Human Dicer (hDicer) is a multi-domain protein belonging to the RNase III family. It plays pivotal roles in small RNA biogenesis during the RNA interference (RNAi) pathway by processing a diverse range of double-stranded RNA (dsRNA) precursors to generate ∼22 nt microRNA (miRNA) or small interfering RNA (siRNA) products for sequence-directed gene silencing. In this work, we solved the cryoelectron microscopy (cryo-EM) structure of hDicer in complex with its cofactor protein TRBP and revealed the precise spatial arrangement of hDicer's multiple domains. We further solved structures of the hDicer-TRBP complex bound with pre-let-7 RNA in two distinct conformations. In combination with biochemical analysis, these structures reveal a property of the hDicer-TRBP complex to promote the stability of pre-miRNA's stem duplex in a pre-dicing state. These results provide insights into the mechanism of RNA processing by hDicer and illustrate the regulatory role of hDicer's N-terminal helicase domain.

Human multidrug resistance protein 4 (hMRP4, also known as ABCC4), with a representative topology of the MRP subfamily, translocates various substrates across the membrane and contributes to the development of multidrug resistance. However, the underlying transport mechanism of hMRP4 remains unclear due to a lack of high-resolution structures. Here, we use cryogenic electron microscopy (cryo-EM) to resolve its near-atomic structures in the apo inward-open and the ATP-bound outward-open states. We also capture the PGE1 substrate-bound structure and, importantly, the inhibitor-bound structure of hMRP4 in complex with sulindac, revealing that substrate and inhibitor compete for the same hydrophobic binding pocket although with different binding modes. Moreover, our cryo-EM structures, together with molecular dynamics simulations and biochemical assay, shed light on the structural basis of the substrate transport and inhibition mechanism, with implications for the development of hMRP4-targeted drugs.

Dysregulation of polyamine homeostasis strongly associates with human diseases. ATP13A2, which is mutated in juvenile-onset Parkinson’s disease and autosomal recessive spastic paraplegia 78, is a transporter with a critical role in balancing the polyamine concentration between the lysosome and the cytosol. Here, to better understand human ATP13A2-mediated polyamine transport, we use single-particle cryo-electron microscopy to solve high-resolution structures of human ATP13A2 in six intermediate states, including the putative E2 structure for the P5 subfamily of the P-type ATPases. These structures comprise a nearly complete conformational cycle spanning the polyamine transport process and capture multiple substrate binding sites distributed along the transmembrane regions, suggesting a potential polyamine transport pathway. Integration of high-resolution structures, biochemical assays, and molecular dynamics simulations allows us to obtain a better understanding of the structural basis of how hATP13A2 transports polyamines, providing a mechanistic framework for ATP13A2-related diseases.

To accomplish concerted physiological reactions, nature has diversified functions of a single hormone at at least two primary levels: 1) Different receptors recognize the same hormone, and 2) different cellular effectors couple to the same hormone–receptor pair [R.P. Xiao, Sci STKE 2001 , re15 (2001); L. Hein, J. D. Altman, B.K. Kobilka, Nature 402 , 181–184 (1999); Y. Daaka, L. M. Luttrell, R. J. Lefkowitz, Nature 390 , 88–91 (1997)]. Not only these questions lie in the heart of hormone actions and receptor signaling but also dissecting mechanisms underlying these questions could offer therapeutic routes for refractory diseases, such as kidney injury (KI) or X-linked nephrogenic diabetes insipidus (NDI). Here, we identified that G s -biased signaling, but not G i activation downstream of EP4, showed beneficial effects for both KI and NDI treatments. Notably, by solving Cryo-electron microscope (cryo-EM) structures of EP3-G i , EP4-G s , and EP4-G i in complex with endogenous prostaglandin E 2 (PGE 2 )or two synthetic agonists and comparing with PGE 2 -EP2-G s structures, we found that unique primary sequences of prostaglandin E2 receptor (EP) receptors and distinct conformational states of the EP4 ligand pocket govern the G s /G i transducer coupling selectivity through different structural propagation paths, especially via TM6 and TM7, to generate selective cytoplasmic structural features. In particular, the orientation of the PGE 2 ω-chain and two distinct pockets encompassing agonist L902688 of EP4 were differentiated by their G s /G i coupling ability. Further, we identified common and distinct features of cytoplasmic side of EP receptors for G s /G i coupling and provide a structural basis for selective and biased agonist design of EP4 with therapeutic potential.