Elad Elkayam

Structure of the active form of human origin recognition complex and its ATPase motor module

Binding of the Origin Recognition Complex (ORC) to origins of replication marks the first step in the initiation of replication of the genome in all eukaryotic cells. Here, we report the structure of the active form of human ORC determined by X-ray crystallography and cryo-electron microscopy. The complex is composed of an ORC1/4/5 motor module lobe in an organization reminiscent of the DNA polymerase clamp loader complexes. A second lobe contains the ORC2/3 subunits. The complex is organized as a double-layered shallow corkscrew, with the AAA+ and AAA+-like domains forming one layer, and the winged-helix domains (WHDs) forming a top layer. CDC6 fits easily between ORC1 and ORC2, completing the ring and the DNA-binding channel, forming an additional ATP hydrolysis site. Analysis of the ATPase activity of the complex provides a basis for understanding ORC activity as well as molecular defects observed in Meier-Gorlin Syndrome mutations. DOI: http://dx.doi.org/10.7554/eLife.20818.001

siRNA carrying an (E)-vinylphosphonate moiety at the 5΄ end of the guide strand augments gene silencing by enhanced binding to human Argonaute-2

Abstract Efficient gene silencing by RNA interference (RNAi) in vivo requires the recognition and binding of the 5΄- phosphate of the guide strand of an siRNA by the Argonaute protein. However, for exogenous siRNAs it is limited by the rapid removal of the 5΄- phosphate of the guide strand by metabolic enzymes. Here, we have determined the crystal structure of human Argonaute-2 in complex with the metabolically stable 5΄-(E)-vinylphosphonate (5΄-E-VP) guide RNA at 2.5-Å resolution. The structure demonstrates how the 5΄ binding site in the Mid domain of human Argonaute-2 is able to adjust the key residues in the 5΄-nucleotide binding pocket to compensate for the change introduced by the modified nucleotide. This observation also explains improved binding affinity of the 5΄-E-VP -modified siRNA to human Argonaute-2 in-vitro, as well as the enhanced silencing in the context of the trivalent N-acetylgalactosamine (GalNAc)-conjugated siRNA in mice relative to the un-modified siRNA.

Structurally modulated codelivery of siRNA and Argonaute 2 for enhanced RNA interference

Significance Small interfering RNA (siRNA) has great potential to specifically target undruggable genes in many diseases. Despite its great promise, few siRNAs have been clinically approved due to limited therapeutic efficacy. We propose that one major barrier to RNA interference (RNAi)-based therapy is that the siRNA must form a bound complex with a “scissor” protein, Argonaute 2 (Ago2), after it is released in the cytoplasm. We take the approach of preassembling the duplex siRNA with Ago2 protein, and then packaging the complex with commercial transfection reagents or structurally defined polycations that specifically enhance the clustering of siRNA with Ago2. This unique approach augments gene silencing in cell lines and a melanoma mouse model, providing a potential translatable platform for RNAi therapies. Small interfering RNA (siRNA) represents a promising class of inhibitors in both fundamental research and the clinic. Numerous delivery vehicles have been developed to facilitate siRNA delivery. Nevertheless, achieving highly potent RNA interference (RNAi) toward clinical translation requires efficient formation of RNA-induced gene-silencing complex (RISC) in the cytoplasm. Here we coencapsulate siRNA and the central RNAi effector protein Argonaute 2 (Ago2) via different delivery carriers as a platform to augment RNAi. The physical clustering between siRNA and Ago2 is found to be indispensable for enhanced RNAi. Moreover, by utilizing polyamines bearing the same backbone but distinct cationic side-group arrangements of ethylene diamine repeats as the delivery vehicles, we find that the molecular structure of these polyamines modulates the degree of siRNA/Ago2-mediated improvement of RNAi. We apply this strategy to silence the oncogene STAT3 and significantly prolong survival in mice challenged with melanoma. Our findings suggest a paradigm for RNAi via the synergistic coassembly of RNA with helper proteins.

Slicer and the Argonautes

Argonautes are the central protein component in small RNA silencing pathways. Of the four human Argonautes (hAgo1-4) only hAgo2 is an active slicer. We have determined structures of the catalytically active hAgo2 as well as the catalytically inactive hAgo1, both bound to discrete miRNAs. The structures are strikingly similar. A conserved catalytic tetrad within the PIWI domain of hAgo2 is required for its slicing activity. Completion of the tetrad combined with a mutation on a loop adjacent to the active site of hAgo1 results in slicer activity that is substantially enhanced by swapping in the N domain of hAgo2. hAgo3, with an intact tetrad, becomes an active slicer by swapping the N domain of hAgo2, without additional mutations. Intriguingly, the elements that make Argonaute an active slicer involve a sophisticated interplay between the active site and more distant regions of the enzyme.