Proteins and their interactions are fundamental to cellular life. Disruption of protein-protein, protein-RNA, or protein-DNA interactions can lead to disease, by affecting the function of protein complexes or by affecting gene regulation. A better understanding of these interactions on the molecular level gives rise to new methods to predict protein interaction, and is critical for the rational design of new therapeutic agents that disrupt disease-causing interactions. This thesis consists of three parts that focus on various aspects of protein interactions and their prediction in the context of disease. In the first part of this thesis, we classify interfaces of protein-protein interactions. We do so by systematically computing all binding sites between protein domains in protein complex structures solved by X-ray crystallography. The result is SCOPPI, the Structural Classification of Protein Protein Interfaces. Clustering and classification of geometrically similar interfaces reveals interesting examples comprising viral mimicry of human interface binding sites, gene fusion events, conservation of interface residues, and diversity of interface localisations. We then develop a novel method to predict protein interactions which is based on these structural interface templates from SCOPPI. The method is applied in three use cases covering osteoclast differentiation, which is relevant for osteoporosis, the microtubule-associated network in meiosis, and proteins found deregulated in pancreatic cancer. As a result, we are able to reconstruct many interactions known to the expert molecular biologist, and predict novel high confidence interactions backed up by structural or experimental evidence. These predictions can facilitate the generation of hypotheses, and provide knowledge on binding sites of promising disease-relevant candidates for targeted drug development. In the second part, we present a novel algorithm to search for protein binding sites in RNA sequences. The algorithm combines RNA structure prediction with sequence motif scanning and evolutionary conservation to identify binding sites on candidate messenger RNAs. It is used to search for binding sites of the PTBP1 protein, an important regulator of glucose secretion in the pancreatic beta cell. First, applied to a benchmark set of mRNAs known to be regulated by PTBP1, the algorithm successfully finds significant binding sites in all benchmark mRNAs. Second, collaborators carried out a screen to identify changes in the proteome of beta cells upon glucose stimulation while inhibiting gene expression. Analysing this set of post-transcriptionally controlled candidate mRNAs for PTBP1 binding, the algorithm produced a ranked list of 11 high confident potential PTBP1 binding sites. Experimental validation of predicted targets is ongoing. Overall, identifying targets of PTBP1 and hence regulators of insulin secretion may contribute to the treatment of diabetes by providing novel protein drug targets or by aiding in the design of novel RNA-binding therapeutics. The third part of this thesis deals with gene regulation in disease. One of the great challenges in medicine is to correlate genotypic data, such as gene expression measurements, and other covariates, such as age or gender, to a variety of phenotypic data from the patient. Here, we address the problem of survival prediction based on microarray data in cancer patients. To this end, a computational approach was devised to find genes in human cancer tissue samples whose expression is predictive for the survival outcome of the patient. The central idea of the approach is the incorporation of background knowledge information in form of a network, and the use of an algorithm similar to Google s PageRank. Applied to pancreas cancer, it identifies a set of eight genes that allows to predict whether a patient has a poor or good prognosis. The approach shows an accuracy comparable to studies that were performed in breast cancer or lymphatic malignancies. Yet, no such study was done for pancreatic cancer. Regulatory networks contain information of transcription factors that bind to DNA in order to regulate genes. We find that including background knowledge in form of such regulatory networks gives highest improvement on prediction accuracy compared to including protein interaction or co-expression networks. Currently, our collaborators test the eight identified genes for their predictive power for survival in an independent group of 150 patients. Under a therapeutic perspective, reliable survival prediction greatly improves the correct choice of therapy. Whereas the live expectancy of some patients might benefit from extensive therapy such as surgery and chemotherapy, for other patients this may only be a burden. Instead, for this group, a less aggressive or different treatment could result in better quality of the remaining lifetime. Conclusively, this thesis contributes novel analytical tools that provide insight into disease-relevant interactions of proteins. Furthermore, this thesis work contributes a novel algorithm to deal with noisy microarray measurements, which allows to considerably improve prediction of survival of cancer patients from gene expression data.

Gegenstand der vorliegenden Arbeit ist das Kerngen CBS2 aus Saccharomyces cerevisiae. Cbs2p wird gemeinsam mit Cbs1p spezifisch für die Translation der Cytochrom b (COB)-mRNA in den Mitochondrien benötigt. Die Untersuchungen konzentrierten sich auf die Charakterisierung funktionell wichtiger Bereiche im N- und C-terminalen Bereich des Proteins, den Nachweis von Protein-Protein Wechelwirkungen, die Assoziation von Cbs2p mit mitochondrialen Ribosomen und die Bedeutung des N-terminalen Bereiches für den Import von Cbs2p in die Mitochondrien. Die aminoterminalen 35 Aminosäuren (As) von Cbs2p genügen, um das Reporterprotein Gfp vollständig in die Mitochondrien zu rekrutieren. Dieses Ergebnis zeigt, dass der N-Terminus von Cbs2p den Import von Proteinen in die Mitochondrien vermitteln kann. Da ein N-terminal um 35 As verkürztes Cbs2-Protein ebenfalls noch in den Mitochondrien nachweisbar ist, besitzt Cbs2p wenigstens ein weiteres, vom N-Terminus unabhängiges, internes oder C-terminal lokalisiertes Importsignal für die Mitochondrien. Aufgrund genetischer Daten ist Abc1p ein potenzieller Interaktionspartner von Cbs2p. Es konnte gezeigt werden, dass eine physikalische Wechselwirkung zwischen beiden Proteinen stattfindet. Mittels Blauer Nativ-Gelelektrophorese wurde Cbs2p in einem höher molekularen Proteinkomplex nachgewiesen. Da Cbs2p in der Lage ist, in vitro Homomere zu bilden, sprechen die Daten dafür, das Cbs2p als Multimer in diesem Komplex vorliegt. Es konnte weiterhin gezeigt werden, dass der N-Terminus von Cbs2p eine essentielle Rolle bei der Homomerisierung des Proteins spielt. Die vorliegenden Ergebnisse erweitern das von Michaelis et al. (1991) entwickelte Modell der Wirkungsweise der Translationsaktivatoren Cbs1p und Cbs2p wie folgt: Cbs1p und Cbs2p sind mit der inneren Membran assoziiert. Beide Proteine könnten die COB-mRNA an die mitochondriale Innenmembran führen. Der Kontakt zwischen der COB-mRNA und der mitochondrialen Translationsmaschinerie könnte durch die Assoziation von Cbs2p mit mitochondrialen Ribosomen hergestellt werden. Nur an der Membran erlaubt die Prozessierungs- und Translationsmaschinerie die Reifung von COB-mRNA und die Synthese und Assemblierung von Cytochrom b. Das Vorliegen von Cbs2p in einem hoch molekularen Komplex und die physikalische Wechselwirkung mit Abc1p könnten ein Hinweis dafür sein, dass die Translation in räumlicher Nähe des Assemblierungsortes von Cytochrom b, dem bc1-Komplex, stattfindet.