In eukaryotic cells, the separation of DNA/transcription from ribosomes/translation by the nuclear envelope necessitates efficient and selective passage of macromolecules through the nuclear pore complex. In human cells, 19 different Karyopherinβ (Kapβs /Importins/Exportins) proteins transport macromolecules in and out of the nucleus by recognizing distinct nuclear localization or export signals (NLSs or NESs). Thus, Kapβs are critically involved in cellular processes such as gene expression, signal transduction, immune response, oncogenesis and viral propagation, all of which require proper nucleocytoplasmic targeting.
Research in our lab is directed towards understanding the physical and cellular mechanisms of Kapβs. We would like to understand how the macromolecular nuclear traffic patterns coordinated by the 19 human Kapβs contribute to overall cellular organization. Why does the cell need so many different Kapβs? Are their collections of cargoes random except for a common signal? Or do individual Kapβs transport cargoes with distinct cellular functions? Our research aims to answer these fundamental questions.
Only two classes of NLS are currently known. The short lysine-rich classical-NLS was discovered in the 1980s and is recognized by Kap&alpha/Kapβ1 (or Importinα/Importin β). In 2006, our lab defined a new NLS class termed the PY-NLS. Using structural, biochemical and bioinformatics approaches, we defined a set of physical rules for PY-NLS recognition by Kapβ2 (or Transportin) and used these to discover new candidate cargoes.
The only known NES is the leucine-rich NES, which is recognized by Kapβ CRM1. We have recently solved the crystal structure of CRM1 bound to leucine-rich NES containing cargo Snurportin 1. The structure explained the recognition of this general export signal, the more complex bipartite recognition of Snurportin 1 by CRM1 and the inhibition of CRM1 by Leptomycin B. The structure also revealed mechanisms of the different steps of nuclear export such as export complex assembly in the nucleus and disassembly in the cytoplasm.
With the exception of Kapβ1, Kapβ2 and CRM1, the other 16 Kapβs are currently known to each recognize only a few diverse sequences, making it extremely difficult to identify their NLSs/NESs. Nevertheless, large sizes, low sequence identities and multiple cargo binding sites for Kapβs all suggest that dozens of NLSs and NESs have yet to be discovered. Our Kapβ2 and CRM1 work now serve as model systems for future discovery of NLSs and NESs. We have discovered complex signals using a collection of physical rules rather than specific sequence motifs alone. This concept could be expanded to study numerous obscure targeting signals in eukaryotic cells and other biological recognition processes that involve linear recognition motifs with weak and obscure consensus sequences.
RESEARCH INTERESTS
Protein transport, nucleocytoplasmic transport, targeting signals
Structural Biology
Biophysics
Bioinformatics
Cell Biology
RECENT PUBLICATIONS
Suel, K. E. and Chook, Y. M., "Kap104p imports the PY-NLS-containing transcription factor Tfg2p into the nucleus." Journal of Biological Chemistry, 284(23):15416-15424, June 2009
Dong, X., Biswas, A., Suel, K. E, Jackson, L. K., Martinez, R., Gu, H. and Chook, Y. M., "Structural basis for leucine-rich nuclear export signal recognition by CRM1." Nature, 458:1136-1142, April 2009
Dong, X., Biswas, A. and Chook, Y. M., "Structural basis for assembly and disassembly of the CRM1 nuclear export complex." Nature Structural and Molecular Biology, 16(5):558-560, April 2009
Zhang, Z. C. and Chook, Y. M., "Structural Analysis of Karyopherin-mediated nucleocytoplasmic transport" Landes Bioscience, Nuclear Transport:edited by Ralph Kehlenbach, 2008
Suel, K. E., Gu, H. and Chook, Y. M., "Modular organization and combinatorial energetics of proline-tyrosine nuclear localization signals." PLoS Biology, 6(6):e137, June 2008
SIGNIFICANT PUBLICATIONS
Cansizoglu, A. E. and Chook, Y. M., "Conformational heterogeneity of Karyopherin-beta2 is segmental." Structure, 15(11):1431-1441, November 2007
Cansizoglu, A. E., Lee, B. J., Zhang, Z. C., Fontoura, B. M. A. and Chook, Y. M., "Structure-based design of a pathway-specific nuclear import inhibitor." Nat. Struc. & Mol. Biol., 14(5):452-454, May 2007
Lee, B. J., Cansizoglu, A. E., Suel, K. E., Louis, T. H., Zichao Zhang and Chook, Y. M., "Rules for nuclear localization sequence recognition by Karyopherinβ2." Cell, 126:543-558, August 2006
Chook, Y.M., Jung, A., Rosen, M.K. and Blobel, G., "Uncoupling Ran binding and substrate dissociation in the Karyopherinβ2 nuclear import pathway." Biochemistry, 41:6955-6966, June 2002
Chook, Y.M. and Blobel, G., "Structure of the nuclear transport complex karyopherinβ2-Ran.GppNHp." Nature, 399:230-237, May 1999
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