The Interplay Between Organelle Form and Function
Cells accomplish the biochemical reactions of life by concentrating their proteins into a variety of subcellular compartments called organelles. Our group explores the relationship between the form of the organelle and the function of its resident macromolecules. How does organelle architecture direct molecular function, and reciprocally, how do macromolecules sculpt and shape organelles?
To investigate these questions, we use focused ion beam (FIB) milling of frozen cells followed by cryo-electron tomography to image macromolecules within their native cellular environment. Through a combination of nanometer-precision localization and high-resolution structural analysis, we aim to chart the molecular landscapes of organelles. Many of our studies use the unicellular green alga, Chlamydomonas reinhardtii, which has intricate and reproducible organelle architecture as well as superb cryo-EM imaging properties. In addition, we are investigating mammalian cells, yeast, several species of marine algae, and the ameoboflagellate Naegleria gruberi.
Recent and ongoing studies in our group include:
We observed that proteasomes tether to two specific sites at the nuclear pore complex (NPC): the inner nuclear membrane and the NPC basket (Albert et al., 2017). Current efforts are focused on identifying the tethering proteins and investigating the functions of these tethered proteasomes. In collaboration with the Beck group (EMBL), we also found that the Chlamydomonas NPC has an unprecedented oligomeric state, raising questions about the evolution of this fundamental eukaryotic structure (Mosalaganti et al., under review).
In Chlamydomonas, we resolved a structure of the ribosome bound to the native ER translocon. Comparison of this structure to the human translocon-bound ribosome revealed differences in the TRAP complex that allowed us to dissect the molecular architecture of human TRAP (Pfeffer et al., Nat Comm 2017). We are currently investigating cytoplasmic regions adjacent to the ER membrane that are concentrated with proteasomes and cdc48, which work together to perform ER-associated degradation.
We identified arrays of linker proteins within the Golgi cisternae that likely maintain Golgi architecture and direct cargo transport (Engel et al., PNAS 2015). In collaboration with the Briggs group (MRC-LMB), we resolved the native structure of the COPI membrane coat bound to cargo, and found that its architecture is highly conserved between eukaryotes. (Bykov et al., eLife 2017).
In an earlier study, we visualized the native architecture of thylakoid membranes and described tubule structures that connect the thylakoids and stroma with the matrix of the pyrenoid, which is densely packed with CO2-fixing rubisco enzymes (Engel et al., eLife 2015). We are building on this study in a few ways: 1) We are mapping all four major complexes of the photosynthetic light reactions into the native thylakoid architecture. 2) In collaboration with the Jonikas group (Princeton), we analyzed the packing of rubisco within the pyrenoid and discovered that the pyrenoid behaves like a phase-separated liquid droplet (Freeman Rosenzweig et al., Cell 2017). 3) As pyrenoids have independently evolved multiple times, we are interested in exploring pyrenoid organization in other species, including diatoms and dinoflagellates.
Centriole and Cilium
Using Naegleria, Chlamydomonas, and mammalian cells, we are exploring how centrioles assemble and how the ciliary pore gates the transit of both soluble and membrane proteins between the cell and the cilium.
Albert S., Schaffer M., Beck F., Mosalaganti S., Asano S., Thomas H.F., Plitzko J.M., Beck M., Baumeister W., Engel B.D.: Proteasomes tether to two distinct sites at the nuclear pore complex. PNAS, in press, 2017. Full Paper MPI Press Release
Bykov Y.S., Schaffer M., Dodonova S.O., Albert S., Plitzko J.M., Baumeister W., Engel B.D., Briggs J.A.G.: The structure of the COPI coat determined within the cell. eLife. 6:e32493, 2017. Full Paper Commentary in eLife Commentary in Science
Freeman Rosenzweig E.S., Xu B., Kuhn Cuellar L., Martinez-Sanchez A., Schaffer M., Strauss M., Cartwright H.N., Ronceray P., Plitzko J.M., Förster F., Wingreen N.S., Engel B.D., Mackinder L.C.M., Jonikas M.C.: The eukaryotic CO2-concentrating organelle is liquid-like and exhibits dynamic reorganization. Cell. 171:148-162.e19, 2017. Full Paper Commentary in Cell MPI Press Release Princeton Press Release
Pfeffer S., Dudek J., Schaffer M., Ng B.G., Albert S., Plitzko J.M., Baumeister W., Zimmermann R., Freeze H.H., Engel B.D., Förster F.: Dissecting the molecular organization of the translocon-associated protein complex. Nature Communications. 8:14516, 2017. Full Paper Utrecht Press Release
Schaffer M., Mahamid J., Engel B.D., Laugks T., Baumeister W., Plitzko J.: Optimized cryo-focused ion beam sample preparation aimed at in situ structural studies of membrane proteins. Journal of Structural Biology. 197:73-82, 2017. Full Paper
Asano S., Engel B.D., Baumeister W.: In situ cryo-electron tomography: a post-reductionist approach to structural biology. Journal of Molecular Biology. 428: 332–343, 2016. Full Paper
Schaffer M., Engel B.D., Laugks T., Mahamid J., Plitzko J.M., Baumeister W.: Cryo-focused ion beam sample preparation for imaging vitreous cells by cryo-electron tomography. Bio-protocol. 5:e1575, 2015. Full Paper
Engel B.D., Schaffer M., Albert S., Asano S., Plitzko J.M., Baumeister W.: In situ structural analysis of Golgi intracisternal protein arrays. PNAS. 112: 11264-11269, 2015. Full Paper MPI Press Release
Engel B.D., Schaffer M., Cuellar Kuhn L., Villa E., Plitzko J.M., Baumeister W.: Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography. eLife. 4: e04889, 2015. Full Paper Commentary in eLife MPI Press Release