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Original Article |
Correspondence to: Lilly Y.W. Bourguignon, Department of Cell Biology and Anatomy, University of Miami Medical School, 1600 N.W. 10th Avenue, Miami, FL 33101. Tel:(305) 547-6691 Fax:(305) 545-7166 E-mail:Lbourgui{at}med.miami.edu.
Tiam1 (T-lymphoma invasion and metastasis 1) is one of the known guanine nucleotide (GDP/GTP) exchange factors (GEFs) for Rho GTPases (e.g., Rac1) and is expressed in breast tumor cells (e.g., SP-1 cell line). Immunoprecipitation and immunoblot analyses indicate that Tiam1 and the cytoskeletal protein, ankyrin, are physically associated as a complex in vivo. In particular, the ankyrin repeat domain (ARD) of ankyrin is responsible for Tiam1 binding. Biochemical studies and deletion mutation analyses indicate that the 11amino acid sequence between amino acids 717 and 727 of Tiam1 (717GEGTDAVKRS727L) is the ankyrin-binding domain. Most importantly, ankyrin binding to Tiam1 activates GDP/GTP exchange on Rho GTPases (e.g., Rac1).
Using an Escherichia coliderived calmodulin-binding peptide (CBP)tagged recombinant Tiam1 (amino acids 393728) fragment that contains the ankyrin-binding domain, we have detected a specific binding interaction between the Tiam1 (amino acids 393738) fragment and ankyrin in vitro. This Tiam1 fragment also acts as a potent competitive inhibitor for Tiam1 binding to ankyrin. Transfection of SP-1 cell with Tiam1 cDNAs stimulates all of the following: (1) Tiam1ankyrin association in the membrane projection; (2) Rac1 activation; and (3) breast tumor cell invasion and migration. Cotransfection of SP1 cells with green fluorescent protein (GFP)tagged Tiam1 fragment cDNA and Tiam1 cDNA effectively blocks Tiam1ankyrin colocalization in the cell membrane, and inhibits GDP/GTP exchange on Rac1 by ankyrin-associated Tiam1 and tumor-specific phenotypes. These findings suggest that ankyrinTiam1 interaction plays a pivotal role in regulating Rac1 signaling and cytoskeleton function required for oncogenic signaling and metastatic breast tumor cell progression.
Key Words: Tiam1, ankyrin, Rac1 signaling, invasion/migration, metastatic breast tumor cells
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