Project description:The yeast Hsp70 chaperone Ssb interacts with ribosomes and nascent chains to co-translationally assist protein folding. Here, we present a proteome-wide analysis of Hsp70 function during translation, based on in vivo selective ribosome profiling, that reveals mechanistic principles coordinating translation with chaperone-assisted protein folding. Ssb binds most cytosolic, nuclear, and mitochondrial proteins and a subset of ER proteins, supporting its general chaperone function. Position-resolved analysis of Ssb engagement reveals compartment- and protein-specific nascent chain binding profiles that are coordinated by emergence of positively charged peptide stretches enriched in aromatic amino acids. Ssbs’ function is temporally coordinated by RAC but independent from NAC. Analysis of ribosome footprint densities along orfs reveals that ribosomes translate faster at times of Ssb binding. This is coordinated by biases in mRNA secondary structure, and codon usage as well as the action of Ssb, suggesting chaperones may allow higher protein synthesis rates by actively coordinating protein synthesis with co-translational folding.
Project description:The rates at which domains fold and codons are translated are important factors in determining whether a nascent protein will co-translationally fold and function or misfold and malfunction. In this study, we develop a chemical kinetic model that calculates a protein domain’s co-translational folding curve using only the domain’s bulk folding and unfolding rates and codon translation rates. We show that this model accurately predicts the course of co-translational folding measured in vivo for four different protein molecules. As part of our chemical kinetic model, we assume that there is steady-state translation kinetics through the time-course of the experiment. To prove that this assumption is valid, we performed Ribo-Seq experiments on two biological replicates of yeast cells to compare their ribosome profiles. For genes with sufficient coverage across the open reading frames in both the replicates, we compare the ribosome profiles from both replicates and see a strong correlation. This implies there is a steady-state and our assumption is valid.
Project description:Codon usage bias is a universal feature of eukaryotic and prokaryotic genomes and has been proposed to regulate translation efficiency, accuracy and protein folding based on the assumption that codon usage affects translation dynamics. The role of codon usage in regulating translation, however, is not clear and has been challenged by recent ribosome profiling studies. Here we used a Neurospora cell-free translation system to directly monitor the velocity of mRNA translation. We demonstrated that the use of preferred codons enhances the rate of translation elongation, whereas non-optimal codons slow translation. In addition, codon usage regulates ribosome traffic on the mRNA. These conclusions were supported by ribosome profiling results in vitro and in vivo with substrate mRNAs manipulated to increase signal over background noise. We further show that codon usage plays an important role in regulating protein function by affecting co-translational protein folding. Together, these results resolve a long-standing fundamental question and demonstrate the importance of codon usage on protein folding.
Project description:Protein synthesis by ribosomes takes place on a linear substrate but at variable speeds. Transient pausing of ribosomes can impact a variety of co-translational processes, including protein targeting and folding. These pauses are influenced by the sequence of the mRNA. Thus redundancy in the genetic code allows the same protein to be translated at different rates. However, our knowledge of both the position and the mechanism of translational pausing in vivo is highly limited. Here we present a genome-wide analysis of translational pausing in bacteria using ribosome profiling-deep sequencing of ribosome-protected mRNA fragments. This approach enables high-resolution measurement of ribosome density profiles along most transcripts at unperturbed, endogenous expression levels. Unexpectedly, we found that codons decoded by rare tRNAs do not lead to slow translation under nutrient-rich conditions. Instead, Shine-Dalgarno-(SD) like features within coding sequences cause pervasive translational pausing. Using an orthogonal ribosome possessing an altered anti-SD sequence, we demonstrated that pausing is due to hybridization between mRNA and the 16S rRNA of the translating ribosome. In protein coding sequences, internal SD sequences are disfavoured, which leads to biased usage, avoiding codons and codon pairs that resemble canonical SD sites. Our results indicate that internal SD-like sequences are a major determinant of translation rates and a global driving force for the coding of bacterial genomes. Identification of translation pause sites in vivo using ribosome profiling
Project description:A comprehensive analysis of Sox9 binding profiles in developing chondrocytes identified marked enrichment of an AP-1-like motif (Ohba et al. 2015). Here, we have explored the functional interplay between Sox9 and AP-1 in mammalian chondrocyte development. Among AP-1 family members, Jun and Fosl2 were highly expressed within prehypertrophic and early hypertrophic chondrocytes. Chromatin immunoprecipitation followed by DNA sequencing (ChIP-seq) showed a striking overlap in Jun- and Sox9-bound regions throughout the chondrocyte genome, a reflection of direct binding of each factor to target motifs in shared enhancers, and physical interactions of AP-1 with Sox9. In vitro expression analysis indicates that direct co-binding of Sox9 and AP-1 at target motifs enhanced target gene expression, while protein-protein interactions suppressed AP-1- and Sox9-driven transcription. Analysis of prehypertrophic chondrocyte removal of Sox9 demonstrated Sox9 was essential for hypertrophic chondrocyte development, while in vitro and ex vivo analyses showed AP-1 promotes chondrocyte hypertrophy. Sox9 and Jun co-bound and co-activated a Col10a1 enhancer in Sox9 and AP-1 motif-dependent manners consistent with their combined action promoting hypertrophic gene expression. Together, the data support a model where AP-1-family members promote Sox9-action in the transition of chondrocytes to a terminal hypertrophic program. Intersection of ChIP-seq data from Sox9 and AP-1 factor Jun, RNA-seq data from developing rib chondrocytes and Col10a1mCherry positive hypertrophic chondrocytes in neonatal mice to uncover regulation of Sox9 by AP-1 factors during chondrocyte hypertrophy.
Project description:A comprehensive analysis of Sox9 binding profiles in developing chondrocytes identified marked enrichment of an AP-1-like motif (Ohba et al. 2015). Here, we have explored the functional interplay between Sox9 and AP-1 in mammalian chondrocyte development. Among AP-1 family members, Jun and Fosl2 were highly expressed within prehypertrophic and early hypertrophic chondrocytes. Chromatin immunoprecipitation followed by DNA sequencing (ChIP-seq) showed a striking overlap in Jun- and Sox9-bound regions throughout the chondrocyte genome, a reflection of direct binding of each factor to target motifs in shared enhancers, and physical interactions of AP-1 with Sox9. In vitro expression analysis indicates that direct co-binding of Sox9 and AP-1 at target motifs enhanced target gene expression, while protein-protein interactions suppressed AP-1- and Sox9-driven transcription. Analysis of prehypertrophic chondrocyte removal of Sox9 demonstrated Sox9 was essential for hypertrophic chondrocyte development, while in vitro and ex vivo analyses showed AP-1 promotes chondrocyte hypertrophy. Sox9 and Jun co-bound and co-activated a Col10a1 enhancer in Sox9 and AP-1 motif-dependent manners consistent with their combined action promoting hypertrophic gene expression. Together, the data support a model where AP-1-family members promote Sox9-action in the transition of chondrocytes to a terminal hypertrophic program.