Transmission and Signalling Across Membranes

A. Wittinghofer, M.R. Ahmadian, K. Scheezek	Structural and Mechanistic Studies on the GTPase of the Oncoprotein Ras	A
K. Rittinger, P. Walker, J. Eccleston, A. Tarricone, S. Gamblin, S. Smerdon	Structural Studies of rho-family Small G-Protien Activation	A
E. Baraldi, M. Hyvönen, M. Saraste, K. Djinovic	Crystal Structure of the Bruton's Tyrosine Kinase PH Domain in Complex with Ins(1,3,4,5)P4	A
O. Mayans, M. Gautel, M. Wilmanns	The Crystal Structure of the Serine/Threonine Kinase Domain of the Giant Muscle Protein Titin Reveals Unique Features for Tight Regulation and ATP Binding	A
D. Bossemeyer, A. Girod, V. Kinzel, R. Engh, R. Huber, L. Prade	cAMP-Dependent Protein Kinase Conformational Changes Induced by Staurosporine and Other Inhibitors	A
H.J. Snijder, N. Dekker, I. Ubarretxena, M. Blaauw, K.H. Kalk, H.M. Verheij, B.W. Dijkstra	Outer Membrane Phospholipase, an Integral Membrane Enzyme	A

This session was the first of a series, devoted to biology applications in X-ray crystallography. It started with a review by Alfred Wittinghofer (Dortmund, Germany) on structural studies on the regulation of the proto oncogene ras by GAP proteins, resulting in an acceleration of its the GTPase activity. The recent structure of the Ras-RasGAP complex shows how R789 of GAP (the "arginine finger") inserts into the switch II region of Ras. It triggers a conformational change in Ras, pivoted at Gly12 and Gln61, which in turn enhances GTP catalysis. Stephen J. Smerdon (Mill Hill, UK) described complimentary work on the regulation of the rho-family small G-proteins by rhoGAP proteins. Like the Dortmund group they have solved two structures of RhoGAP in complex with cdc42 and rhoA as well. The work of both groups has established a profound understanding on the activation mechanism of Ras and Ras related proteins, whose oncogenic forms are the cause of a significant fraction of human tumors. In the second part of the session Elena Baraldi (EMBL Heidelberg, Germany) described a complex structure of the Btk PH domain in complex with IP4. Mutants of Btk cause X-linked immunodeficiency (XLA) in humans. A "gain of function" mutant of this PH domain (E41K) surprisingly displays two IP4 binding sites, where the second one directly involves K41. Olga Mayans (EMBL Outstation, Hamburg, Germany) presented the crystal structure of the autoinhibited form of the serine kinase of the giant muscle protein titin. The structure has revealed blockage of the catalytic aspartate, common to all known protein kinases, by a tyrosine not from the activation segment but from the so-called P+1 loop. The structure has provided the basis to reveal the regulation of this kinase, a two-step activation mechanism by binding of Ca2+ / calmodulin and by specific phosphorylation of the tyrosine from the P+1 loop by a so far unknown upstream kinase. Because of the high potential of protein kinases for tumors and genetic diseases they represent an important target for the design for specific drugs. RICK ENGH (Martinsried, Germany) gave insight into structural studies on specific staurosporine inhibitors for cAMP dependent protein kinase, demonstrating the importance of experimental structural biology for effective modeling of inhibitors. Finally, H.J. Snijder (Groningen, The Netherlands) presented the first structure of a phospholipase that is an integrated membrane protein (OMPLA). In this structure the catalytic triad, thoroughly characterised for soluble lipases, is well conserved. The overall fold of this protein is formed by a beta-barrel of 12 strands. This topology is reminiscent of another family of integral membrane proteins, the porins with 16 or more strands. The audience was fascinated to see how biology has transfered the same catalytic activity into a completely different protein context (here the surface of the membrane).