The Mechanism for Regulation of the F-actin Binding Activity of IQGAP1 by Calcium/Calmodulin

• Published in: The Journal of biological chemistry 2002-04, Vol.277 (14), p.12324-12333
• Main Authors: Mateer, Scott C., McDaniel, Amanda E., Nicolas, Valérie, Habermacher, Geoffrey M., Lin, Mei-Jung Sun, Cromer, Damond A., King, Michelle E., Bloom, George S.
• Format: Article
• Language: English
• Subjects: 3T3 Cells; Actins - metabolism; Amino Acid Motifs; Animals; Calcimycin - pharmacology; Calcium - metabolism; Calmodulin - metabolism; Carrier Proteins - metabolism; Cells, Cultured; Dimerization; DNA, Complementary - metabolism; Glutathione Transferase - metabolism; Humans; Immunoblotting; IQGAP1 protein; Mice; Microscopy, Electron; Models, Biological; Mutagenesis, Site-Directed; Plasmids - metabolism; Polymerase Chain Reaction; Precipitin Tests; Protein Binding; Protein Structure, Tertiary; ras GTPase-Activating Proteins; Recombinant Proteins - metabolism; Time Factors
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M109535200

IQGAP1 colocalizes with actin filaments in the cell cortex and binds in vitro to F-actin and several signaling proteins, including calmodulin, Cdc42, Rac1, and β-catenin. It is thought that the F-actin binding activity of IQGAP1 is regulated by its reversible association with these signaling molecules, but the mechanisms have remained obscure. Here we describe the regulatory mechanism for calmodulin. Purified adrenal IQGAP1 was found to consist of two distinct protein pools, one of which bound F-actin and lacked calmodulin, and the other of which did not bind F-actin but was tightly associated with calmodulin. Based on this finding we hypothesized that calmodulin negatively regulates binding of IQGAP1 to F-actin. This hypothesis was tested in vitro using recombinant wild type and mutated IQGAP1s and in live cells that transiently expressed IQGAP1-YFP. In vitro, the affinity of wild type IQGAP1 for F-actin decreased with increasing concentrations of calmodulin, and this effect was dramatically enhanced by Ca2+ and required the IQ domains of IQGAP1. In addition, we found that calmodulin bound wild type IQGAP1 much more efficiently in the presence of Ca2+ than EGTA, and all 8 IQ motifs in each IQGAP1 dimer could bind calmodulin simultaneously. In live cells, IQGAP1-YFP localized to the cell cortex, but elevation of intracellular Ca2+ reversibly induced the fluorescent fusion protein to become diffusely distributed. Taken together, these results support a model in which a rise in free intracellular Ca2+ promotes binding of calmodulin to IQGAP1, which in turn inhibits IQGAP1 from binding to cortical actin filaments.