Xu Lab of Structural and Molecular Biochemistry

Department of Molecular and Structural Biochemistry, North Carolina State University, Raleigh

Research areas

Pathogen-host interactions, mechanism of signal transduction in the innate immune response, X-ray crystallography.

Research in our lab focuses on elucidating structural mechanism of protein kinase activation. Protein kinases are the main signal transmitters and play essential roles in almost all aspects of biological processes. The long term objective of the lab is to understand the structural mechanism of two aspects of kinase activation in signal transduction:

  • How extracellular ligands induce the activation of receptor kinases on the cell membrane
  • How extracellular signals lead to activation of intracellular kinases for signal transmission and amplification

There are two ongoing projects in our lab. First, we use IkB kinase complex as an example to investigate the mechanism of intracellular kinase activation during signal transduction. Nuclear factor (NF)-kB transcription factors are the master regulators of a variety of cellular processes, particularly immune and inflammatory responses. Although many different inducers activate NF-kB, almost all the signalling pathways converge on the activation of the inhibitor of kB kinase (IKK). IKK is a large protein complex that comprises the kinase subunits IKKa and IKKβ, and a regulatory subunit NF-kB essential modifier (NEMO, also known as IKKγ). IKK harbours an N-terminal kinase domain (KD), an ubiquitin-like domain (ULD), a scaffold and dimerization domain (SDD), and a C-terminal NEMO-binding domain (NBD). It contributes to the majority of the IkB kinase activity of IKK and plays a dominant role in the canonical NF-κB pathway by phosphorylating IkBα. IKKα shares more than 50% sequence identity with IKK and plays an indispensable role in the non-canonical NF-kB pathway.

Activation of IKK requires its interaction with NEMO between the NBD of IKK and the kinase-binding domain (KBD) of NEMO that probably facilitates its oligomerization and trans-autophosphorylation. Some upstream kinases such as TAK1 can also activate IKK in a cell-type specific manner. In contrast, NEMO is dispensable for the activation of IKKa; instead it depends on an active upstream kinase NIK1. Oligomerization-mediated trans-autophosphorylation and phosphorylation by upstream kinases lead to the phosphorylation of two serine residues on the activation segment of both IKKa and IKK, which dictates their activation.

We intend to address two fundamental questions on IKK:

  • What is the mechanism of IKK activation?
  • What are the mechanisms of IKK substrate specificity and catalysis?

First, our preliminary data have indicated that NEMO forms a large molecular assembly with IKK. We propose to determine the structures of the pre- and post-activation NEMO-IKKholo-complexes to reveal NEMO-mediated IKK trans-activation. Second, we propose to unveil the structural basis of the docking interaction between IKK and IkB and investigate the influence of docking interaction on IKK substrate specificity and catalytic mechanism. These studies will not only aid the optimization of current IKK inhibitors but also lead to novel strategies of targeting IKK-substrate docking interaction.

The second ongoing project in the lab is to use plant leucine-rich repeat receptor kinases (LRR-RKs) as examples to elucidate the role of ligand-induced receptor kinase activation. Membrane integral receptor kinases (RKs) play essential roles in mediating intercellular communications in almost all multicellular organisms. Activation of RKs is usually induced by binding of ligands to their extracellular domain(s), which induce either oligomerization or conformational changes of the RKs, leading to the activation of their intracellular kinase domain to initiate signal transduction into the cells. Genetic aberration or dysregulation of RKs are associated with numerous common human diseases such as many types of cancer, diabetes, and autoimmune diseases, among others.

Receptor kinase contains an extracellular domain that is usually ligand-binding domain (ECD), a single-helix transmembrane domain (TM), and a cytoplasmic region with a kinase domain (KD). Some RKs have a short extracellular jaxamembrane domain (eJM) between the ECD and TM, and an intracellular JM between the TM and the KD. Following the kinase domain, there is a C-terminal tail (CT) that often provides autophosphosphorylation sites for the activated kinase domain. Some membrane integral RKs exist as monomers. Ligand binding induces it dimerization or oligomerization, which eventually triggers KD trans-autophosphorylation and activation. While other RKs form dimer in the absence of ligand. Ligand binding may rearrange the domain conformations of the RK leading to KD trans-autophosphorylation and activation. Both the ECD and the KD may mediate either the preformed or ligand induced RK dimerization. In addition, the TM, JMs, and CT can also contribute RK activation. The CT of some RKs contains Serine, Threonine, or Tyrosine sites that are autophosphorylated by the activated KD. The phosphorylated sites on CT provide docking sites for downstream phosphor-Ser, Thr, or Tyr binding domain containing signalling proteins to elicit signal transduction.

Receptor kinases perceive a large variety of ligands such as proteins and small peptides, among many others. The most common RK ligands are proteins, such as Insulin, insulin-like growth factors (IGFs), epidermal growth factors (EGFs), etc. In plants, many modified small peptide ligands are perceived by a family of leucine-rich repeats RKs (LRR-RKs), which consists of more than 200 members in Arabidopsis thaliana and makes up the largest family of the plant RKs known so far.

Dr. Guozhou Xu, Assistant Professor of Biochemistry, Department of Molecular and Structural Biochemistry, North Carolina State University

Education
  • PhD: Weill Medical College of Cornell University
  • Postdoctoral: Harvard Medical School
Contact
  • Office: Polk Hall
  • Phone: 919-515-0835 (Office)
  • Email: gxu3@ncsu.edu
Other members
  • Zhijie Li, Postdoctoral Fellow
  • Sayan Chakraborty, Graduate Student
  • Nour Saleh, Undergraduate Student (2014 - 2015)
  • Elizabeth Brown, Undergraduate Student (2015)
  • Veronica Emmerich, Undergraduate Student (2015)
  • Jerrin Gause, Undergraduate Student (2015)

Our lab is always interested in recruiting outstanding research fellows, Postdoctoral associates, graduate students and undergraduate students.

For Postdoctoral positions and Graduate studies please contact Dr. Xu.

For undergraduate students please set up an appointment with Dr. Xu by an email.