Deregulation of the ubiquitin ligase E6 associated protein (E6AP) encoded by the UBE3A gene has been associated with three different clinical pictures. Hijacking of E6AP by the E6 oncoprotein of distinct human papillomaviruses (HPV) contributes to the development of cervical cancer, whereas loss of E6AP expression or function is the cause of Angelman syndrome, a neurodevelopmental disorder, and increased expression of E6AP has been involved in autism spectrum disorders. Although these observations indicate that the activity of E6AP has to be tightly controlled, only little is known about how E6AP is regulated at the posttranslational level. Here, we provide evidence that the hydrophobic patch of ubiquitin comprising Leu-8 and Ile-44 is important for E6AP-mediated ubiquitination, whereas it does not affect the catalytic properties of the isolated catalytic HECT domain of E6AP. Furthermore, we show that the HPV E6 oncoprotein rescues the disability of full-length E6AP to use a respective hydrophobic patch mutant of ubiquitin for ubiquitination and that it stimulates E6AP-mediated ubiquitination of Ring1B, a known substrate of E6AP, in vitro and in cells. Based on these data, we propose that E6AP exists in at least two different states, an active and a less active or latent one, and that the activity of E6AP is controlled by noncovalent interactions with ubiquitin and allosteric activators such as the HPV E6 oncoprotein.I n eukaryotes, posttranslational modification of proteins by ubiquitin plays a pivotal role in the regulation of many cellular processes, including cell cycle, DNA metabolism (e.g., DNA repair, transcription), and various signal transduction pathways (1-4). The specificity of the ubiquitin-conjugation system is mainly ensured by E3 ubiquitin ligases, which mediate the recognition of target proteins. Based on the presence of distinct domains and their mode of action, E3 proteins can be grouped into three families, RING/RING-like E3s, RING-in-between-RING (RBR) E3s, and HECT E3s (5-7). All E3s have interaction sites for both substrate proteins and E2 ubiquitinconjugating enzymes. However, whereas in the case of RBR E3s and HECT E3s, ubiquitin is transferred from the E3 to substrates, RING/RING-like E3s function as adaptors between substrates and E2s (i.e., ubiquitin is transferred from the E2 to the substrate).E6AP, the founding member of the HECT E3 family, was originally identified as an interacting protein of the E6 oncoprotein of cancer-associated human papillomaviruses (HPVs) (8, 9). The E6-E6AP complex targets the tumor suppressor p53 and other proteins-which in the absence of E6 are not targeted by E6AP-for ubiquitination and degradation thereby contributing to HPV-induced cervical carcinogenesis (10, 11). In 1997, it was recognized that alterations in the UBE3A gene, which encodes E6AP, resulting in loss of E6AP expression or in the expression of E6AP variants with compromised E3 activity, are the cause of the Angelman syndrome (AS), a neurodevelopmental disorder (12-14). Recently, it was rep...

Ubiquitylation is a complex posttranslational protein modification and deregulation of this pathway has been associated with different human disorders. Ubiquitylation comes in different flavors: Besides mono-ubiquitylation, ubiquitin chains of various topologies are formed on substrate proteins. The fate of ubiquitylated proteins is determined by the linkage-type of the attached ubiquitin chains, however, the underlying mechanism is poorly characterized. Herein, we describe a new method based on codon expansion and click-chemistry-based polymerization to generate linkage-defined ubiquitin chains that are resistant to ubiquitin-specific proteases and adopt native-like functions. The potential of these artificial chains for analyzing ubiquitin signaling is demonstrated by linkage-specific effects on cell-cycle progression.

Cancer is a leading cause of death worldwide. Functional inactivation of tumor suppressor proteins, mainly by mutations in the corresponding genes, is a key event in cancer development. The fragile histidine triade protein (Fhit) is a tumor suppressor that is frequently affected in different cancer types. Fhit possesses diadenosine triphosphate hydrolase activity, but although reduction of its enzymatic activity appears to be important for exerting its tumor suppressor function, the regulation of Fhit activity is poorly understood. Here, we introduce a novel fluorogenic probe that is suited to selectively analyze the enzymatic activity of Fhit in extracts derived from human cells. This novel method will allow in-depth insight into the mechanisms involved in Fhit regulation in biologically relevant setups and, thus, into its role in the development of cancer.

The Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC) has become increasingly important in the conjugation chemistry of biomolecules. For example, it is an efficient and convenient method to generate defined ubiquitin-protein conjugates. Here, we investigate the effect of surfactants on the efficiency of CuAAC for chemical protein ubiquitylation. We found that anionic surfactants enhance conjugate formation by up to 10-fold resulting in high yields even at low (i.e., micromolar) concentrations of the reactants. Notably, the herein investigated conjugates are functional and thus properly folded.

Krebs ist eine der Haupttodesursachen weltweit. Die funktionelle Inaktivierung von Tumorsuppressoren, hauptsächlich durch Mutationen in den entsprechenden Genen, ist ein Schlüsselereignis in der Krebsentstehung. Das "fragile histidine triade protein" (Fhit) ist ein Tumorsuppressor, der in verschiedenen Krebsarten verändert ist. Fhit hat Diadenosintriphosphat-Hydrolase-Aktivität, aber obwohl die Reduktion dieser enzymatischen Aktivität wichtig für seine Tumorsuppressor-Aktivität zu sein scheint, ist deren Regulation unzureichend verstanden. In dieser Arbeit stellen wir eine neuartige fluorogene Sonde vor, die geeignet ist, die enzymatische Aktivität von Fhit in Extrakten humaner Zelllinien zu untersuchen. Diese neue Methode wird es ermçglichen, tiefgreifende Einblicke in die Mechanismen, die an der Regulation von Fhit beteiligt sind, zu erhalten und somit seine Rolle in der Krebsentstehung besser zu verstehen.