Assistant Professor, Molecular & Cellular Physiology
Honors & Awards
Harold M. Weintraub Graduate Student Award, Fred Hutchinson Cancer Research Center (2006)
Chinese Government Award for Outstanding Self-Financed Students Abroad, Chinese Government (2006)
Porter Ogden Jacobus Honorific Fellowship, Princeton University (2005)
PhD, Princeton University (2006)
MPhil, HKUST (2002)
B.S., Tsinghua University (1999)
Current Research and Scholarly Interests
Biological membranes act as selective barriers that separate the interior of cells from their outside environment. Membrane proteins play crucial roles in a wide range of biological and physiological processes and are targeted by a large number of pharmacologically active compounds including ~ 50% of the drugs in use today. Lack of high-resolution structural information has become a bottleneck for the mechanistic understanding of membrane proteins and hinders potential drug development. Our research interest lies primarily in understanding the mechanism and regulation of these dynamic membrane proteins, and developing small molecule modulators based on their structures and functions. We are taking a multi-disciplinary approach, employing X-ray crystallography to capture the high-resolution picture of different states, functional assays and biophysical methods to dissect the information inferred from the structure and computational approaches to understand their dynamics and kinetics.
- How Cells Work: Energetics, Compartments, and Coupling in Cell Biology
MCP 156, MCP 256 (Win)
- Imaging: Biological Light Microscopy
BIO 152, MCP 222 (Win)
Independent Studies (7)
- Directed Investigation
BIOE 392 (Aut, Win, Spr, Sum)
- Directed Reading in Biophysics
BIOPHYS 399 (Aut, Win, Spr)
- Directed Reading in Molecular and Cellular Physiology
MCP 299 (Spr, Sum)
- Graduate Research
BIOPHYS 300 (Aut, Win, Spr)
- Graduate Research
MCP 399 (Aut, Win, Spr, Sum)
- Medical Scholars Research
MCP 370 (Spr, Sum)
- Undergraduate Research
MCP 199 (Aut, Spr, Sum)
- Directed Investigation
- Structures of bacterial homologues of SWEET transporters in two distinct conformations NATURE 2014; 515 (7527): 448-?
Structure of a Eukaryotic CLC Transporter Defines an Intermediate State in the Transport Cycle
2010; 330 (6004): 635-641
CLC proteins transport chloride (Cl(-)) ions across cell membranes to control the electrical potential of muscle cells, transfer electrolytes across epithelia, and control the pH and electrolyte composition of intracellular organelles. Some members of this protein family are Cl(-) ion channels, whereas others are secondary active transporters that exchange Cl(-) ions and protons (H(+)) with a 2:1 stoichiometry. We have determined the structure of a eukaryotic CLC transporter at 3.5 angstrom resolution. Cytoplasmic cystathionine beta-synthase (CBS) domains are strategically positioned to regulate the ion-transport pathway, and many disease-causing mutations in human CLCs reside on the CBS-transmembrane interface. Comparison with prokaryotic CLC shows that a gating glutamate residue changes conformation and suggests a basis for 2:1 Cl(-)/H(+) exchange and a simple mechanistic connection between CLC channels and transporters.
View details for DOI 10.1126/science.1195230
View details for Web of Science ID 000283580600038
View details for PubMedID 20929736
Structure and metal exchange in the cadmium carbonic anhydrase of marine diatoms
2008; 452 (7183): 56-U3
Carbonic anhydrase, a zinc enzyme found in organisms from all kingdoms, catalyses the reversible hydration of carbon dioxide and is used for inorganic carbon acquisition by phytoplankton. In the oceans, where zinc is nearly depleted, diatoms use cadmium as a catalytic metal atom in cadmium carbonic anhydrase (CDCA). Here we report the crystal structures of CDCA in four distinct forms: cadmium-bound, zinc-bound, metal-free and acetate-bound. Despite lack of sequence homology, CDCA is a structural mimic of a functional beta-carbonic anhydrase dimer, with striking similarity in the spatial organization of the active site residues. CDCA readily exchanges cadmium and zinc at its active site--an apparently unique adaptation to oceanic life that is explained by a stable opening of the metal coordinating site in the absence of metal. Given the central role of diatoms in exporting carbon to the deep sea, their use of cadmium in an enzyme critical for carbon acquisition establishes a remarkable link between the global cycles of cadmium and carbon.
View details for DOI 10.1038/nature06636
View details for Web of Science ID 000253671900045
View details for PubMedID 18322527
Structure of a site-2 protease family intramembrane metalloprotease
2007; 318 (5856): 1608-1612
Regulated intramembrane proteolysis by members of the site-2 protease (S2P) family is an important signaling mechanism conserved from bacteria to humans. Here we report the crystal structure of the transmembrane core domain of an S2P metalloprotease from Methanocaldococcus jannaschii. The protease consists of six transmembrane segments, with the catalytic zinc atom coordinated by two histidine residues and one aspartate residue approximately 14 angstroms into the lipid membrane surface. The protease exhibits two distinct conformations in the crystals. In the closed conformation, the active site is surrounded by transmembrane helices and is impermeable to substrate peptide; water molecules gain access to zinc through a polar, central channel that opens to the cytosolic side. In the open conformation, transmembrane helices alpha1 and alpha6 separate from each other by 10 to 12 angstroms, exposing the active site to substrate entry. The structure reveals how zinc embedded in an integral membrane protein can catalyze peptide cleavage.
View details for DOI 10.1126/science.1150755
View details for Web of Science ID 000251421700041
View details for PubMedID 18063795
Structure of oxidized alpha-haemoglobin bound to AHSP reveals a protective mechanism for haem
2005; 435 (7042): 697-701
The synthesis of haemoglobin A (HbA) is exquisitely coordinated during erythrocyte development to prevent damaging effects from individual alpha- and beta-subunits. The alpha-haemoglobin-stabilizing protein (AHSP) binds alpha-haemoglobin (alphaHb), inhibits the ability of alphaHb to generate reactive oxygen species and prevents its precipitation on exposure to oxidant stress. The structure of AHSP bound to ferrous alphaHb is thought to represent a transitional complex through which alphaHb is converted to a non-reactive, hexacoordinate ferric form. Here we report the crystal structure of this ferric alphaHb-AHSP complex at 2.4 A resolution. Our findings reveal a striking bis-histidyl configuration in which both the proximal and the distal histidines coordinate the haem iron atom. To attain this unusual conformation, segments of alphaHb undergo drastic structural rearrangements, including the repositioning of several alpha-helices. Moreover, conversion to the ferric bis-histidine configuration strongly and specifically inhibits redox chemistry catalysis and haem loss from alphaHb. The observed structural changes, which impair the chemical reactivity of haem iron, explain how AHSP stabilizes alphaHb and prevents its damaging effects in cells.
View details for DOI 10.1038/nature03609
View details for Web of Science ID 000229476200055
View details for PubMedID 15931225
Molecular mechanism of AHSP-mediated stabilization of alpha-hemoglobin
2004; 119 (5): 629-640
Hemoglobin A (HbA), the oxygen delivery system in humans, comprises two alpha and two beta subunits. Free alpha-hemoglobin (alphaHb) is unstable, and its precipitation contributes to the pathophysiology of beta thalassemia. In erythrocytes, the alpha-hemoglobin stabilizing protein (AHSP) binds alphaHb and inhibits its precipitation. The crystal structure of AHSP bound to Fe(II)-alphaHb reveals that AHSP specifically recognizes the G and H helices of alphaHb through a hydrophobic interface that largely recapitulates the alpha1-beta1 interface of hemoglobin. The AHSP-alphaHb interactions are extensive but suboptimal, explaining why beta-hemoglobin can competitively displace AHSP to form HbA. Remarkably, the Fe(II)-heme group in AHSP bound alphaHb is coordinated by the distal but not the proximal histidine. Importantly, binding to AHSP facilitates the conversion of oxy-alphaHb to a deoxygenated, oxidized [Fe(III)], nonreactive form in which all six coordinate positions are occupied. These observations reveal the molecular mechanisms by which AHSP stabilizes free alphaHb.
View details for Web of Science ID 000225389500009
View details for PubMedID 15550245
Molecular mechanism of proton transport in CLC Cl-/H+ exchange transporters
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2012; 109 (29): 11699-11704
CLC proteins underlie muscle, kidney, bone, and other organ system function by catalyzing the transport of Cl(-) ions across cell and organellar membranes. Some CLC proteins are ion channels while others are pumps that exchange Cl(-) for H(+). The pathway through which Cl(-) ions cross the membrane has been characterized, but the transport of H(+) and the principle by which their movement is coupled to Cl(-) movement is not well understood. Here we show that H(+) transport depends not only on the presence of a specific glutamate residue but also the presence of Cl(-) ions. H(+) transport, however, can be isolated and analyzed in the absence of Cl(-) by mutating the glutamate to alanine and adding carboxylate-containing molecules to solution, consistent with the notion that H(+) transfer is mediated through the entry of a carboxylate group into the anion pathway. Cl(-) ions and carboxylate interact with each other strongly. These data support a mechanism in which the glutamate carboxylate functions as a surrogate Cl(-) ion, but it can accept a H(+) and transfer it between the external solution and the central Cl(-) binding site, coupled to the movement of 2 Cl(-) ions.
View details for DOI 10.1073/pnas.1205764109
View details for Web of Science ID 000306837100047
View details for PubMedID 22753511
A cis-Proline in alpha-Hemoglobin Stabilizing Protein Directs the Structural Reorganization of alpha-Hemoglobin
JOURNAL OF BIOLOGICAL CHEMISTRY
2009; 284 (43): 29462-29469
alpha-Hemoglobin (alphaHb) stabilizing protein (AHSP) is expressed in erythropoietic tissues as an accessory factor in hemoglobin synthesis. AHSP forms a specific complex with alphaHb and suppresses the heme-catalyzed evolution of reactive oxygen species by converting alphaHb to a conformation in which the heme is coordinated at both axial positions by histidine side chains (bis-histidyl coordination). Currently, the detailed mechanism by which AHSP induces structural changes in alphaHb has not been determined. Here, we present x-ray crystallography, NMR spectroscopy, and mutagenesis data that identify, for the first time, the importance of an evolutionarily conserved proline, Pro(30), in loop 1 of AHSP. Mutation of Pro(30) to a variety of residue types results in reduced ability to convert alphaHb. In complex with alphaHb, AHSP Pro(30) adopts a cis-peptidyl conformation and makes contact with the N terminus of helix G in alphaHb. Mutations that stabilize the cis-peptidyl conformation of free AHSP, also enhance the alphaHb conversion activity. These findings suggest that AHSP loop 1 can transmit structural changes to the heme pocket of alphaHb, and, more generally, highlight the importance of cis-peptidyl prolyl residues in defining the conformation of regulatory protein loops.
View details for DOI 10.1074/jbc.M109.027045
View details for Web of Science ID 000270896800028
View details for PubMedID 19706593
Enzymatic analysis of a rhomboid intramembrane protease implicates transmembrane helix 5 as the lateral substrate gate
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2007; 104 (20): 8257-8262
Intramembrane proteolysis is a core regulatory mechanism of cells that raises a biochemical paradox of how hydrolysis of peptide bonds is accomplished within the normally hydrophobic environment of the membrane. Recent high-resolution crystal structures have revealed that rhomboid proteases contain a catalytic serine recessed into the plane of the membrane, within a hydrophilic cavity that opens to the extracellular face, but protected laterally from membrane lipids by a ring of transmembrane segments. This architecture poses questions about how substrates enter the internal active site laterally from membrane lipid. Because structures are static glimpses of a dynamic enzyme, we have taken a structure-function approach analyzing >40 engineered variants to identify the gating mechanism used by rhomboid proteases. Importantly, our analyses were conducted with a substrate that we show is cleaved at two intramembrane sites within the previously defined Spitz substrate motif. Engineered mutants in the L1 loop and active-site region of the GlpG rhomboid protease suggest an important structural, rather than dynamic, gating function for the L1 loop that was first proposed to be the substrate gate. Conversely, three classes of mutations that promote transmembrane helix 5 displacement away from the protease core dramatically enhanced enzyme activity 4- to 10-fold. Our functional analyses have identified transmembrane helix 5 movement to gate lateral substrate entry as a rate-limiting step in intramembrane proteolysis. Moreover, our mutagenesis also underscores the importance of other residue interactions within the enzyme that warrant further scrutiny.
View details for DOI 10.1073/pnas.0700814104
View details for Web of Science ID 000246599900014
View details for PubMedID 17463085
Structural analysis of a rhomboid family intramembrane protease reveals a gating mechanism for substrate entry
NATURE STRUCTURAL & MOLECULAR BIOLOGY
2006; 13 (12): 1084-1091
Intramembrane proteolysis regulates diverse biological processes. Cleavage of substrate peptide bonds within the membrane bilayer is catalyzed by integral membrane proteases. Here we report the crystal structure of the transmembrane core domain of GlpG, a rhomboid-family intramembrane serine protease from Escherichia coli. The protein contains six transmembrane helices, with the catalytic Ser201 located at the N terminus of helix alpha4 approximately 10 A below the membrane surface. Access to water molecules is provided by a central cavity that opens to the extracellular region and converges on Ser201. One of the two GlpG molecules in the asymmetric unit has an open conformation at the active site, with the transmembrane helix alpha5 bent away from the rest of the molecule. Structural analysis suggests that substrate entry to the active site is probably gated by the movement of helix alpha5.
View details for DOI 10.1038/nsmb1179
View details for Web of Science ID 000242655600013
View details for PubMedID 17099694
Simple bioseparations using self-cleaving elastin-like polypeptide tags
2005; 2 (9): 659-661
We introduce a new method for the purification of recombinant proteins expressed in Escherichia coli using self-cleaving elastin-like polypeptide (ELP) fusion tags without the need for affinity chromatography or proteolytic tag removal. Using this method we obtained high purity, activity and reasonable yields for ten diverse target proteins.
View details for Web of Science ID 000235261900017
View details for PubMedID 16074986
A repetitive element containing a critical tyrosine residue is required for transcriptional activation by the EWS/ATF1 oncogene
2001; 20 (31): 4161-4168
Chromosomal fusion of the N-terminal region of the Ewings Sarcoma Oncogene (EWS-activation-domain, EAD) to the DNA-binding domains of a variety of cellular transcription factors produce oncogenic proteins (EWS-fusion proteins (EFPs)) that cause distinct malignancies. In EFPs, the EAD acts as a potent transcriptional activation domain and this ability is repressed in the context of normal, non-tumorigenic, EWS. Trans-activation by the EAD is therefore a specific characteristic of EFPs and it is thought that EFPs induce tumorigenesis via improper transcriptional activation of cellular genes. Functional elements required for transcriptional activation are dispersed throughout the EAD, as are thirty-one copies of a Degenerate Hexapeptide Repeat (DHR, consensus SYGQQS). This suggests that the EAD contains a highly reiterated functional element related to DHRs. Here we show that in the context of EWS/ATF1, the EFP that causes malignant melanoma of soft parts, trans-cooperation by small regions of the EAD ( approximately 30 residues) results in potent transcriptional activation dependent on the conserved tyrosine residues present in DHRs. These findings provide the first evidence for a role of DHRs in EAD-mediated trans-activation and demonstrate that the EAD represents a novel tyrosine-dependent transcriptional activation domain.
View details for Web of Science ID 000169857200006
View details for PubMedID 11464282