Biology subjects

Carlstrom, A.

Publications and source records attributed to Carlstrom, A..

3 recordsLinked to original sources

bioRxiv2025

A molecular switch at the yeast mitoribosomal tunnel exit controls cytochrome b synthesis

Mitochondrial gene expression needs to be balanced with cytosolic translation to produce oxidative phosphorylation complexes. In yeast, translational feedback loops involving lowly expressed proteins called translational activators help to achieve this balance. Synthesis of cytochrome b (Cytb or COB), a core subunit of complex III in the respiratory chain, is controlled by three translational activators and the assembly factor Cbp3-Cbp6. However, the molecular interface between the COB translational feedback loop and complex III assembly is yet unknown. Here, using protein-proximity mapping combined with selective mitoribosome profiling, we reveal the components and dynamics of the molecular switch controlling COB translation. Specifically, we demonstrate that Mrx4, a previously uncharacterized ligand of the mitoribosomal polypeptide tunnel exit, interacts with either the assembly factor Cbp3-Cbp6 or with the translational activator Cbs2. These reciprocal interactions determine whether the translational activator complex with bound COB mRNA can interact with the mRNA channel exit on the small ribosomal subunit for translation initiation. Organization of the feedback loop at the tunnel exit therefore orchestrates mitochondrial translation with respiratory chain biogenesis.

molecular biology↗

bioRxiv2025

Translational activators align mRNAs at the small mitoribosomal subunit for translation initiation

Mitochondrial gene expression is essential for oxidative phosphorylation. Mitochondrial-encoded mRNAs are translated by dedicated mitochondrial ribosomes (mitoribosomes), whose regulation remains elusive. In the bakers yeast Saccharomyces cerevisiae, nuclear-encoded mitochondrial translational activators (TAs) facilitate transcript-specific translation by a yet unknown mechanism. Here, we investigated the function of TAs containing RNA-binding pentatricopeptide repeats (PPRs) using selective mitoribosome profiling and cryo-EM structural analysis. These analyses revealed that TAs exhibit strong selectivity for mitoribosomes initiating on their target transcripts. Moreover, TA-mitoribosome footprints indicated that TAs recruit mitoribosomes proximal to the start codon. Two cryo-EM structures of mRNA-TA complexes bound to post-initiation/pre-elongation-stalled mitoribosomes revealed the general mechanism of TA action. Specifically, the TAs bind to structural elements in the 5 UTR of the client mRNA as well as to the mRNA channel exit to align the mRNA in the small subunit during initiation. Our findings provide a mechanistic basis for understanding how mitochondria achieve transcript-specific translation initiation without relying on general sequence elements to position mitoribosomes at start codons.

molecular biology↗

bioRxiv2024

Bioengineered yeast tethered respiratory supercomplexes reveal mechanisms governing efficient substrate utilization

The mitochondrial respiratory chain (MRC) enzymatic complexes, essential for aerobic energy transduction in eukaryotic cells, are organized into evolutionarily conserved higher-order structures known as supercomplexes (SCs). The elucidation of the physiological relevance of respiratory SCs is essential for our understanding of mitochondrial function and cellular bioenergetics, yet it has been severely hampered by the limited availability of experimental models isolating SC formation as the sole variable. In the yeast Saccharomyces cerevisiae, where SCs are formed by the association of complexes III and IV into III2IV1 and III2IV2 configurations, compelling evidence suggests that SCs confer a competitive advantage by facilitating cytochrome c diffusion along the SC surface and enhancing respiratory rates. However, the significance of the proposed MRC plasticity and the role of distinct SC conformations in substrate utilization remain unresolved, leaving critical gaps in our understanding of mitochondrial bioenergetics and the adaptive evolution of energy transduction. To address these open questions, we engineered a yeast strain expressing a covalently linked III2IV2 SC, whose high-resolution structure is virtually identical to wild-type. Exclusive expression of this tethered SC supports robust overall respiratory activity but selectively affects mitochondrial respiration of cytosolically-generated NADH. This is attributable to the preferential interaction of distinct SC species with mitochondrial NADH dehydrogenases. We propose that in yeast mitochondria, substrate-driven formation of defined respirasome-like SC organizations contributes to the optimization of electron fluxes across the MRC and support metabolic plasticity.

biochemistry↗