Axin contains three separable domains that confer intramolecular, homodimeric, and heterodimeric interactions involved in distinct functions.
Axin is a major scaffold protein, interacting with diverse molecules involved in a number of signaling pathways. Axin can undergo dimer/oligomerization via its DIX domain. Here we show that whereas deletion of the DIX domain at the C terminus rendered Axin incapable of forming dimer, a larger deletion of the C-terminal region restored the ability of Axin to form dimers. Detailed analyses revealed that Axin actually contains two separate domains (D and I) in addition to the DIX domain for homodimerization. The D, I, and DIX domains alone can form homodimers. Interestingly, D and I domains strongly interact with each other, suggesting that Axin can form an intramolecular structure through D and I interaction in the absence of DIX. We also found that DIX-DIX homodimeric interaction is weak but that point mutations in the DIX domain abolished Axin homodimerization. We propose a model to suggest that Axin forms homodimeric interactions through three domains, D, I, and DIX. More importantly, lack of DIX-DIX interaction caused by point mutations in the DIX domain or deletion causes Axin to form an intramolecular loop through the D and I domains, disallowing homodimer formation. Ccd1 interacts with Axin D domain yet fails to interact with AxinDeltaDIX, confirming that D is masked after D-I looping. The Axin mutants that are defective in homodimer formation fail to activate JNK but have no effect on beta-catenin signaling. Our findings have thus provided a structural basis of conformational changes in Axin, which may underlie the diversity of Axin functions.
Luo
Wen
W
Department of Biomedical Sciences and the Key Laboratory of the Ministry of Education for Cell Biology and Tumor Cell Engineering, School of Life Sciences, Xiamen University, Xiamen, Fujian 361005, China.
Zou
Haiying
H
Jin
Lihua
L
Lin
Shuyong
S
Li
Qinxi
Q
Ye
Zhiyun
Z
Rui
Hongliang
H
Lin
Sheng-Cai
SC
D060466:Axin Protein; D015153:Blotting, Western; D002460:Cell Line; D003598:Cytoskeletal Proteins / Q000378:metabolism; D019281:Dimerization; D017353:Gene Deletion; D017930:Genes, Reporter; D006801:Humans; D047468:Immunoprecipitation; D047908:Intracellular Signaling Peptides and Proteins; D008156:Luciferases / Q000378:metabolism; D008840:Microfilament Proteins; D008958:Models, Molecular; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D017354:Point Mutation; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D012097:Repressor Proteins / Q000737:chemistry*; D015534:Trans-Activators / Q000378:metabolism; D014162:Transfection; D051176:beta Catenin
D060466:Axin Protein; C495270:CTNNB1 protein, human; D003598:Cytoskeletal Proteins; C513335:DIXDC1 protein, human; D047908:Intracellular Signaling Peptides and Proteins; D008840:Microfilament Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; D051176:beta Catenin; D008156:Luciferases