Structures of the human mitochondrial ribosome in native states of assembly

Area of Science:

• Cell Biology
• Molecular Biology
• Structural Biology

Background:

• Mammalian mitochondrial ribosomes (mitoribosomes) exhibit significant structural divergence from bacterial ribosomes, featuring reduced rRNA content and additional proteins.
• The intricate process of mitoribosomal biogenesis is not fully understood at the molecular level, posing challenges for studying its unique features.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying the final stages of human mitoribosomal large subunit (mt-LSU) assembly.
• To identify novel protein factors involved in mitoribosomal biogenesis and understand their roles.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine the structures of late-stage mt-LSU assembly intermediates.
• Native pools of intermediates were isolated from a human cell line for structural analysis.

Main Results:

• Two distinct late-stage mt-LSU assembly intermediates were resolved to approximately 3-Å resolution.
• Insights into the temporal order of rRNA folding and protein integration during mitoribosome maturation were gained.
• The ribosome silencing factor (RsfS) family was redefined as multifunctional biogenesis factors.
• Two previously uncharacterized assembly factors, L0R8F8 and mt-ACP, were identified.

Conclusions:

• The study provides crucial structural snapshots of mitoribosomal biogenesis, highlighting evolutionary adaptations in this complex process.
• The identification of new assembly factors and the functional redefinition of RsfS expand the known molecular machinery of mitoribosome production.