Our Science – Ji Website

Xinhua Ji, Ph.D.

Macromolecular Crystallography Laboratory Head, Biomolecular Structure Section Senior Investigator Building 539 Room 124 Frederick, MD 21702-1201 Phone:   301-846-5035 Fax:   301-846-6073 E-Mail:   jix@ncifcrf.gov Link: Other Homepage

Biography

Dr. Ji obtained his Ph.D. from the University of Oklahoma in 1990 and received postdoctoral training at the University of Maryland Biotechnology Institute, where he became a Research Assistant Professor before joining the National Cancer Institute (NCI). At NCI-Frederick, Dr. Ji established his laboratory in the ABL-Basic Research Program in 1995, moved to the Center for Cancer Research as a Section Chief in 1999, and in 2001 gained tenure as an NIH Senior Investigator. Dr. Ji is also an Ajdunct Professor of Biochemistry and Molecular Biology in the School of Medicine and Health Sciences at the George Washington University in Washington DC.

Research

Structural biology of gene expression control with an emphasis on RNA-processing proteins (see RNase III) and RNA polymerase-associated transcription factors (see RapA); structure-based development of anticancer and antimicrobial agents (see PABA/NO).

RNase III: RNA interference is mediated by small interfering RNAs produced by members of the RNase III family, including Dicer. For mechanistic studies, bacterial RNase III has been a valuable model system for the entire family. Previously, we have shown how the dimerization of the endonuclease domain of the enzyme creates a catalytic valley where two catalytic sites are located, how the catalytic valley accommodates a dsRNA in a manner such that each of the two RNA strands is aligned with one of the two catalytic sites, how the hydrolysis of each strand involves both subunits (residues from one subunit are involved in the selection of the scissile bond, while those from the partner subunit are involved in the cleavage chemistry), and how RNase III uses the two catalytic sites to create the 2-nucleotide 3' overhangs in its products. Recently, we have demonstrated how Mg2+ is essential for the formation of a catalytically competent protein-RNA complex, how the use of two Mg2+ ions can drive the hydrolysis of each phosphodiester bond, and how conformational changes in both the substrate and the protein are critical elements for assembling the catalytic complex. Moreover, we have modeled a protein-substrate complex and a protein-reaction intermediate (transition state) complex in a meaningful way. Together, the models and crystal structures suggest a stepwise mechanism for the enzyme to execute the phosphoryl transfer reaction. The structural information of protein-dsRNA interactions and the mechanism of dsRNA processing by bacterial RNase III can be extrapolated to other family members, including eukaryotic Rnt1p, Drosha and Dicer.

RapA: RapA, as abundant as sigma 70 in the cell, is an RNA polymerase (RNAP)-associated Swi2/Snf2 protein with ATPase activity. It stimulates RNAP recycling during transcription. Recently, we reported the first structure of RapA, which is also the first full-length structure for the entire Swi2/Snf2 family. RapA contains seven domains, two of which exhibit novel protein folds. Our model of RapA in complex with ATP and double-stranded (ds) DNA suggests that RapA may bind to and translocate on dsDNA. Our kinetic template-switching assay shows that RapA facilitates the release of sequestered RNAP from a posttranscrption/posttermination complex (PTC) for transcription reinitiation. Our in vitro competition experiment indicates that RapA binds to core RNAP only but is readily displaceable by sigma 70. RapA is likely another general transcription factor, the structure of which provides a framework for future studies of this bacterial Swi2/Snf2 protein and its important roles in RNAP recycling during transcription.

PABA/NO: Glutathione S-transferase (GST) is a superfamily of detoxification enzymes, represented by GST-alpha, GST-mu, GST-pi, etc. GST-alpha is the predominant isoform of GST in human liver, playing important roles for our well being. GST-pi is overexpressed in many forms of cancer, thus presenting an opportunity for selective targeting of cancer cells. Our structure-based design of prodrugs intended to release cytotoxic levels of nitric oxide in GST-pi-overexpressing cancer cells yielded PABA/NO, which exhibited anticancer activity both in vitro and in vivo with a potency similar to that of cisplatin. The design was based on GST structures at both ground state and transition state. The ground-state structures outlined the shape and property of the substrate-binding site in different isozymes, and the structural information at the transition-state provided guidance for structural modifications of the prodrug molecules. Two key alterations of a GST-alpha-selective compound led to the GST-pi-selective PABA/NO.

Our collaborators include Drs. Donald Court, Dimiter Dimitrov, Ding Jin, Larry Keefer, Joseph Tropea, and David Waugh (NCI) and Professors Shivendra Singh (University of Pittsburgh) and Honggao Yan (Michigan State University).

This page was last updated on 9/23/2008.