title: Biochemical and functional characterization of six SIX1 Branchio-oto-renal syndrome mutations.
abstract: Branchio-oto-renal syndrome (BOR) is an autosomal dominant developmental disorder characterized by hearing loss, branchial arch defects, and renal anomalies. Recently, eight mutations in the SIX1 homeobox gene were discovered in BOR patients. To characterize the effect of SIX1 BOR mutations on the EYA-SIX1-DNA complex, we expressed and purified six of the eight mutants in Escherichia coli. We demonstrate that only the most N-terminal mutation in SIX1 (V17E) completely abolishes SIX1-EYA complex formation, whereas all of the other mutants are able to form a stable complex with EYA. We further show that only the V17E mutant fails to localize EYA to the nucleus and cannot be stabilized by EYA in the cell. The remaining five SIX1 mutants are instead all deficient in DNA binding. In contrast, V17E alone has a DNA binding affinity similar to that of wild type SIX1 in complex with the EYA co-factor. Finally, we show that all SIX1 BOR mutants are defective in transcriptional activation using luciferase reporter assays. Taken together, our experiments demonstrate that the SIX1 BOR mutations contribute to the pathology of the disease through at least two different mechanisms that involve: 1) abolishing the formation of the SIX1-EYA complex or 2) diminishing the ability of SIX1 to bind DNA. Furthermore, our data demonstrate for the first time that EYA: 1) requires the N-terminal region of the SIX1 Six domain for its interaction, 2) increases the level of the SIX1 protein within the cell, and 3) increases the DNA binding affinity of SIX1.
authors.affiliation: Program in Molecular Biology, University of Colorado Denver School of Medicine, Aurora, Colorado 80045, USA.
authors.forename: Aaron N
authors.initials: AN
authors.lastname: Patrick
authors.forename: Barbara J
authors.initials: BJ
authors.lastname: Schiemann
authors.forename: Kui
authors.initials: K
authors.lastname: Yang
authors.forename: Rui
authors.initials: R
authors.lastname: Zhao
authors.forename: Heide L
authors.initials: HL
authors.lastname: Ford
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D019280:Branchio-Oto-Renal Syndrome / Q000235:genetics*; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism; D004247:DNA / Q000378:metabolism; D018398:Homeodomain Proteins / Q000737:chemistry / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000737:chemistry / Q000378:metabolism; D050505:Mutant Proteins / Q000737:chemistry / Q000302:isolation & purification; D009154:Mutation / Q000187:drug effects*; D009687:Nuclear Proteins / Q000737:chemistry / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding; D055550:Protein Stability; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D017027:Protein Tyrosine Phosphatases / Q000737:chemistry / Q000378:metabolism; D015533:Transcriptional Activation / Q000235:genetics
chemical_list: D018398:Homeodomain Proteins; D047908:Intracellular Signaling Peptides and Proteins; D050505:Mutant Proteins; D009687:Nuclear Proteins; C099236:SIX1 protein, human; D004247:DNA; C106938:EYA2 protein, human; D017027:Protein Tyrosine Phosphatases