Pingping Cao

All Publications

• Review Structural advances in sterol-sensing domain-containing proteins TRENDS IN BIOCHEMICAL SCIENCES Wu, X., Yan, R., Cao, P., Qian, H., Yan, N. 2022; 47 (4): 289-300

  Abstract

  The sterol-sensing domain (SSD) is present in several membrane proteins that function in cholesterol metabolism, transport, and signaling. Recent progress in structural studies of SSD-containing proteins, such as sterol regulatory element-binding protein (SREBP)-cleavage activating protein (Scap), Patched, Niemann-Pick disease type C1 (NPC1), and related proteins, reveals a conserved core that is essential for their sterol-dependent functions. This domain, by its name, 'senses' the presence of sterol substrates through interactions and may modulate protein behaviors with changing sterol levels. We summarize recent advances in structural and mechanistic investigations of these proteins and propose to divide them to two classes: M for 'moderator' proteins that regulate sterol metabolism in response to membrane sterol levels, and T for 'transporter' proteins that harbor inner tunnels for cargo trafficking across cellular membranes.

  View details for DOI 10.1016/j.tibs.2021.12.005

  View details for Web of Science ID 000776181200004

  View details for PubMedID 35012873

• Structural basis for sterol sensing by Scap and Insig CELL REPORTS Yan, R., Cao, P., Song, W., Li, Y., Wang, T., Qian, H., Yan, C., Yan, N. 2021; 35 (13): 109299

  Abstract

  The sterol regulatory element-binding protein (SREBP) pathway monitors the cellular cholesterol level through sterol-regulated association between the SREBP cleavage-activating protein (Scap) and the insulin-induced gene (Insig). Despite structural determination of the Scap and Insig-2 complex bound to 25-hydroxycholesterol, the luminal domains of Scap remain unresolved. In this study, combining cryogenic electron microscopy (cryo-EM) analysis and artificial intelligence-facilitated structural prediction, we report the structure of the human Scap/Insig-2 complex purified in digitonin. The luminal domain loop 1 and a co-folded segment in loop 7 of Scap resemble those of the luminal/extracellular domain in NPC1 and related proteins, providing clues to the cholesterol-regulated interaction of loop 1 and loop 7. An additional luminal interface is observed between Scap and Insig. We also show that Scap(D428A), which inhibits SREBP activation even under sterol depletion, exhibits an identical conformation with the wild-type protein when complexed with Insig-2, and its constitutive suppression of the SREBP pathway may also involve a later step in protein trafficking.

  View details for DOI 10.1016/j.celrep.2021.109299

  View details for Web of Science ID 000668072600008

  View details for PubMedID 34192549

• A structure of human Scap bound to Insig-2 suggests how their interaction is regulated by sterols SCIENCE Yan, R., Cao, P., Song, W., Qian, H., Du, X., Coates, H. W., Zhao, X., Li, Y., Gao, S., Gong, X., Liu, X., Sui, J., Lei, J., Yang, H., Brown, A. J., Zhou, Q., Yan, C., Yan, N. 2021; 371 (6533): 1012-+

  Abstract

  The sterol regulatory element-binding protein (SREBP) pathway controls cellular homeostasis of sterols. The key players in this pathway, Scap and Insig-1 and -2, are membrane-embedded sterol sensors. The 25-hydroxycholesterol (25HC)-dependent association of Scap and Insig acts as the master switch for the SREBP pathway. Here, we present cryo-electron microscopy analysis of the human Scap and Insig-2 complex in the presence of 25HC, with the transmembrane (TM) domains determined at an average resolution of 3.7 angstrom. The sterol-sensing domain in Scap and all six TMs in Insig-2 were resolved. A 25HC molecule is sandwiched between the S4 to S6 segments in Scap and TMs 3 and 4 in Insig-2 in the luminal leaflet of the membrane. Unwinding of the middle of the Scap-S4 segment is crucial for 25HC binding and Insig association.

  View details for DOI 10.1126/science.abb2224

  View details for Web of Science ID 000625876100046

  View details for PubMedID 33446483

• Inhibition of tetrameric Patched1 by Sonic Hedgehog through an asymmetric paradigm NATURE COMMUNICATIONS Qian, H., Cao, P., Hu, M., Gao, S., Yan, N., Gong, X. 2019; 10: 2320

  Abstract

  The Hedgehog (Hh) pathway controls embryonic development and postnatal tissue maintenance and regeneration. Inhibition of Hh receptor Patched (Ptch) by the Hh ligands relieves suppression of signaling cascades. Here, we report the cryo-EM structure of tetrameric Ptch1 in complex with the palmitoylated N-terminal signaling domain of human Sonic hedgehog (ShhNp) at a 4:2 stoichiometric ratio. The structure shows that four Ptch1 protomers are organized as a loose dimer of dimers. Each dimer binds to one ShhNp through two distinct inhibitory interfaces, one mainly through the N-terminal peptide and the palmitoyl moiety of ShhNp and the other through the Ca2+-mediated interface on ShhNp. Map comparison reveals that the cholesteryl moiety of native ShhN occupies a recently identified extracellular steroid binding pocket in Ptch1. Our structure elucidates the tetrameric assembly of Ptch1 and suggests an asymmetric mode of action of the Hh ligands for inhibiting the potential cholesterol transport activity of Ptch1.

  View details for DOI 10.1038/s41467-019-10234-9

  View details for Web of Science ID 000468857900027

  View details for PubMedID 31127104

  View details for PubMedCentralID PMC6534611

• Structural basis for the recognition of Sonic Hedgehog by human Patchedl SCIENCE Gong, X., Qian, H., Cao, P., Zhao, X., Zhou, Q., Lei, J., Yan, N. 2018; 361 (6402)

  Abstract

  The Hedgehog (Hh) pathway involved in development and regeneration is activated by the extracellular binding of Hh to the membrane receptor Patched (Ptch). We report the structures of human Ptch1 alone and in complex with the N-terminal domain of human Sonic hedgehog (ShhN) at resolutions of 3.9 and 3.6 angstroms, respectively, as determined by cryo-electron microscopy. Ptch1 comprises two interacting extracellular domains, ECD1 and ECD2, and 12 transmembrane segments (TMs), with TMs 2 to 6 constituting the sterol-sensing domain (SSD). Two steroid-shaped densities are resolved in both structures, one enclosed by ECD1/2 and the other in the membrane-facing cavity of the SSD. Structure-guided mutational analysis shows that interaction between ShhN and Ptch1 is steroid-dependent. The structure of a steroid binding-deficient Ptch1 mutant displays pronounced conformational rearrangements.

  View details for DOI 10.1126/science.aas8935

  View details for Web of Science ID 000441202700042

  View details for PubMedID 29954986

• Structure of the Human Lipid Exporter ABCA1 CELL Qian, H., Zhao, X., Cao, P., Lei, J., Yan, N., Gong, X. 2017; 169 (7): 1228-+

  Abstract

  ABCA1, an ATP-binding cassette (ABC) subfamily A exporter, mediates the cellular efflux of phospholipids and cholesterol to the extracellular acceptor apolipoprotein A-I (apoA-I) for generation of nascent high-density lipoprotein (HDL). Mutations of human ABCA1 are associated with Tangier disease and familial HDL deficiency. Here, we report the cryo-EM structure of human ABCA1 with nominal resolutions of 4.1 Å for the overall structure and 3.9 Å for the massive extracellular domain. The nucleotide-binding domains (NBDs) display a nucleotide-free state, while the two transmembrane domains (TMDs) contact each other through a narrow interface in the intracellular leaflet of the membrane. In addition to TMDs and NBDs, two extracellular domains of ABCA1 enclose an elongated hydrophobic tunnel. Structural mapping of dozens of disease-related mutations allows potential interpretation of their diverse pathogenic mechanisms. Structural-based analysis suggests a plausible "lateral access" mechanism for ABCA1-mediated lipid export that may be distinct from the conventional alternating-access paradigm.

  View details for DOI 10.1016/j.cell.2017.05.020

  View details for Web of Science ID 000403332400012

  View details for PubMedID 28602350

• Structural Insights into the Niemann-Pick C1 (NPC1)-Mediated Cholesterol Transfer and Ebola Infection CELL Gong, X., Qian, H., Zhou, X., Wu, J., Wan, T., Cao, P., Huang, W., Zhao, X., Wang, X., Wang, P., Shi, Y., Gao, G. F., Zhou, Q., Yan, N. 2016; 165 (6): 1467-1478

  Abstract

  Niemann-Pick disease type C (NPC) is associated with mutations in NPC1 and NPC2, whose gene products are key players in the endosomal/lysosomal egress of low-density lipoprotein-derived cholesterol. NPC1 is also the intracellular receptor for Ebola virus (EBOV). Here, we present a 4.4 Å structure of full-length human NPC1 and a low-resolution reconstruction of NPC1 in complex with the cleaved glycoprotein (GPcl) of EBOV, both determined by single-particle electron cryomicroscopy. NPC1 contains 13 transmembrane segments (TMs) and three distinct lumenal domains A (also designated NTD), C, and I. TMs 2-13 exhibit a typical resistance-nodulation-cell division fold, among which TMs 3-7 constitute the sterol-sensing domain conserved in several proteins involved in cholesterol metabolism and signaling. A trimeric EBOV-GPcl binds to one NPC1 monomer through the domain C. Our structural and biochemical characterizations provide an important framework for mechanistic understanding of NPC1-mediated intracellular cholesterol trafficking and Ebola virus infection.

  View details for DOI 10.1016/j.cell.2016.05.022

  View details for Web of Science ID 000377045400019

  View details for PubMedID 27238017

  View details for PubMedCentralID PMC7111323