FAM46B is a prokaryotic-like cytoplasmic poly(A) polymerase essential in human embryonic stem cells

Area of Science:

• Biochemistry and Molecular Biology
• Developmental Biology
• Structural Biology

Background:

• Family with sequence similarity (FAM46) proteins are metazoan-specific poly(A) polymerases (PAPs) with largely unknown functions and architectures.
• FAM46 proteins were predicted as Gld-2-like eukaryotic non-canonical PAPs, but their structural and functional details remained elusive.

Purpose of the Study:

• To elucidate the structure and function of FAM46B, a novel eukaryotic non-canonical poly(A) polymerase.
• To investigate the role of FAM46B in human pluripotent stem cells and early embryonic development.

Main Methods:

• X-ray crystallography to determine the 3D structure of FAM46B.
• Biochemical assays to characterize FAM46B's enzymatic activity and substrate preference.
• Expression analysis, subcellular localization studies, and gene manipulation (knock-out/knock-down) in human embryonic stem cells.

Main Results:

• The crystal structure of FAM46B reveals a unique architecture with a large N-terminal catalytic domain and a C-terminal helical domain, resembling prokaryotic PAPs.
• FAM46B functions as an active poly(A) polymerase with a preference for adenosine-rich RNA substrates.
• FAM46B is highly expressed in human pre-implantation embryos and pluripotent stem cells, localizes to the nucleus and cytosol, and is essential for cell viability.
• FAM46B deficiency leads to apoptosis and restricted protein synthesis, highlighting its critical role in maintaining translational efficiency.

Conclusions:

• FAM46B represents a distinct class of eukaryotic PAPs with structural similarities to prokaryotic enzymes.
• FAM46B is a pluripotent stem cell-specific enzyme indispensable for early embryonic development by maintaining translational efficiency.
• The findings provide crucial insights into the function of FAM46 proteins in high eukaryotes.