Kinetics and Mechanism of Mammalian Mitochondrial Ribosome Assembly

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Mammalian mitochondria contain a unique genetic system encoded by mitochondrial DNA (mtDNA).
• Mitochondrial ribosomes (mitoribosomes) synthesize all 13 essential proteins encoded by mtDNA, crucial for cellular respiration.
• While mitoribosome structure is known, its assembly pathway remains elusive.

Purpose of the Study:

• To elucidate the mechanism and kinetics of mammalian mitoribosome assembly.
• To identify distinct stages and protein binding dynamics during mitoribosome biogenesis.
• To correlate assembly steps with clinically relevant mutations in mitoribosomal proteins (MRPs).

Main Methods:

• Stable Isotope Labeling by Amino acids in Cell culture (SILAC) pulse-labeling experiments were employed.
• Quantitative analysis of mitochondrial protein import and assembly rates was performed.
• Cryoelectron microscopy (cryo-EM) structural data provided context for assembly stages.

Main Results:

• Mitoribosome assembly is a slow process, initiated at the mtDNA nucleoid.
• Individual MRPs are synthesized in excess, driving the initial assembly steps.
• MRPs associated tightly in the final structure often assemble coordinately.
• Clinically significant mutations in MRPs affect proteins involved in early assembly stages.

Conclusions:

• A working model for mitoribosome assembly has been generated, distinguishing early and late-binding MRPs.
• The study provides insights into the temporal regulation of mitoribosome biogenesis.
• Early-acting MRPs are critical, as evidenced by their association with disease phenotypes.