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The tailspike protein of Salmonella phage P22 is a viral adhesion protein with both receptor binding and destroying activities. It recognises the O-antigenic repeating units of cell surface lipopolysaccharide of serogroup A, B and D1 as receptor, but also inactivates its receptor by endoglycosidase (endorhamnosidase) activity. In the final step of bacteriophage P22 assembly six homotrimeric tailspike molecules are non-covalently attached to the DNA injection apparatus, mediated by their N-terminal, head-binding domains. We report the crystal structure of the head-binding domain of P22 tailspike protein at 2.3 A resolution, solved with a recombinant telluromethionine derivative and non-crystallographic symmetry averaging. The trimeric dome-like structure is formed by two perpendicular beta-sheets of five and three strands, respectively in each subunit and caps a three-helix bundle observed in the structure of the C-terminal receptor binding and cleaving fragment, reported here after full refinement at 1.56 A resolution. In the central part of the receptor binding fragment, three parallel beta-helices of 13 complete turns are associated side-by-side, while the three polypeptide strands merge into a single domain towards their C termini, with close interdigitation at the junction to the beta-helix part. Complex structures with receptor fragments from S. typhimurium, S. enteritidis and S. typhi253Ty determined at 1.8 A resolution are described in detail. Insertions into the beta-helix form the O-antigen binding groove, which also harbours the active site residues Asp392, Asp395 and Glu359. In the intact structure of the tailspike protein, head-binding and receptor-binding parts are probably linked by a flexible hinge whose function may be either to deal with shearing forces on the exposed, 150 A long tailspikes or to allow them to bend during the infection process.

Diese Arbeit beschäftigt sich mit granularen Medien geringer Dichte - den so genannten granularen Gasen. Die Dynamik dieser dissipativen Systeme ist durch komplexe Strukturbildung gekennzeichnet: Aus einem anfänglich homogenen System bilden sich mit der Zeit Dichte- und Temperaturschwankungen heraus, die immer weiter anwachsen, bis sie schließlich Systemgröße erreicht haben. Im ersten Teil der Arbeit stellen wir mit der parallelisierten Ereignisdynamik ein Verfahren vor, mit dem sich granulare Gase effizient simulieren lassen. Im zweiten Teil der Arbeit untersuchen wir die Eigenschaften granularer Gase zum einen mit Hilfe theoretischer Konzepte wie kinetischer Gastheorie, linearer Stabilitätsanalyse der hydrodynamischen Bewegungsgleichungen, einer Clusterpopulationsdynamik und der Perkolationstheorie. Zum anderen führen wir umfangreiche Computersimulationen durch, um die Clusterbildung besser zu verstehen. Insgesamt erhalten wir mit Hilfe dieser Untersuchungsmethoden einen dreistufigen Verlauf des Clusterwachstums: Zunächst das homogene Regime das gut durch die Vorhersagen der kinetischen Gastheorie beschrieben wird, dann das Clusterwachstum, in dem sich ein großer Perkolationscluster herausbildet und schließlich das Sättigungsregime, wenn dieser Systemgröße erreicht hat., This thesis deals with granular gases -granular media of low density. The initial homogeneous cooling state of granular gases becomes unstable to perturbations ans the system develops inhomogenities in temperature and density, Clusters of particles emerge and start to grow, until finally, they reach system size. The first part of this thesis is concerned with event-driven molecular dynamics and its parallelization. The second part deals with the theoretical concepts and computer simulations of granular gases. The theoretical methods include kinetic theory, linear stability analysis of the hydrodynamical equations of motion, cluster population dynamics and percolation theory. Furthermore we use extensive computer simulations to gain deeper insight into the evolution of the clusters. The evolution of the clusters can be described in terms of three regimes, i.e. homogeneous cooling regime, cluster growth regime, and saturation regime.