Welcome to the homepage of the Ungermann lab. Our research is focussed on the machinery responsible for membrane dynamics (fission and fusion) at endosomes and lysosomes/yeast vacuoles. Our main interests are small regulators, the Rab GTPases Rab5 (Vps21) and Rab7 (Ypt7), and their interaction partners, in particular the tethering complexes CORVET and HOPS. A second major focus are membrane contact sites of vacuoles, in particular with mitochondria and the ER, which also depend on the Rab7-like Ypt7. 


















Endosomes and lysosomes are part of a very dynamic membrane system. During endocytosis, cell surface proteins (shown in red and green on the left figure) are transported to the lysosome/vacuole (labeled in blue in the right figure), which carry the Rab7-like Ypt7 protein (green). They pass first through the early endosome, which converts into the late endosome that then fuses with the lysosome. Defects in the endolysosomal system result in several serious diseases such as neuronal defects or bleeding disorders. An understanding of the underlying machinery thus offers opportunities to interfere with such defects.


To explore the role of factors involved in endosome and lysosome biogenesis, we follow different approaches, including in vivo assays to trace proteins in the cell, the isolation of large complexes, their biochemical and biophysical characterization, in vitro assays to measure organelle fusion and enzymatic activities, structural analyses and genetics. Main collaborators include Fulvio Reggiori (Groningen), Stefan Raunser (Dortmund), and Daniel Kümmel (Osnabrück). We profit from excellent support by the SFB 944 collaborative research center in Osnabrück and Münster and the CellNanOs groups with its excellent imaging facilities to monitor cellular microcompartments in vivo and in vitro.


Ongoing projects of the lab are:


* Function and structure of tethering complexes at endosome and vacuole

* Control of Rab5 and Rab7 activity by GEFs and GAPs

* Mechanism of endosomal maturation

* Physiology and function of vacuole membrane contact zones

  1. *Biogenesis of autophagosomes






Further reading: Endosomal and lysosomal biogenesis























Endosomes are fueled with membranes (via vesicles) by the endocytic and biosynthetic transport routes, which deliver both hydrolases, membrane proteins and fluids. The endosomes then undergo a dramatic alteration in their shape - they form internal vesicles by invaginating their own surface. This process sorts membrane, but also membrane proteins. The resulting organelle, the multivesicular body (MVB), finally fuses with the lysosome. During this process, endosomes do not only change their shape, but also their surface composition, they ‘mature’. This means that they loose for instance their initial Rab GTPase Rab5 (Vps21) and replace it by Rab7 (Ypt7), which is a prerequisite for its fusion with the lysosome. This process is highly cooperative and regulated, though the details of the coordination are poorly understood. It is possible that MVBs will only fuse with the lysosome if the ubiquitin-tagged membrane proteins have vanished from their surface - a process that requires the endosomal ESCRT complexes. Prior to this fusion, receptors that brought proteins to the MVB need to get back to the Golgi apparatus, which requires the retromer complex. It is not yet clear how endosomes coordinate recycling with fusion.




















Figure legend: Structure and interactions of the HOPS complex. (A) Purification of the HOPS complex from yeast cells. (B) Structure of the HOPS complex from negative stain electron microscopy. (C) Localization of Ypt7 on the HOPS identifies two Rab-binding sites. (D) Overall structure and subunit localization within the HOPS complex. (From Bröcker et al., 2012, in collaboration with Anne Kuhlee, Christos Gatsogiannis and Stefan Raunser, MPI Dortmund)


We are particularly interested in the mechanism and function of membrane dynamics at endosomes and lysosomes that underlie these transport processes. A main interest of the lab are Rab GTPases (the Rab5-like Vps21 and the Rab7-like Ypt7 proteins), which interact with tethering complexes like HOPS and CORVET. Over the last years, our lab has provided important insights into the machinery of endosomal and vacuolar membrane fusion. We identified and characterized the CORVET tethering complex (Peplowska et al., 2007; Balderhaar et al., 2013), the GEF for Ypt7, called Mon1-Ccz1 (Nordmann et al., 2010, Cabrera et al., 2014), the GAP module for Vps21 (Lachmann et al., 2012; John Peter et al., 2013), and solved the molecular architecture of the HOPS complex (Bröcker et al., 2012).


Recently, we identified novel links between the Ypt7 GTPase and organelle contact sites of the vacuole. In parallel to Maya Schuldiner’s group (Elbaz-Alon et al., 2014), we identified a contact zone of vacuoles with mitochondria, termed vCLAMPs (Hönscher et al., 2014). Another Ypt7-interactor is the Ivy1 protein, and inverted BAR protein that is found in sterol-rich domains of the vacuole, which is found proximal to a major metabolic regulator, the TOR complex (Numrich et al., 2015). The function of these sites are only slowly emerging, but may reveal novel concepts of organelle membrane homeostasis.


Our approaches


We use yeast as our model system to study membrane dynamics in the endomembrane system. Recently, we identified the CORVET tethering complex at the endosome, and could show that it shares four subunits (the Class C subunits Vps11, 16, 18 and 33) with the vacuolar HOPS complex (Peplowska et al., 2007; Balderhaar et al., 2013). These complexes share several activities, including Rab binding and activation, and SNARE binding. Recent evidence also points to a cross-talk between the Tor signaling pathway and the HOPS complex (Binda et al., 2009).


Maturation of endosomes therefore should include the coordination of several machineries - the turn-over of Rabs and tethers (and potentially their re-assembly), the recruitment and turn-over of ESCRTs, the retrieval of receptors via retromer, the coordination with the Tor machinery, and fusion with lysosomes. We are presently addressing the underlying molecular events and how these processes are coordinated.


Additional interest in the lab exists for palmitoylated proteins of the vacuole membrane, which are involved in regulation of autophagy, endosomal biogenesis and the generation of membrane contact sites (see LaGrassa et al., 2005; Subramanian et al., 2006; Cabrera et al., 2009; Hou et al., 2009). We are interested how their lipidation is mediated and palmitoylation guides their functionality.


How can we study the endosome and vacuole biogenesis? The model system of choice is the yeast vacuole. It is large, can be purified to large quantities and visualized by fluorescent dyes in vivo and in vitro. We employ established assays for follow fusion in vitro and monitor vacuole morphology with marker proteins by fluorescence microscopy. Assays are available to follow the sorting of proteins to the vacuole, and the dynamics of proteins in vivo and in vitro. Importantly, the facile generation of mutants and the conservation of the proteins among eukaryots makes yeast one of the most attractive model systems to unravel molecular mechanisms in cell biology.

Christian Ungermann


Professor of Biochemistry


Department of Biology/Chemistry

Biochemistry Section

University of Osnabrück

49082 Osnabrück, Germany


phone: +49-541-969-2884/2752

email: cu@uos.de


Office hour: Thursday 12-1 pm


CV


  1. SFB 944 (Cellular Microcompartments)

Exploring the mechanism of organelle identity of endosomes and lysosomes