George T. Lountos, Ph.D.
Dr. Lountos is currently involved in a number of highly collaborative research projects within NCI at Frederick involving the structural determination of proteins engaged in cancer and infectious diseases with particular emphasis on the structure-based design of novel small molecular inhibitors. A variety of targets are currently under investigation, including protein kinases and phosphatases, viral proteases, the SUMO-conjugating enzyme, Ubc9, tyrosyl-DNA phosphodiesterase I (TDP1) and other enzymes. Dr. Lountos played a pivotal role in solving high-resolution crystal structures of human checkpoint kinase 2 (Chk2) in complex with various inhibitors that have enabled the rational design of potent and specific inhibitors with anti-cancer activity.
1) crystallization of macromolecules including protein-ligand complexes, 2) X-ray diffraction data collection, 3) structure determination of macromolecules, 4) structure-based drug design, 5) fragment-based drug discovery, 6) protein expression and purification
Structure-Based Drug Design. Under the direction of Dr. David Waugh, a variety of drug targets involved in cancer and infectious disease are currently being studied by X-ray crystallography in collaboration with medicinal chemists and biochemists within the CCR. Our goal is to elucidate the structural basis for inhibitor binding as a first step towards the optimization of these inhibitors by rational design. In addition to studying protein-inhibitor complexes of compounds identified from high-throughput screening, our group is leading fragment-based drug discovery efforts for drug targets by screening libraries of small drug-like fragments by crystallographic methods. X-ray crystallography is a very powerful tool that can be used to identify and confirm binding of low-affinity fragments to proteins that are not detectable by other screening methods. Structural information gleaned from these studies can be used to optimize low-affinity fragments into inhibitors with higher potency while retaining favorable drug-like properties.
Current targets under collaborative investigation using fragment-screening methods include the SUMO-conjugating enzyme, Ubc9 (Dr. Jay Schneekloth, Chemical Biology Laboratory) and tyrosyl-DNA phosphodiesterase I (Dr. Yves Pommier, LMP). Very recently, we have solved the first high-resolution crystal structure of the main protease (3CLpro) from the highly pathogenic Middle East Respiratory Syndrome (MERS) virus and are initiating fragment screening for this important drug target as well. In collaboration with Dr. Terrence Burke (Chemical Biology Laboratory) and Dr. Robert Ulrich (USAMRIID), we have used structure-based design methods to develop highly potent and specific inhibitors of the Yersinia pestis protein tyrosine phosphatase, YopH. Our group also has played a pivotal role in the rational design of human checkpoint kinase 2 inhibitors in collaboration with Dr. Yves Pommier and Dr. Robert Shoemaker (NCI Division of Cancer Prevention). Currently, we are extending structure-based drug design efforts to identify new inhibitors of the protein tyrosine phosphatase, PTPepsilon, in collaboration with Dr. Terrence Burke and Dr. Robert Ulrich.
Structure-Function Studies of Biological Macromolecules. The human dual-specificity phosphatases (DUSPs) are a subclass of the protein tyrosine phosphatase (PTP) family with the ability to dephosphorylate both phosphotyrosine and phosphoserine/phosphothreonine residues. In collaboration with Dr. Robert Ulrich (USAMRIID) and the MCL Protein Purification Core, we are studying the structures of several DUSPs by X-ray crystallography to complement biochemical studies of interactions with substrates. We are developing novel assays to study protein-ligand interactions as well as high-throughput screening methods for inhibitor discovery. To this end, we have solved the first crystal structures of several human DUSPs including DUSP7, DUSP14, DUSP27 as well as co-crystal structure of DUSP22 in complex with a substrate analog.
Our group is also engaged in applying several protein engineering and crystallization rescue strategies such as reductive methylation, surface-entropy reduction mutagenesis and protein construct optimization to achieve the crystallization of macromolecules for structural studies.
Selected Key Publications
Structure of the Trypanosoma cruzi protein tyrosine phosphatase TcPTP1, a potential therapeutic target for Chagas’ disease.Mol. Biochem. Parasitol. 187: 1-8, 2013. [ Journal Article ]
Structural characterization of inhibitor complexes with checkpoint kinase 2 (Chk2), a drug target for cancer therapy.J. Struct. Biol. 176: 292-301, 2011. [ Journal Article ]
Utilization of nitrophenylphosphates and oxime-based ligation for development of nanomolar affinity inhibitors of the Yersinia pestis outer protein H (YopH) phosphatase .J. Med. Chem. 54: 2933-43, 2011. [ Journal Article ]
Cellular inhibition of checkpoint kinase 2 (Chk2) and potentiation of camptothecins and radiation by the novel Chk2 inhibitor PV1019.J. Pharmacol. Exp. Ther. 331: 816-26, 2009. [ Journal Article ]
Crystal structure of checkpoint kinase 2 in complex with NSC 109555, a potent and selective inhibitor.Protein Sci. 18: 92-100, 2009. [ Journal Article ]
Dr. Lountos received his undergraduate degree in chemistry with high honors(2000) from the Georgia Institute of Technology where he performed research in organic chemistry in the laboratory of Prof. Suzy Beckham. He obtained a Ph.D. in chemistry (2005) from the School of Chemistry & Biochemistry, Georgia Institute of Technology, under the direction of Prof. Allen Orville where he conducted crystallographic studies of flavin-dependent enzymes. From 2006-2011, he was a postdoctoral fellow in the Protein Engineering Section of the Macromolecular Crystallography Laboratory, under the direction of Dr. David Waugh. His postdoctoral research focused on structural studies of proteins involved in the Yersinia pestis type III secretion system and structure-based drug design of inhibitors of the human checkpoint kinase 2 and Yersinia pestis YopH,
Dr. Lountos was the recipient of a Federal Technology Transfer Award in 2008 and the NIH Fellows Award for Research Excellence (FARE) in 2009 and in 2010. In 2011, he was appointed as Scientist I with the Basic Research Program, Leidos Biomedical Research, Inc., where he works in support of the Macromolecular Crystallography Laboratory. Dr. Lountos is a member of the American Crystallographic Association (ACA) and is currently serving as chair of the ACA Young Scientist Scientific Interest Group (2015).