2012 Theses Doctoral

Structural Determinants of DNA-binding Specificity for Hox Proteins

Liu, Peng

Hox proteins are a group of homeodomain-containing transcription factors that define the body plan in both vertebrates and invertebrates. Mutations in Hox proteins lead to limb malformations or cancer in humans. Despite having homeodomains with similar sequences and structures, the eight Hox proteins in Drosophila exhibit a variety of DNA-binding specificities when they are in complex with their cofactor Extradenticle (Exd), raising the question of how such diverse specificity is generated.
We have identified DNA minor groove shape as a structural determinant for Hox specificity. Using Monte Carlo simulations, we predicted the minor groove widths for Hox-binding sites obtained from a high-throughput experiment - Systematic Evolution of Ligands by Exponential Enrichment with massive parallel sequencing (SELEX-seq). We found that DNA sites selected by anterior Hox proteins have two narrow regions in the minor groove where Hox-Exd binds. In contrast, DNA sites favored by posterior Hox proteins have only one narrow region. Moreover, clustering of Hox proteins based on their preference of DNA minor groove shape reproduced the ordering of Hox genes along the chromosome, suggesting a striking relationship between body axis morphogenesis and nuances in DNA shape.
Intrigued by the question of how DNA shape is recognized, we studied the interactions between an anterior Hox protein, Sex combs reduced (Scr), and its preferential DNA sites identified from SELEX-seq. Through structure-based homology modeling, we found that two Arg residues on the N-terminal arm of Scr specifically recognize the two narrow regions in the minor groove of Scr-favored sites, regardless of their nucleotide identities. Our work leads to a new understanding of the structural basis of specific DNA-binding for Drosophila Hox proteins, linking preference of DNA-binding sites to DNA minor groove shape.
Our studies on Hox-cofactor-DNA structures revealed highly conserved features of protein-DNA recognition, e.g. Hox's Asn51 forms hydrogen bonds to an adenine, which are essential for Hox-DNA binding. In order to automatically identify this type of important interactions, we developed a computational module based on the functional annotation server MarkUs. This module displays a variety of protein-DNA interactions inside query structure and illustrates their degrees of conservation by comparing query structure with its structural homologs. This functional annotation module provides an effective way to analyze protein-DNA recognition and to identify essential interactions.
In this dissertation, Chapter 1 introduces the field of protein-DNA specific recognition from the perspectives of three-dimensional structures, high-throughput experiments, and computational modeling approaches. Chapter 2 introduces the biological background of Hox proteins, focusing on their biological functions, three-dimensional structures, and previous studies on their DNA-binding specificity. Chapter 3 presents the investigation of DNA-binding specificity for Hox-Exd complexes. The role of DNA minor groove width as a structural determinant is demonstrated through Monte Carlo simulations. Chapter 4 describes the homology modeling method for studying DNA minor groove recognition for Scr. The recognition mode of Scr-favored SELEX-seq sequences is inferred through protein-DNA docking and interface optimization. Chapter 5 elucidates the functional annotation module for protein-DNA structures. The functions and features of this module are demonstrated through a case study on a Scr-Exd-DNA structure. Chapter 6 summarizes my research projects described in this dissertation and proposes future directions for studying specific protein-DNA recognition.