Regulation in Cellular Signal Transduction


Cells respond to a large variety of signals using only a limited set of signaling modules which are organized in complex networks. The activity of these signaling switches is spatially and temporally regulated, and this regulation governs normal development and cellular homeostasis. Consequently, uncontrolled activation is a hallmark of many diseases such as cancer. It is therefore not surprising that the cell has intricate regulatory mechanisms in place to control signal transduction.

Our lab is interested in deciphering the basic regulatory principles in signal transduction networks on a molecular level using a combination of X-ray crystallography, biophysical and cellular approaches. We focus on two main areas of research: Bacterial signaling controlling biofilm formation and pathogenicity, and endocytosis in eukaryotes.


  1. -More details coming soon! -


Major support:

 




Past research (carried out in the Kuriyan Lab)


The Ras superfamily comprises signaling switches which first were discovered as productive oncogenes. In eukaryotic cells, proper activation of Ras is crucial for cell mobility, differentiation and survival. Ras is kept under strict control, and its unregulated activation is a consistent hallmark of many cancers (Coleman et al., 2004). Anchored to the plasma membrane by lipid modifications, Ras switches between inactive GDP-bound and active GTP-bound states, catalyzed by nucleotide exchange factors (RasGEFs) and GTPase activating proteins (RasGAPs), respectively. The classic model for Ras activation focuses primarily on the recruitment of the RasGEF Son of Sevenless (SOS) to the membrane where it engages Ras, and the basic molecular mechanism of this activation is well understood (Boriack-Sjodin et al., 1998). Recent studies have revealed a more sophisticated network of regulatory mechanisms which inhibit or stimulate Ras signaling, or fine-tune its response, involving allosteric modulation and spatial segregation of Ras and its effectors (Margarit et al., 2003; Murakoshi et al., 2004; Prior et al., 2003).


SOS is a large multi-domain protein containing an N-terminal Histone fold, a Dbl/Pleckstrin homology (DH/PH) module and a catalytic unit for Ras nucleotide exchange. During my postdoctoral research I solved several structures of SOS and Ras:SOS complexes using X-ray crystallography and, in combination with kinetic assays, dissected molecular mechanisms underlying the regulation of SOS. This work led to the discovery of a previously unexpected positive feedback loop, in which activated Ras allosterically upregulates the nucleotide exchange activity of SOS. In addition to binding to the classical active site of SOS, crystallographic analysis revealed that Ras can simultaneously associate with a second site on the catalytic module (Margarit et al., 2003). The crystal structure of an autoregulated SOS in conjunction with extensive mutagenesis studies demonstrated that this distal Ras-binding site on SOS is of fundamental importance for SOS activity (Sondermann et al., 2004).


Finally, I determined the crystal structure of the N-terminal domain of SOS which resembles the structure of a histone dimer (Sondermann et al., 2003). Using isothermal titration calorimetry, I could show that the histone domain binds with high affinity to a fragment of SOS lacking the histone domain, an interaction which, in full-length SOS, occurs intramolecularly. To analyze this supramolecular assembly, we combined high resolution structures, computational docking and small angle X-ray scattering, which provided novel insight into the architecture and regulation of SOS (Sondermann et al., in press). We identified the binding site of the histone domain in SOS by computational docking, and validated the results by mutagenesis, binding assays, and solution small angle X-ray scattering.


In the docked model, the histone fold domain bridges the DH, PH and catalytic domain by binding to the helical linker connecting the PH and catalytic domains. This places a patch of the histone domain with positive electrostatic potential next to the phosphatidylinositol binding site of SOS. We propose a model in which the histone domain serves as an additional anchor point for membrane interactions of SOS. Interestingly, the predicted membrane interaction site corresponds to the surface in nucleosomal histones used for DNA binding suggesting the conservation of functionality of those surface patches. The orientation and position of the histone domain and its predicted membrane interaction will help to explain how membrane-dependent activation signals can potentially release the autoinhibition of SOS.



  1. •Boriack-Sjodin, P. A., Margarit, S. M., Bar-Sagi, D., and Kuriyan, J. (1998). Nature 394, 337-343.

  2. •Coleman, M. L., Marshall, C. J., and Olson, M. F. (2004). Nat Rev Mol Cell Biol 5, 355-366.

  3. •Margarit, S. M., Sondermann, H., Hall, B. E., Nagar, B., Hoelz, A., Pirruccello, M., Bar-Sagi, D., and Kuriyan, J. (2003). Cell 112, 685-695.

  4. •Murakoshi, H., Iino, R., Kobayashi, T., Fujiwara, T., Ohshima, C., Yoshimura, A., and Kusumi, A. (2004). Proc Natl Acad Sci USA 101, 7317-7322.

  5. •Prior, I. A., Muncke, C., Parton, R. G., and Hancock, J. F. (2003). J Cell Biol 160, 165-170.

  6. •Sondermann, H.*, Soisson, S.M.*, Boykevisch, S., Yang, S.-S., Bar-Sagi, D., and Kuriyan, J. (2004). Cell 119(3):393-405. (*contributed equally)

  7. •Sondermann, H., Soisson, S.M., Bar-Sagi, D., and Kuriyan, J. (2003). Structure 11(12):1583-1593.

  8. •Sondermann, H., Nagar, B., Bar-Sagi, D., and Kuriyan, J. (2005). Proc Natl Acad Sci USA: 102(46):16632-16637.