Associate Professor of Biology
Diploma Federal Institute of Technology, Switzerland
Ph.D. Federal Institute of Technology, Switzerland
Markus Babst's Lab Page
Markus Babst's PubMed Literature Search
Molecular Biology Program
Biological Chemistry Program
In eukaryotic cells the majority of transmembrane proteins are degraded in the lumen of the lysosome. The topological problem of delivering transmembrane proteins into the lumen of lyosomes is solved by the formation of multivesicular bodies (MVBs) (Figure A). These endosomal structures package proteins destined for degradation into vesicles that bud from the limiting membrane into the lumen of the compartment. Mature MVBs fuse with lysosmes and release their vesicles into the hydrolytic lysosomal lumen where both lipids and proteins are degraded.
The MVB pathway is responsible for the turnover of plasma membrane proteins and thus plays an essential role in maintaining proper cell surface protein composition. Furthermore the function of many plasma membrane proteins is regulated in part by their signal-induced degradation via the MVB pathway. For example, activated growth factor receptors are rapidly endocytosed and delivered to the lysosome for degradation. This downregulation of signaling cell surface receptors plays a key role in regulating the cellular response to the external signals and when disrupted can lead to uncontrolled cellular proliferation and cancer.
Both the sorting of MVB cargo and the formation of MVB vesicles depend on three conserved protein complexes, called ESCRT-I (Endosomal Sorting Complex required for Transport-I), ESCRT-II and ESCRT-III. A current model for the function of these protein complexes is shown in Figure 1B. The mechanism of vesicle formation remains unknown because it is unique in that it is directed away from the cytoplasm and towards the lumen of compartment, whereas vesicles originating from the plasma membrane, Golgi and endoplasmic reticulum are budding towards the cytoplasm. Therefore the mechanisms of membrane deformation and pinching-off of the MVB vesicles differ from the well-studied systems such as the clathrin-dependent vesicle formation during endocytosis. Interestingly, the ESCRT machinery has been implicated in two other membrane fusion events that have the same topology as the fission event that occurs during MVB vesicle formation. Retroviruses such as HIV-1 form new virus particles at the plasma membrane by an ESCRT-dependent budding reaction and the final step in cytokinesis requires the ESCRT-dependent fission of the plasma membrane in order to form two separate cells.
The goal of my lab is to understand the molecular mechanisms involved in the formation of MVBs and the sorting of transmembrane proteins into the MVB vesicles. My lab uses a combination of yeast genetics, cell biology and biochemistry to gain insight into the function of the ESCRT proteins. We utilize the powerful genetic system of yeast to obtain interesting new mutants that are deficient in certain aspects of the MVB pathway. The function and interactions of the proteins encoded by these genes are then characterized by cell biological methods and biochemistry. Our eventual goal is to purify the proteins required for MVB formation and biochemically reconstitute their function in vitro.
Figure A: Protein transport pathways in eukaryotic cells. Endocytosed surface proteins and a subset of proteins from the secretory pathway are delivered to the endosomal system. Transmembrane proteins destined for degradation in the lysosome are sorted into lumenal vesicles of multivesicular bodies (MVBs). Mature MVBs fuse with the lysosome and deliver the MVB vesicle to the lysosomal lumen where the vesicles and cargo are degraded.
Figure B: Model for the ubiquitin-dependent sorting of transmembrane proteins by the ESCRT machinery. At the endosome Vps27 binds to an endosome specific lipid, phosphatidylinositol 3-phosphate (P), and associates with monoubiquitinated cargo proteins. ESCRT-I and ESCRT-II are recruited from the cytoplasm to the endosome by interacting either with Vps27 or phosphatidylinositol 3-phosphate, respectively. Both ESCRT complexes bind to monoubiquitinated cargo proteins. ESCRT-I activates ESCRT-II, which in turn initiates the formation of ESCRT-III. The ESCRT-III complex concentrates the MVB cargo and recruits additional factors such as the deubiquitinating enzyme Doa4 and the AAA-type ATPase Vps4. Vps4 dissociates the ESCRT machinery and releases the ESCRTs for further rounds of sorting.
- Babst, M. 2014. Quality control at the plasma membrane: one mechanism does not fit all. J. Cell Biol.205(1): 11-20.
- Mageswaran, S.K., M.G. Dixon, M. Curtiss, J.P. Keener, and M. Babst. 2013. Binding to any ESCRT can mediate ubiquitin-independent cargo sorting. Traffic15(2): 212-229.
- Jones, C.B., E.M. Ott, J.M. Keener, M. Curtiss, V. Sandrin, and M. Babst. 2012. Regulation of protein degradation by starvation-response pathways. Traffic13(3): 468-82.
- Shestakova, A., A. Hanono, S. Drosner, M. Curtiss, B.A. Davies, D.J. Katzmann, and M. Babst. 2010. Assembly of the AAA ATPase Vps4 on ESCRT-III. Mol Biol Cell21(6): 1059-71.
- Dimaano, C., C.B. Jones, A. Hanono, M. Curtiss, and M. Babst. 2008. Ist1 Regulates Vps4 Localization and Assembly. Mol Biol Cell19(2): 465-74.
- Curtiss, M., C. Jones, and M. Babst. 2007. Efficient cargo sorting by ESCRT-I and the subsequent release of ESCRT-I from MVBs requires the subunit Mvb12. Mol Biol Cell18(2): 636-45.
- Babst, M. 2005. A protein's final ESCRT. Review. Traffic 6(1): 2-9.