Dokument

Summary:
In den meisten Prokaryoten erfolgt die Zellteilung mittels eines Multi-Proteinkomplexes, dem Divisom. Das Grundgerüst des Divisoms bildet der sogenannte Z-Ring, eine ringähnliche Struktur, die durch die Polymerisation von einzelnen FtsZ-Molekülen in der Zellmitte gebildet wird. Eine erfolgreiche Zellteilung hängt maßgeblich von der zeitlichen und räumlichen Regulation der Entstehung des Z-Rings sowie der Assemblierung und Stabilisierung des Divisoms ab. Es wurden bereits Faktoren, die zur positiven Regulation des Zellteilungsapparates beitragen wurden bereits identifiziert. Jedoch scheinen diese in den meisten Fällen eine speziesspezifische Funktion während der Zellteilung auszuführen. Es stellt sich daher die Frage, welche zusätzlichen Faktoren in C. crescentus für eine effiziente Zellteilung verantwortlich sind. Im Rahmen dieser Arbeit konnte das neue Zellteilungsprotein CedX (cell divison protein X) identifiziert werden. CedX ist ein prolinreiches Membranprotein, welches in Abhängigkeit von FtsZ und FtsN zur Zellteilungsebene rekrutiert wird. Interessanterweise inhibierte eine Überproduktion von CedX die Zellteilung und führte gleichzeitig zur Entstehung von mehreren Z Ringen in den Zellfilamenten. Diese Z-Ringe waren jedoch nicht mehr in der Lage, eine Zellteilung einzuleiten. Eine funktionelle Analyse von verkürzten Proteinvarianten von CedX zeigte, dass sowohl die Membranverankerung als auch die prolinreiche Region für die räumliche Lokalisation und den beobachteten Überproduktionsphänotyp von CedX eine Rolle spielen. Mittels Co-Immunpräzipitation und einer bakteriellen Zwei-Hybrid-Analyse konnte nachgewiesen werden, dass CedX nicht nur mit den essentiellen Zellteilungsproteinen FtsZ und FtsN, sondern auch mit weiteren, späten Zellteilungsproteinen interagiert. Co-Lokalisationsversuche mit fluoreszenz-markierten Derivaten von FtsZ, FtsA, FtsN und CedX bestätigten, dass CedX zu den späten Zellteilungsproteinen zählt. Bis zum jetzigen Zeitpunkt konnten jedoch keine Bedingungen ermittelt werden, unter denen CedX essentiell für die Zellteilung in C. crescentus wird. Zusammengefasst lässt sich feststellen, dass CedX ein akzessorisches Zellteilungsprotein ist, welches wahrscheinlich den Einbau von späteren Zellteilungskomponenten in den Zellteilungsapparat unterstützt. Dabei wird die stabilisierende Wirkung des Proteins wahrscheinlich über den unstrukturierten, prolinreichen Abschnitt im C-terminalen Bereich vermittelt. Neben der Zellenteilung ist die Ausbildung einer Prostheka, auch Stiel genannt, eine weitere charakteristische Morphologieänderung. Der Stiel von C. crescentus ist eine dünne Verlängerung des Zellkörpers, welche weder DNA noch Ribosomen enthält und weitestgehend frei von zytoplasmatischen Proteinen ist. Darüber hinaus wird der Stiel in unregelmäßigen Abständen von scheibenartigen Strukturen, sogenannten Querbalken, segmentiert. Es wird vermutet, dass Crossbands eine strukturelle oder stabilisierende Funktion im Stiel übernehmen. Obwohl schon mehrere Studien zur Aufklärung der physiologischen Bedeutung des Stiels durchgeführt wurden, ist bis jetzt nur wenig über die Bildung und Funktion der Prosthecae sowie der rätselhaften Querbalken bekannt. Auf der Suche nach Faktoren, die für die Biogenese und/oder Morphogenese des Stiels in C. crescentus verantwortlich sind, konnten im zweiten Teil dieser Arbeit die vier neuen Stielproteine StpABCD (stalk proteins ABCD) identifiziert werden. Es konnte gezeigt werden, dass die Synthese von StpABCD mit der Initiierung des Stielwachstums erfolgt. Im Zuge der Assemblierung der Querbalken im periplasmatischen Raum des Stiels übernimmt StpA eine entscheidende „Ankerfunktion“ für die korrekte Lokalisierung von StpBCD. Darüber hinaus wurde mithilfe von Co-Immunpräzipitation nachgewiesen, dass StpABCD in einem Proteinkomplex vorliegen. Mittels Fluoreszenzmikroskopie konnte gezeigt werden, dass StpABCD eine charakteristische subzelluläre Position einnehmen, die mit der natürlichen Verteilung der Querbalken in den Prosthecae korreliert. Wurden in Zellen die Gene für stpAB deletiert, konnten keine Querbalken mehr mittels Kryo-Elektronenmikroskopie nachgewiesen werden. Um festzustellen, ob Zellen mit bzw. ohne Querbalken intrazelluläre Kompartimentierung aufweisen, wurde die Mobilität von fluoreszenz-markierten Proteinen mithilfe von „FLIP“ und „pulse-labeling“ Experimenten getestet. Diese Versuche verdeutlichten, dass Crossbands in der Tat für die Kompartimentierung von inneren und äußeren Membranproteinen sowie periplasmatischen Proteinen verantwortlich sind. Aus diesen Ergebnissen lässt sich daher schließen, dass StpABCD für die Synthese der Querbalken im Stiel von C. crescentus verantwortlich sind. Diese Strukturen wirken dabei als Diffusionsbarrieren, welche den periplasmatischen Raum zwischen Stiel und Zellkörper separieren.

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