Jochen Zimmer

Education

  • Diploma, ETH Zurich
  • Dphil, University of Oxford
  • Postdoc, Harvard Medical School

Primary Appointment

  • Assistant Professor, Molecular Physiology and Biological Physics

Contact

Research Interest(s)

Transport of biopolymers across biological membranes with a particular interest in polysaccharide and protein translocation.

Research Description

Under certain conditions, all essential biopolymers, such as polypeptides, nucleic acids and polysaccharides, have to cross at least one membrane to reach their final destinations. Examples include the secretion of antibodies, export of RNA from the nucleus and the deposition of polysaccharides in the extracellular matrix. While our knowledge has expended significantly on the translocation and membrane integration of polypeptides, very little is known about how one of the most hydrophilic polymers, the polysaccharide, can cross the hydrophobic barrier surrounding every cell. Polysaccharides come in many forms and serve a multitude of different functions. For example, starch and glycogen are means of energy storage, cellulose and chitin confer structural stability to the cell, and hyaluronan modulates cell differentiation, proliferation and migration in vertebrates. To serve their ultimate purpose, many of these polysaccharides, which can be several microns in length, have to be transported across lipid bilayers. In many cases, membrane embedded processive polysaccharide synthases connect intracellular building blocks (activated sugars) and translocate the growing polysaccharide chain across the membrane. This process requires (1) the recognition and binding of the activated sugars by intracellular catalytic domains, (2) a glycosyltransferase activity to covalently link the building blocks, (3) the formation of a transmembrane channel, and (4) the translocation of the growing polysaccharide chain through the channel. Combined, this process couples the biosynthesis of extremely long linear polysaccharides with their membrane translocation, catalyzed by a single enzyme. Studying the translocation of polysaccharides across biological membranes necessitates a broad spectrum of techniques. Initially, target proteins will be cloned, expressed, and purified. With high quality material in hand, biophysical and biochemical studies follow. A major aim of the laboratory is the structural characterization of glycosyltransferases involved in hyaluronan, cellulose and curdlan biosynthesis. Simultaneously, we strive to establish in vitro polysaccharide translocation assays by reconstituting purified enzymes into proteoliposomes, which are a convenient tool to study membrane transport processes. This will allow us to (1) reproduce the biosynthesis and translocation of the polymer outside the cell, (2) develop assays that identify the path of the polysaccharide through the membrane channel, and (3) identify conditions that will trap translocation intermediates for structural studies. As an interdisciplinary approach, we will also use electron microscopy to study the supra-molecular assemblies some of the glycosyltransferases form in native membranes as well as cell biology techniques to study the surface expression of matrix polysaccharides in a physiological context. Techniques to be applied:

Recombinant DNA and molecular biology techniques

Heterologous protein expression in bacterial and eukaryotic systems

Reconstitution of proteins into lipid vesicles

Enzyme activity assays

Protein purification (metal affinity-, ion exchange- and gel filtration chromatography)

X-ray crystallography

Selected Publications

  • Morgan J, McNamara J, Zimmer J. Mechanism of activation of bacterial cellulose synthase by cyclic di-GMP. Nature structural & molecular biology. 2014;21(5): 489-96. PMID: 24704788 | PMCID: PMC4013215
  • Omadjela O, Narahari A, Strumillo J, Mélida H, Mazur O, Bulone V, Zimmer J. BcsA and BcsB form the catalytically active core of bacterial cellulose synthase sufficient for in vitro cellulose synthesis. Proceedings of the National Academy of Sciences of the United States of America. 2013;110(44): 17856-61. PMID: 24127606 | PMCID: PMC3816479
  • Hubbard C, McNamara J, Azumaya C, Patel M, Zimmer J. The hyaluronan synthase catalyzes the synthesis and membrane translocation of hyaluronan. Journal of molecular biology. 2012;418(1): 21-31. PMID: 22343360
  • Morgan J, Strumillo J, Zimmer J. Crystallographic snapshot of cellulose synthesis and membrane translocation. Nature. 2012;493(7431): 181-6. PMID: 23222542 | PMCID: PMC3542415
  • Mazur O, Zimmer J. Apo- and cellopentaose-bound structures of the bacterial cellulose synthase subunit BcsZ. The Journal of biological chemistry. 2011;286(20): 17601-6. PMID: 21454578 | PMCID: PMC3093835