Kai Sheng

Investigating the intricate and rapid folding kinetics of large RNA-protein complexes (RNPs), like the bacterial ribosome, remains a formidable challenge in structural biology. Previous genetic approaches to probe assembly have focused on modulating the expression of either r-proteins or assembly factors. Here, anti-sense oligonucleotides (ASOs) were used to disrupt native RNA/RNA and RNA/protein interactions, in order to generate previously uncharacterized folding intermediates. In an in vitro co-transcriptional ribosome assembly assay, 10 assembly inhibitor ASOs were identified. Using cryo-electron microscopy, 38 intermediate structures were determined resulting from the specific inhibitions generated by 6 inhibitory ASOs. A notable intermediate class provided compelling evidence for independent rRNA domain folding before proper interdomain docking. Three PNAs targeting domain-I of 23S rRNA further subdivide the previously identified assembly core into smaller blocks representing the earliest steps in assembly. The resulting assembly graph reveals template-directed RNA docking of defined regions as foldons, and domain consolidation, which provides a hierarchical view of the RNP assembly process. This approach not only identifies potential targets for antibiotic development but also establishes a platform for probing the structure and dynamics of RNP assemblies.

Investigating the intricate and rapid folding kinetics of large RNA-protein complexes (RNPs), like the bacterial ribosome, remains a formidable challenge in structural biology. Previous genetic approaches to probe assembly have focused on modulating the expression of either r-proteins or assembly factors. Here, anti-sense oligonucleotides (ASOs) were used to disrupt native RNA/RNA and RNA/protein interactions, in order to generate novel folding intermediates. In an in vitro co-transcriptional assembly assay, 8 assembly inhibitor ASOs were identified. Using cryo-electron microscopy, 38 new intermediate structures were determined resulting from the specific inhibitions. In particular a novel intermediate class provided compelling evidence of independent rRNA domain folding before proper interdomain docking. Three PNAs targeting domain-I of 23S- rRNA further subdivided the previously identified assembly core into smaller blocks representing the earliest steps in assembly. The resulting assembly graph reveals template-directed RNA foldon docking and domain consolidation, which provides a hierarchical view of the RNP assembly process.

Co-transcriptional assembly is an integral feature of the formation of RNA-protein complexes that mediate translation. For ribosome synthesis, prior studies have indicated that the strict order of transcription of rRNA domains may not be obligatory during bacterial ribosome biogenesis, since a series of circularly permuted rRNAs are viable. In this work, we report the insights into assembly of the bacterial ribosome large subunit (LSU) based on cryo-EM density maps of intermediates that accumulate during in vitro ribosome synthesis using a set of circularly permuted (CiPer) rRNAs. The observed ensemble of twenty-three resolved ribosome large subunit intermediates reveals conserved assembly routes with an underlying hierarchy among cooperative assembly blocks. There are intricate interdependencies for the formation of key structural rRNA helices revealed from the circular permutation of rRNA. While the order of domain synthesis is not obligatory, the order of domain association does appear to proceed with a particular order, likely due to the strong evolutionary pressure on efficient ribosome synthesis. This work reinforces the robustness of the known assembly hierarchy of the bacterial large ribosomal subunit, and offers a coherent view of how efficient assembly of CiPer rRNAs can be understood in that context.

Mass photometry (MP) was used to investigate the assembly of myristoylated full-length HIV-1 Gag (myr-Gag) and vRNA 5’ UTR fragment in a supported lipid bilayer (SLB) model system. The MP trajectories demonstrated that Gag trimerization on the membrane is a key step of early Gag assembly in the presence of vRNA. Growth of myr-Gag oligomers requires vRNA, occuring by addition of 1 or 2 monomers at a time from solution. These data support a model where formation of the Gag hexamers characteristic of the immature capsid lattice occurs by a gradual edge expansion, following a trimeric nucleation event. These dynamic single molecule data involving protein, RNA, and lipid components together, provide novel and fundamental insights into the initiation of virus capsid assembly.

Understanding the assembly principles of biological macromolecular complexes remains a significant challenge, due to the complexity of the systems and the difficulties in developing experimental approaches. As a ribonucleoprotein complex, the ribosome serves as an ideal model system for the profiling of macromolecular complex assembly. In this work, we report an ensemble of large ribosomal subunit intermediate structures that accumulate during synthesis in a near-physiological and co-transcriptional in vitro reconstitution system. Thirteen pre-50S intermediate maps covering the whole assembly process were resolved using cryo-EM single particle analysis and heterogeneous subclassification. Segmentation of the set of density maps reveals that the 50S ribosome intermediates assemble based on fourteen cooperative assembly blocks, including the smallest assembly core reported up to now, which is composed of a 600-nucleotide-long folded rRNA and three ribosomal proteins. The cooperative blocks assemble onto the assembly core following a defined set of dependencies, revealing the parallel assembly pathways at both early and late assembly stages of the 50S subunit.

Ribosome biogenesis is a complex but efficient process in rapidly growing bacteria, where assemble a functional 70S ribosome takes ~ 2 min, involving participation of 3 rRNAs, 50 r-proteins and dozens of assembly factors. In vitro reconstitution using different subsets of large subunit (50S, LSU) proteins with rRNAs, pioneered by Nierhaus lab, resulted in the Nierhaus assembly map, embodying the cooperativity and dependency for binding of LSU r-proteins to 23S rRNA. Critically absent from the Nierhaus map is the underlying folding of the rRNA that creates the binding sites for the r-proteins. In addition, the relationship of the observed cooperativity in vitro to the co-transcriptional assembly in cells remains to be determined. Pre-50S intermediates accumulate at low temperature in ΔdeaD, a DEAD-box helicase implicated in 50S assembly. We solved 21 pre-50S density maps from intermediate-containing fractions using cryo-EM. In the newly solved maps, we discovered the earliest intermediate ever reported, consisting of domain I at the 5’-end of 23S rRNA. To probe the mechanism behind the maps during assembly, we developed a novel density map segmentation and dependency analysis method. Ten cooperative assembly blocks were identified from segmentation of the maps, and these were organized into a block dependency map. This is the first time the dependencies on folding of rRNA helices and ribosomal protein binding could be integrated into a complete assembly map. In addition, we showed how the exit tunnel is folded on the solvent side, serving as a scaffold for 50S maturation. Using this new segmentation analysis method, we revisited previously reported bL17-depletion and ΔsrmB datasets. Most remarkably, the other two datasets are also consistent with the block dependency, implying a unified early assembly pathway and flexible maturation landscape in early 50S biogenesis.

Previously, we evolved a DNA polymerase, SFM4-3, for the recognition of substrates modified at their 2′ positions with a fluoro, O-methyl, or azido substituent. Here we use SFM4-3 to synthesize 2′-azido-modified DNA; we then use the azido group to attach different, large hydrophobic groups via click chemistry. We show that SFM4-3 recognizes the modified templates under standard conditions, producing natural DNA and thereby allowing amplification. To demonstrate the utility of this remarkable property, we use SFM4-3 to select aptamers with large hydrophobic 2′ substituents that bind human neutrophil elastase or the blood coagulation protein factor IXa. The results indicate that SFM4-3 should facilitate the discovery of aptamers that adopt novel and perhaps more protein-like folds with hydrophobic cores that in turn allow them to access novel activities.