enhancing DHX9's liquid-liquid phase separation, thereby coordinating nucleolar formation.

Conclusions:

• NISM is a novel regulator of nucleolar integrity and ribosome biogenesis.
• Disordered microproteins can drive the formation of membraneless organelles like the nucleolus.
• NISM's function highlights a new mechanism linking nucleolar stress, protein interactions, and cellular fate.