Filled pockets: Epothilones initiate apoptosis in eukaryotic cells by inducing extensive polymerization of αβ-tubulin. Until recently, little was known about the binding mode of epothilones within the tubulin binding cavity. The structure of the epothilone A/tubulin complex (see picture) determined by electron crystallography has dramatically reversed the situation.
Im Zusammenhang mit der Beantragung von Sozialleistungen spielen der Umgang mit Anträgen, wie auch die Beratung und Begleitung im Vorfeld eines Antrags eine bedeutsame Rolle, wie auch eben Verfahrensvorschriften, insbesondere des Sozialgesetzbuchs I zeigen. Es kommt hierbei mitunter zu Fehlern, die hier u. a. beschrieben werden und zu Korrekturmöglichkeiten bei Fehlberatung. Im nachfolgenden Beitrag werden Fragestellungen in diesen Zusammenhängen aufgegriffen.
The facultative intracellular human pathogenic bacterium Listeria monocytogenes actively recruits host actin to its surface to achieve motility within infected cells. The bacterial surface protein ActA is solely responsible for this process by mimicking fundamental steps of host cell actin dynamics. ActA, a modular protein, contains an N-terminal actin nucleation site and a central proline-rich motif of the 4-fold repeated consensus sequence FPPPP (FP4). This motif is specifically recognized by members of the Ena/VASP protein family. These proteins additionally recruit the profilin-G-actin complex increasing the local concentration of G-actin close to the bacterial surface. By using analytical ultracentrifugation, we show that a single ActA molecule can simultaneously interact with four Ena/VASP homology 1 (EVH1) domains. The four FP4 sites have roughly equivalent affinities with dissociation constants of about 4 µm. Mutational analysis of the FP4 motifs indicate that the phenylalanine is mandatory for ActA-EVH1 interaction, whereas in each case exchange of the third proline was tolerated. Finally, by using sedimentation equilibrium centrifugation techniques, we demonstrate that ActA is a monomeric protein. By combining these results, we formulate a stoichiometric model to describe how ActA enables Listeriato utilize efficiently resources of the host cell microfilament for its own intracellular motility.
The lysozyme from bacteriophage T4 is being used as a model system to determine the roles of individual amino acids in the folding and stability of a typical globular protein. One general finding is that the protein is very adaptable, being able to accommodate many potentially destabilizing replacements.In order to determine the importance of 'α-helix propensity' in protein stability, different replacements have been made within α-helical segments of T4 lysozyme. Several such substitutions of the form Xaa → Ala increase the stability of the protein, supporting the idea that alanine is a strongly helix-favouring amino acid. It is possible to engineer a protein that has up to ten alanines in succession, yet still folds and has normal activity. This illustrates the redundancy that is present in the amino acid sequence.A number of 'cavity-creating' mutants of the form Leu → Ala have been constructed to understand better the nature of hydrophobic stabilization. The structural consequences of these mutations differ from site to site. In some cases the protein structure hardly changes at all; in other cases removal of the wild-type side-chain allows surrounding atoms to move in and occupy the vacated space, although a cavity always remains. The destabilization of the protein associated with these cavity-creating mutations also varies from case to case. The results suggest how to reconcile recent conflicting reports concerning the strength of the hydrophobic effect in proteins.
