ERC FUNDED PROJECTS

Cell volume regulation is crucial for any living cell because changes in volume determine the metabolic activity through e.g. changes in ionic strength, pH, macromolecular crowding and membrane tension. These physical chemical parameters influence interaction rates and affinities of biomolecules, folding rates, and fold stabilities in vivo. Understanding of the underlying volume regulatory mechanisms has immediate application in biotechnology and health, yet these factors are generally ignored in systems analyses of cellular functions. My team has uncovered a number of mechanisms and insights of cell volume regulation. The next step forward is to elucidate how the components of a cell volume regulatory circuit work together and control the physicochemical conditions of the cell. I propose construction of a synthetic cell in which an osmoregulatory transporter and mechanosensitive channel form a minimal volume regulatory network. My group has developed the technology to reconstitute membrane proteins into lipid vesicles (synthetic cells). One of the challenges is to incorporate into the vesicles an efficient pathway for ATP production and maintain energy homeostasis while the load on the system varies. We aim to control the transmembrane flux of osmolytes, which requires elucidation of the molecular mechanism of gating of the osmoregulatory transporter. We will focus on the glycine betaine ABC importer, which is one of the most complex transporters known to date with ten distinct protein domains, transiently interacting with each other. The proposed synthetic metabolic circuit constitutes a fascinating out-of-equilibrium system, allowing us to understand cell volume regulatory mechanisms in a context and at a level of complexity minimally needed for life. Analysis of this circuit will address many outstanding questions and eventually allow us to design more sophisticated vesicular systems with applications, for example as compartmentalized reaction networks.

2 247 231 €

Start date: 2015-07-01, End date: 2020-06-30

Algebras of operators on Hilbert spaces were originally introduced as the right framework for the mathematical description of quantum mechanics. In modern mathematics the scope has much broadened due to the highly versatile nature of operator algebras. They are particularly useful in the analysis of groups and their actions. Amenability is a finiteness property which occurs in many different contexts and which can be characterised in many different ways. We will analyse amenability in terms of approximation properties, in the frameworks of abstract C*-algebras, of topological dynamical systems, and of discrete groups. Such approximation properties will serve as bridging devices between these setups, and they will be used to systematically recover geometric information about the underlying structures. When passing from groups, and more generally from dynamical systems, to operator algebras, one loses information, but one gains new tools to isolate and analyse pertinent properties of the underlying structure. We will mostly be interested in the topological setting, and in the associated C*-algebras. Amenability of groups or of dynamical systems then translates into the completely positive approximation property. Systems of completely positive approximations store all the essential data about a C*-algebra, and sometimes one can arrange the systems so that one can directly read of such information. For transformation group C*-algebras, one can achieve this by using approximation properties of the underlying dynamics. To some extent one can even go back, and extract dynamical approximation properties from completely positive approximations of the C*-algebra. This interplay between approximation properties in topological dynamics and in noncommutative topology carries a surprisingly rich structure. It connects directly to the heart of the classification problem for nuclear C*-algebras on the one hand, and to central open questions on amenable dynamics on the other.

1 596 017 €

Start date: 2019-10-01, End date: 2024-09-30

The analysis of geometric and functional inequalities naturally leads to consider the extremal cases, thus looking for optimal sets, or optimal functions, or optimal constants. The most classical examples are the (different versions of the) isoperimetric inequality and the Sobolev-like inequalities. Much is known about equality cases and best constants, but there are still many questions which seem quite natural but yet have no answer. For instance, it is not known, even in the 2-dimensional space, the answer of a question by Brezis: which set, among those with a given volume, has the biggest Sobolev-Poincaré constant for p=1? This is a very natural problem, and it appears reasonable that the optimal set should be the ball, but this has never been proved. The interest in problems like this relies not only in the extreme simplicity of the questions and in their classical flavour, but also in the new ideas and techniques which are needed to provide the answers. The main techniques that we aim to use are fine arguments of symmetrization, geometric constructions and tools from mass transportation (which is well known to be deeply connected with functional inequalities). These are the basic tools that we already used to reach, in last years, many results in a specific direction, namely the search of sharp quantitative inequalities. Our first result, together with Fusco and Maggi, showed what follows. Everybody knows that the set which minimizes the perimeter with given volume is the ball. But is it true that a set which almost minimizes the perimeter must be close to a ball? The question had been posed in the 1920's and many partial result appeared in the years. In our paper (Ann. of Math., 2007) we proved the sharp result. Many other results of this kind were obtained in last two years.

540 000 €

Start date: 2010-08-01, End date: 2015-07-31

This proposal aims to unravel mysteries at the frontier of number theory and other areas of mathematics and physics. The main focus will be to understand and exploit “modularity” of q-hypergeometric series. “Modular forms are functions on the complex plane that are inordinately symmetric.” (Mazur) The motivation comes from the wide-reaching applications of modularity in combinatorics, percolation, Lie theory, and physics (black holes). The interplay between automorphic forms, q-series, and other areas of mathematics and physics is often two-sided. On the one hand, the other areas provide interesting examples of automorphic objects and predict their behavior. Sometimes these even motivate new classes of automorphic objects which have not been previously studied. On the other hand, knowing that certain generating functions are modular gives one access to deep theoretical tools to prove results in other areas. “Mathematics is a language, and we need that language to understand the physics of our universe.”(Ooguri) Understanding this interplay has attracted attention of researchers from a variety of areas. However, proofs of modularity of q-hypergeometric series currently fall far short of a comprehensive theory to describe the interplay between them and automorphic forms. A recent conjecture of W. Nahm relates the modularity of such series to K-theory. In this proposal I aim to fill this gap and provide a better understanding of this interplay by building a general structural framework enveloping these q-series. For this I will employ new kinds of automorphic objects and embed the functions of interest into bigger families A successful outcome of the proposed research will open further horizons and also answer open questions, even those in other areas which were not addressed in this proposal; for example the new theory could be applied to better understand Donaldson invariants.

1 240 500 €

Start date: 2014-01-01, End date: 2019-04-30

Argonaute nucleases are key players of the eukaryotic RNA interference (RNAi) system. Using small RNA guides, these Argonaute (Ago) proteins specifically target complementary RNA molecules, resulting in regulation of a wide range of crucial processes, including chromosome organization, gene expression and anti-virus defence. Since 2010, my research team has studied closely-related prokaryotic Argonaute (pAgo) variants. This has revealed spectacular mechanistic variations: several thermophilic pAgos catalyse DNA-guided cleavage of double stranded DNA, but only at elevated temperatures. Interestingly, a recently discovered mesophilic Argonaute (CbAgo) can generate double strand DNA breaks at moderate temperatures, providing an excellent basis for this ARGO project. In addition, genome analysis has revealed many distantly-related Argonaute variants, often with unique domain architectures. Hence, the currently known Argonaute homologs are just the tip of the iceberg, and the stage is set for making a big leap in the exploration of the Argonaute family. Initially we will dissect the molecular basis of functional and mechanistic features of uncharacterized natural Argonaute variants, both in eukaryotes (the presence of an Ago-like subunit in the Mediator complex, strongly suggests a regulatory role of an elusive non-coding RNA ligand) and in prokaryotes (selected Ago variants possess distinct domains indicating novel functionalities). After their thorough biochemical characterization, I aim at engineering the functionality of the aforementioned CbAgo through an integrated rational & random approach, i.e. by tinkering of domains, and by an unprecedented in vitro laboratory evolution approach. Eventually, natural & synthetic Argonautes will be selected for their exploitation, and used for developing original genome editing applications (from silencing to base editing). Embarking on this ambitious ARGO expedition will lead us to many exciting discoveries.

2 177 158 €

Start date: 2019-07-01, End date: 2024-06-30

Supramolecular assemblies – formed by the self-assembly of hundreds of protein subunits – are part of bacterial nanomachines involved in key cellular processes. Important examples in pathogenic bacteria are pili and type 3 secretion systems (T3SS) that mediate adhesion to host cells and injection of virulence proteins. Structure determination at atomic resolution of such assemblies by standard techniques such as X-ray crystallography or solution NMR is severely limited: Intact T3SSs or pili cannot be crystallized and are also inherently insoluble. Cryo-electron microscopy techniques have recently made it possible to obtain low- and medium-resolution models, but atomic details have not been accessible at the resolution obtained in these studies, leading sometimes to inaccurate models. I propose to use solid-state NMR (ssNMR) to fill this knowledge-gap. I could recently show that ssNMR on in vitro preparations of Salmonella T3SS needles constitutes a powerful approach to study the structure of this virulence factor. Our integrated approach also included results from electron microscopy and modeling as well as in vivo assays (Loquet et al., Nature 2012). This is the foundation of this application. I propose to extend ssNMR methodology to tackle the structures of even larger or more complex homo-oligomeric assemblies with up to 200 residues per monomeric subunit. We will apply such techniques to address the currently unknown 3D structures of type I pili and cytoskeletal bactofilin filaments. Furthermore, I want to develop strategies to directly study assemblies in a native-like setting. As a first application, I will study the 3D structure of T3SS needles when they are complemented with intact T3SSs purified from Salmonella or Shigella. The ultimate goal of this proposal is to establish ssNMR as a generally applicable method that allows solving the currently unknown structures of bacterial supramolecular assemblies at atomic resolution.

1 456 000 €

Start date: 2014-05-01, End date: 2019-04-30

Autophagy, a lysosomal degradation pathway in which the cell digests its own components, is an essential biological pathway that promotes organismal health and longevity and helps combat cancer and neurodegenerative diseases. Accordingly, the 2016 Nobel Prize in Physiology or Medicine was awarded for research in autophagy. Although autophagy has been extensively studied from yeast to mammals, the molecular events that underlie its induction and progression remain elusive. A highly conserved protein kinase, Atg1, plays a unique and essential role in initiating autophagy, yet despite this pivotal importance it has taken over twenty years for its first downstream target to be discovered. However, whilst our identification of the autophagy related membrane protein Atg9 as the first Atg1 substrate is an important advance, the molecular mechanisms that enable the extensive remodelling of cellular membranes that occurs during autophagy is still completely undefined. A detailed knowledge of the inputs and outputs of the Atg1 kinase will enable us to provide a definitive mechanistic understanding of autophagy. We have devised a novel permeabilized cell assay that reconstitutes the pathway in vitro, allowing us to recapitulate key steps in the autophagic process and thereby determine how the individual steps that lead up to autophagy are controlled. We will use this system to dissect the functional role of Atg1 kinase in autophagosome-vacuole fusion (Objective 1), and to determine the origin of the autophagic membrane and the role of Atg1 in expanding these (Objective 2). To reveal how Atg1/ULK1 kinase is activated in mammalian cells, we will apply the unique and carefully tailored synthetic in vivo approaches that we have recently developed (Objective 3). By focusing on the activation of the Atg1 kinase and the molecular events that it executes, we will be able to explain its central role in regulating the autophagic process and define the mechanistic steps in the pathway.

1 955 666 €

Start date: 2018-06-01, End date: 2023-05-31

The Endoplasmic Reticulum (ER) membrane in all eukaryotic cells has an intricate protein network that facilitates protein biogene-sis and homeostasis. The molecular complexity and sophisticated regulation of this machinery favours study-ing it in its native microenvironment by novel approaches. Cryo-electron tomography (CET) allows 3D im-aging of membrane-associated complexes in their native surrounding. Computational analysis of many sub-tomograms depicting the same type of macromolecule, a technology I pioneered, provides subnanometer resolution insights into different conformations of native complexes. I propose to leverage CET of cellular and cell-free systems to reveal the molecular details of ER protein bio-genesis and homeostasis. In detail, I will study: (a) The structure of the ER translocon, the dynamic gateway for import of nascent proteins into the ER and their maturation. The largest component is the oligosaccharyl transferase complex. (b) Cotranslational ER import, N-glycosylation, chaperone-mediated stabilization and folding as well as oligomerization of established model substrate such a major histocompatibility complex (MHC) class I and II complexes. (c) The degradation of misfolded ER-residing proteins by the cytosolic 26S proteasome using cytomegalovirus-induced depletion of MHC class I as a model system. (d) The structural changes of the ER-bound translation machinery upon ER stress through IRE1-mediated degradation of mRNA that is specific for ER-targeted proteins. (e) The improved ‘in silico purification’ of different states of native macromolecules by maximum likelihood subtomogram classification and its application to a-d. This project will be the blueprint for a new approach to structural biology of membrane-associated processes. It will contribute to our mechanistic understanding of viral immune evasion and glycosylation disorders as well as numerous diseases involving chronic ER stress including diabetes and neurodegenerative diseases.

2 496 611 €

Start date: 2017-04-01, End date: 2022-03-31

Transcription in single cells is a stochastic process that arises from the random collision of molecules, resulting in heterogeneity in gene expression in cell populations. This heterogeneity in gene expression influences cell fate decisions and disease progression. Interestingly, gene expression variability is not the same for every gene: noise can vary by several orders of magnitude across transcriptomes. The reason for this transcript-specific behavior is that genes are not transcribed in a continuous fashion, but can show transcriptional bursting, with periods of gene activity followed by periods of inactivity. The noisiness of a gene can be tuned by changing the duration and the rate of switching between periods of activity and inactivity. Even though transcriptional bursting is conserved from bacteria to yeast to human cells, the origin and regulators of bursting remain largely unknown. Here, I will use cutting-edge single-molecule RNA imaging techniques to directly observe and measure transcriptional bursting in living yeast cells. First, bursting properties will be quantified at different endogenous and mutated genes to evaluate the contribution of cis-regulatory promoter elements on bursting. Second, the role of trans-regulatory complexes will be characterized by dynamic depletion or gene-specific targeting of transcription regulatory proteins and observing changes in RNA synthesis in real-time. Third, I will develop a new technology to visualize the binding dynamics of single transcription factor molecules at the transcription site, so that the stability of upstream regulatory factors and the RNA output can directly be compared in the same cell. Finally, I will examine the phenotypic effect of different bursting patterns on organismal fitness. Overall, these approaches will reveal how bursting is regulated at the molecular level and how different bursting patterns affect the heterogeneity and fitness of the organism.

1 950 775 €

Start date: 2018-01-01, End date: 2022-12-31