Xiaolei Su

Assistant Professor of Cell Biology, Yale School of Medicine

 

B.S.           Peking University, 2006

Ph.D.        Harvard University, 2012

Postdoc   University of California, San Francisco 2017

Student    Physiology Course, Marine Biological Laboratory, 2009

My initial interest in biology originates from my love for railways. Since my childhood, I have been keen on taking train trips and watching locomotives and cars. Listening to the clicks between rails and trains is one of my favorites. Not until I attended college and took cell biology course did I learn there is a similar railway system in cells but at the molecular scale. Molecular motors such as kinesins and dyneins move along microtubule tracks and transport cargoes. Amazingly, these tracks are also dynamic, undergoing frequent remodeling through polymerization and depolymerization. The curiosity for this molecular railway system drives me to join Dr. Jianguo Chen and Junlin Teng's Laboratory at Peking University. With the support of a university fellowship for undergraduates, I took an independent project aiming on identifying microtubule associated proteins in brains through a proteomic approach. I was pretty excited about making lysates from calf brain and performing microtubule assembly-disassembly assay. Luckily I identified several new hits which helped to explain microtubule stability regulation in neurons. This opened the door for me to cell biology research and gave me the initial training in biochemistry. 

 

Continuing my love for microtubules and motors, I attended graduate school at Harvard and joined David Pellman’s Laboratory. My graduate work aimed to address two long-standing questions in the field of cytoskeleton: 1. how are lengths of microtubule polymers matched to their cellular functions? 2. how are individual microtubules organized into complex structures like mitotic spindles? I focused on a universal microtubule length regulator kinesin-8. Kinesin-8 is unique among all the kinesin families because it not only contains the conventional walking activity along microtubules, but also acquires the ability to affect microtubule end dynamics. However, the effects of kinesin-8 on microtubule dynamics are puzzling. Both microtubule stabilizing and destabilizing effects had been documented before. I proposed a dosage-dependent model to explain this puzzle and was able to demonstrate that kinesin-8 destabilizes microtubules at high concentrations but stabilizes microtubules at low concentrations. The dose-dependent effect of kinesin-8 narrows down the length distribution of microtubules and explains how kinesin-8 promotes chromosome congression during mitosis (1). The other part of my graduate work reveals a novel function of kinesin-8 in organizing microtubules and promoting spindle assembly. Using combined approaches of biochemistry, single molecule imaging and genetics, I found kinesin-8 contains a secondary microtubule binding domain on the tail, which mediates kinesin-8’s microtubule sliding activity in vitro and spindle pole separating function in vivo (2). Like a Swiss Army Knife, kinein-8 integrates the microtubule-based walking, end-modulating, and sliding activities. My discovery raises interesting questions about how these individual activities are coordinated to regulate microtubule dynamics throughout the cell cycle.

As a postdoctoral fellow in Ron Vale's Lab at UCSF, I continued to study the spatial organization of molecules, but this time in the two-dimensional context of plasma membranes. In the past decade, many microscopy-based studies have reported that signaling molecules form discrete “microclusters” on the plasma membrane. However, beyond these phenomenological observations, experimental evidence illuminating how clustering influences signaling has been lacking. The TCR pathway represents a specific instance of this general phenomenon in cell biology - the organization of proteins and nucleic acids into micron-sized (or meso-scale) clusters (e.g. focal adhesion, P bodies, stress granules, Cajal bodies, nucleolus, etc). While recent studies have suggested that these objects arise through a process of phase separation, the functional consequences of forming phase separated domains remain unclear. The compositional heterogeneity, complex networks of molecular interactions, and genetic redundancy created major challenges in understanding the roles of microclusters in cell physiology. I aimed to develop a system that allows me to have a quantitative control over the system and make perturbations to address specific questions. I had the luck to join the HHMI summer institute at the Marine Biological Laboratroy at Woods Hole Massachustettes. In a wonderful collaboration with Jon Ditlev from Mike Rosen's Lab, we have been able to reconstitute the T cell microclusters and a TCR pathway using 12 purified components on model membranes, starting from the activation of the TCR and ending up with actin polymerization (3). Using this approach, we demonstrated that the microclusters are phase separated structures assembled through multivalent protein-protein interactions. They display liquid-like properties, enrich kinases but exclude phosphatases, thus promoting phosphorylation and actin polymerization. These results demonstrated that protein phase separation can create a distinct physical and biochemical compartment that facilitates signaling (4). This provides a new perspective to understand the regulatory mechanisms of protein activities as well as signal transduction. Similar principles may be applied to other types of signaling pathways involving multivalent protein interactions, including EGF, Ephrin, and FAS receptors, and reconstitution could be a powerful approach towards understanding these signaling pathways.

References:

  1. Su X, Qiu W, Gupta ML Jr, Pereira-Leal JB, Reck-Peterson SL, Pellman D.
    Mechanisms underlying the dual-mode regulation of microtubule dynamics by kip3/kinesin-8.
    Molecular Cell. 2011 Sep 2;43(5):751-63.

  2. Su X, Arellano-Santoyo H, Portran D, Gaillard J, Vantard M, Thery M, and Pellman D.
    Microtubule sliding activity of a kinesin-8 promotes spindle assembly and spindle length control.
    Nature Cell Biology. 2013 Aug; 15(8): 948-57.

  3. Su X, Ditlev JA, Rosen MK, Vale RD.
    Reconstitution of TCR Signaling Using Supported Lipid Bilayers.
    Methods in Molecular Biology. 2017;1584:65-76.

  4. Su X*, Ditlev JA*, Hui E, Xing W, Banjade S, Okrut J, King DS, Taunton J, Rosen MK$, and Vale RD$.
    Phase separation of signaling molecules promotes T cell receptor signal transduction
    Science. 2016 Apr; 352(6285):595-9. (*Co-first author, $Co-corresponding author)