Professor Liz Carpenter
|Research Area:||Protein Science and Structural Biology|
|Technology Exchange:||Crystallography, Drug discovery and Protein interaction|
|Scientific Themes:||Protein Science & Structural Biology and Physiology, Cellular & Molecular Biology|
|Keywords:||Membrane proteins, X-ray crystallography, Protein structure, ion channels, high throughput methods and drug design|
Membrane proteins are the gateways to the cell. All cells and organelles are surrounded by an oily, impermeable lipid bilayer and many small molecules can only cross this barrier by passing through protein molecules embedded in the bilayer. Many nutrients, ions, waste products and even DNA and proteins enter and leave cells only via proteins which are tightly controlled, thus maintaining the integrity of the cell. Communication between cells is also mediated by these proteins often by binding signaling molecules outside cells and amplifying the signal by triggering chemical reactions inside the cell. These diverse functions are fulfilled by a huge variety of membrane proteins, in fact approximately 15% of all the genes in the human genome code for these proteins. Given their location on the surfaces of cells, it is not surprising that membrane proteins are often found to be the targets for drugs, such as the calcium channel blockers used to treat heart disease and potassium channel blockers which are used in diabetes treatment. Indeed membrane proteins are involved in the development of many diseases, including heart disease, cancer, cystic fibrosis, Alzheimer’s, Parkinson’s and other neurological diseases, kidney disease and epilepsy.
My group at the structural Genomics Consortium in Oxford aims to solve the three dimensional structures of human membrane proteins using X-ray crystallography. We purify proteins, pursuad them to form crystals, and then expose them to a beam of X-rays. The resulting diffraction patterns can then be used to understand the positions of all the atoms in the protein. We then study the structures in complex with inhibitors and drugs, using this information to improve and extend the available treatments for disease. There are less than 50 structures of human membrane proteins known and we therefore seak to develop methods to make this process more efficient. In the past four years we have established a working high-throughput system for the producing human membrane proteins for structural studies.
The IMP group at the SGC studies proteins from a variety membrane protein families, including ion channels enzymes and ABC transporters. To date we have solved structures of of proteins in three different areas:
1. We solved the first structure of a human ABC transporter, ABCB10, a mitochondrial protein which is important for heme production and for resistance of mitochondria to oxidative stress.
2. Premature ageing syndromes can be caused by a failure in the processing of the lamin proteins, which form a network of fibres underlying the nuclear membrane within cells. We have solved the structure of a zinc metalloprotease, ZMPSTE24, which is responsible for two steps in this processing. This structure has allowed us to see how mutations in the ZMPSTE24 protein can lead to premature ageing diseases, which provide a model for normal ageing.
3. Recently we have solved and deposited the structure of a human ion channel, TREK-2, one of the family of K2P proteins which are responsible for the background leak current that helps to maintain the membrane potential and also are susceptible to a range of physiological and pharmacological stimuli.
|Professor Juha T Huiskonen||Structural Biology||University of Oxford||United Kingdom|
|Dr Nicola A Burgess-Brown||Structural Genomics Consortium||University of Oxford||United Kingdom|
|Professor Wyatt W Yue||Structural Genomics Consortium||University of Oxford||United Kingdom|
|Dr Brian D Marsden||Structural Genomics Consortium||University of Oxford||United Kingdom|
|Professor Paul Brennan||Target Discovery Institute||University of Oxford||United Kingdom|
|Dr Frank von Delft||Structural Genomics Consortium||University of Oxford||United Kingdom|
|Prof David Beeson (RDM)||Weatherall Institute of Molecular Medicine||University of Oxford||United Kingdom|
A small number of physiologically important ATP-binding cassette (ABC) transporters are found in mitochondria. Most are half transporters of the B group forming homodimers and their topology suggests they function as exporters. The results of mutant studies point towards involvement in iron cofactor biosynthesis. In particular, ABC subfamily B member 7 (ABCB7) and its homologues in yeast and plants are required for iron-sulfur (Fe-S) cluster biosynthesis outside of the mitochondria, whereas ABCB10 is involved in haem biosynthesis. They also play a role in preventing oxidative stress. Mutations in ABCB6 and ABCB7 have been linked to human disease. Recent crystal structures of yeast Atm1 and human ABCB10 have been key to identifying substrate-binding sites and transport mechanisms. Combined with in vitro and in vivo studies, progress is being made to find the physiological substrates of the different mitochondrial ABC transporters. Hide abstract
TREK-2 (KCNK10/K2P10), a two-pore domain potassium (K2P) channel, is gated by multiple stimuli such as stretch, fatty acids, and pH and by several drugs. However, the mechanisms that control channel gating are unclear. Here we present crystal structures of the human TREK-2 channel (up to 3.4 angstrom resolution) in two conformations and in complex with norfluoxetine, the active metabolite of fluoxetine (Prozac) and a state-dependent blocker of TREK channels. Norfluoxetine binds within intramembrane fenestrations found in only one of these two conformations. Channel activation by arachidonic acid and mechanical stretch involves conversion between these states through movement of the pore-lining helices. These results provide an explanation for TREK channel mechanosensitivity, regulation by diverse stimuli, and possible off-target effects of the serotonin reuptake inhibitor Prozac. Hide abstract
There has been exponential growth in the number of membrane protein structures determined. Nevertheless, these structures are usually resolved in the absence of their lipid environment. Coarse-grained molecular dynamics (CGMD) simulations enable insertion of membrane proteins into explicit models of lipid bilayers. We have automated the CGMD methodology, enabling membrane protein structures to be identified upon their release into the PDB and embedded into a membrane. The simulations are analyzed for protein-lipid interactions, identifying lipid binding sites, and revealing local bilayer deformations plus molecular access pathways within the membrane. The coarse-grained models of membrane protein/bilayer complexes are transformed to atomistic resolution for further analysis and simulation. Using this automated simulation pipeline, we have analyzed a number of recently determined membrane protein structures to predict their locations within a membrane, their lipid/protein interactions, and the functional implications of an enhanced understanding of the local membrane environment of each protein. Hide abstract
ABCB10 (ATP binding cassette sub-family B10) is a mitochondrial inner-membrane ABC transporter. ABCB10 has been shown to protect the heart from the impact of ROS during ischemia-reperfusion and to allow for proper hemoglobin synthesis during erythroid development. ABC transporters are proteins that increase ATP binding and hydrolysis activity in the presence of the transported substrate. However, molecular entities transported by ABCB10 and its regulatory mechanisms are currently unknown. Here we characterized ATP binding and hydrolysis properties of ABCB10 by using the 8-azido-ATP photolabeling technique. This technique can identify potential ABCB10 regulators, transported substrates and amino-acidic residues required for ATP binding and hydrolysis. We confirmed that Gly497 and Lys498 in the Walker A motif, Glu624 in the Walker B motif and Gly602 in the C-Loop motif of ABCB10 are required for proper ATP binding and hydrolysis activity, as their mutation changed ABCB10 8-Azido-ATP photo-labeling. In addition, we show that the potential ABCB10 transported entity and heme precursor delta-aminolevulinic acid (dALA) does not alter 8-azido-ATP photo-labeling. In contrast, oxidized glutathione (GSSG) stimulates ATP hydrolysis without affecting ATP binding, whereas reduced glutathione (GSH) inhibits ATP binding and hydrolysis. Indeed, we detectABCB10 glutathionylation in Cys547 and show that it is one of the exposed cysteine residues within ABCB10 structure. In all, we characterize essential residues for ABCB10 ATPase activity and we provide evidence that supports the exclusion of dALA as a potential substrate directly transported by ABCB10. Last, we show the first molecular mechanism by which mitochondrial oxidative status, through GSH/GSSG, can regulate ABCB10. Hide abstract
Mutations in human LMBRD1 and ABCD4 prevent lysosomal export of vitamin B(12) to the cytoplasm, impairing the vitamin B(12)-dependent enzymes methionine synthase and methylmalonyl-CoA mutase. The gene products of LMBRD1 and ABCD4 are implicated in vitamin B(12) transport at the lysosomal membrane and are proposed to act in complex. To address the mechanism for lysosomal vitamin B(12) transport, we report the novel recombinant production of LMBD1 and ABCD4 for detailed biophysical analyses. Using blue native PAGE, chemical crosslinking, and size exclusion chromatography coupled to multi-angle light scattering (SEC-MALS), we show that both detergent-solubilized LMBD1 and detergent-solubilized ABCD4 form homodimers. To examine the functional binding properties of these proteins, label-free surface plasmon resonance (SPR) provides direct in vitro evidence that: (i) LMBD1 and ABCD4 interact with low nanomolar affinity; and (ii) the cytoplasmic vitamin B(12)-processing protein MMACHC also interacts with LMBD1 and ABCD4 with low nanomolar affinity. Accordingly, we propose a model whereby membrane-bound LMBD1 and ABCD4 facilitate the vectorial delivery of lysosomal vitamin B(12) to cytoplasmic MMACHC, thus preventing cofactor dilution to the cytoplasmic milieu and protecting against inactivating side reactions. Hide abstract
ABCB10 is one of the three ATP-binding cassette (ABC) transporters found in the inner membrane of mitochondria. In mammals ABCB10 is essential for erythropoiesis, and for protection of mitochondria against oxidative stress. ABCB10 is therefore a potential therapeutic target for diseases in which increased mitochondrial reactive oxygen species production and oxidative stress play a major role. The crystal structure of apo-ABCB10 shows a classic exporter fold ABC transporter structure, in an open-inwards conformation, ready to bind the substrate or nucleotide from the inner mitochondrial matrix or membrane. Unexpectedly, however, ABCB10 adopts an open-inwards conformation when complexed with nonhydrolysable ATP analogs, in contrast to other transporter structures which adopt an open-outwards conformation in complex with ATP. The three complexes of ABCB10/ATP analogs reported here showed varying degrees of opening of the transport substrate binding site, indicating that in this conformation there is some flexibility between the two halves of the protein. These structures suggest that the observed plasticity, together with a portal between two helices in the transmembrane region of ABCB10, assist transport substrate entry into the substrate binding cavity. These structures indicate that ABC transporters may exist in an open-inwards conformation when nucleotide is bound. We discuss ways in which this observation can be aligned with the current views on mechanisms of ABC transporters. Hide abstract
Mutations in the nuclear membrane zinc metalloprotease ZMPSTE24 lead to diseases of lamin processing (laminopathies), such as the premature aging disease progeria and metabolic disorders. ZMPSTE24 processes prelamin A, a component of the nuclear lamina intermediate filaments, by cleaving it at two sites. Failure of this processing results in accumulation of farnesylated, membrane-associated prelamin A. The 3.4 angstrom crystal structure of human ZMPSTE24 has a seven transmembrane α-helical barrel structure, surrounding a large, water-filled, intramembrane chamber, capped by a zinc metalloprotease domain with the catalytic site facing into the chamber. The 3.8 angstrom structure of a complex with a CSIM tetrapeptide showed that the mode of binding of the substrate resembles that of an insect metalloprotease inhibitor in thermolysin. Laminopathy-associated mutations predicted to reduce ZMPSTE24 activity map to the zinc metalloprotease peptide-binding site and to the bottom of the chamber. Hide abstract
Base excision repair (BER) is a highly conserved DNA repair pathway throughout all kingdoms from bacteria to humans. Whereas several enzymes are required to complete the multistep repair process of damaged bases, apurinic-apyrimidic (AP) endonucleases play an essential role in enabling the repair process by recognizing intermediary abasic sites cleaving the phosphodiester backbone 5' to the abasic site. Despite extensive study, there is no structure of a bacterial AP endonuclease bound to substrate DNA. Furthermore, the structural mechanism for AP-site cleavage is incomplete. Here we report a detailed structural and biochemical study of the AP endonuclease from Neisseria meningitidis that has allowed us to capture structural intermediates providing more complete snapshots of the catalytic mechanism. Our data reveal subtle differences in AP-site recognition and kinetics between the human and bacterial enzymes that may reflect different evolutionary pressures. Hide abstract
The mechanism by which nucleotide-binding domains (NBDs) of ABC transporters power the transport of substrates across cell membranes is currently unclear. Here we report the crystal structure of an NBD, FbpC, from the Neisseria gonorrhoeae ferric iron uptake transporter with an unusual and substantial domain swap in the C-terminal regulatory domain. This entanglement suggests that FbpC is unable to open to the same extent as the homologous protein MalK. Using molecular dynamics we demonstrate that this is not the case: both NBDs open rapidly once ATP is removed. We conclude from this result that the closed structures of FbpC and MalK have higher free energies than their respective open states. This result has important implications for our understanding of the mechanism of power generation in ABC transporters, because the unwinding of this free energy ensures that the opening of these two NBDs is also powered. Hide abstract
Outer membrane proteins are structurally distinct from those that reside in the inner membrane and play important roles in bacterial pathogenicity and human metabolism. X-ray crystallography studies on >40 different outer membrane proteins have revealed that the transmembrane portion of these proteins can be constructed from either beta-sheets or less commonly from alpha-helices. The most common architecture is the beta-barrel, which can be formed from either a single anti-parallel sheet, fused at both ends to form a barrel or from multiple peptide chains. Outer membrane proteins exhibit considerable rigidity and stability, making their study through x-ray crystallography particularly tractable. As the number of structures of outer membrane proteins increases a more rational approach to their crystallization can be made. Herein we analyse the crystallization data from 53 outer membrane proteins and compare the results to those obtained for inner membrane proteins. A targeted sparse matrix screen for outer membrane protein crystallization is presented based on the present analysis. Hide abstract
The nucleobase-cation-symport-1 (NCS1) transporters are essential components of salvage pathways for nucleobases and related metabolites. Here, we report the 2.85-angstrom resolution structure of the NCS1 benzyl-hydantoin transporter, Mhp1, from Microbacterium liquefaciens. Mhp1 contains 12 transmembrane helices, 10 of which are arranged in two inverted repeats of five helices. The structures of the outward-facing open and substrate-bound occluded conformations were solved, showing how the outward-facing cavity closes upon binding of substrate. Comparisons with the leucine transporter LeuT(Aa) and the galactose transporter vSGLT reveal that the outward- and inward-facing cavities are symmetrically arranged on opposite sides of the membrane. The reciprocal opening and closing of these cavities is synchronized by the inverted repeat helices 3 and 8, providing the structural basis of the alternating access model for membrane transport. Hide abstract
Membrane protein structural biology is still a largely unconquered area, given that approximately 25% of all proteins are membrane proteins and yet less than 150 unique structures are available. Membrane proteins have proven to be difficult to study owing to their partially hydrophobic surfaces, flexibility and lack of stability. The field is now taking advantage of the high-throughput revolution in structural biology and methods are emerging for effective expression, solubilisation, purification and crystallisation of membrane proteins. These technical advances will lead to a rapid increase in the rate at which membrane protein structures are solved in the near future. Hide abstract
Electron Microscopy and crystallography of membrane proteins in cancer, metabolic diseases and neuropsychiatric disease
We aim to understand the fundamental causes of diseases such as cancer, metabolic diseases, neuropsychiatric disorders such as autism, bipolar disease and schizophrenia, and inflammation. All of these diseases are caused by failure of cells to make proteins that are properly folded and functional. In the Carpenter lab we study a particularly challenging set of molecules, proteins that are embedded within the membranes of cells and organelles. There are now many studies in progress to identify th ...
Structural studies of human potassium and calcium channels in pain and heart disease
Ion channels are critical for the function of almost every aspect of the human body, playing essential roles in the brain, nervous system and the heart. Defects in these channels lead to a host of medical conditions such as heart disease, cystic fibrosis and depression. We aim to study these complex, membrane embedded proteins using a range of structure determination techniques (X-ray crystallography, serial femtosecond crystallography, Electron microscopy) to solve their three dimensional stru ...
ZMPSTE24, lamin processing and premature ageing: A structure/function study
One of the most serious medical needs for the next 30 years will be to address the adverse consequences of ageing in the general population. A recent UN report predicted that by 2050, 22% of the world’s population will be over age 60. Maintaining physical and mental health, requires increasing intervention with age, with most requiring at least three forms of routine treatment in their 80s. As the population ages the impact on society will increase relentlessly. There is therefore a pressing nee ...