The IQGAP1 Protein Is a Calmodulin-regulated Barbed End Capper of Actin Filaments

Abstract

IQGAP1 is a large modular protein that displays multiple partnership and is thought to act as a scaffold in coupling cell signaling to the actin and microtubule cytoskeletons in cell migration, adhesion, and cytokinesis. However the molecular mechanisms underlying the activities of IQGAP1 are poorly understood in part because of its large size, poor solubility and lack of functional assays to challenge biochemical properties in various contexts. We have purified bacterially expressed recombinant human IQGAP1. The protein binds Cdc42, Rac1, and the CRIB domain of N-WASP in a calmodulin-sensitive fashion. We further show that in addition to bundling of filaments via a single N-terminal calponin-homology domain, IQGAP1 actually regulates actin assembly. It caps barbed ends, with a higher affinity for ADP-bound terminal subunits (K B ‫؍‬ 4 nM). The barbed end capping activity is inhibited by calmodulin, consistent with calmodulin binding to IQGAP1 with a K C of 40 nM, both in the absence and presence of Ca 2؉ ions. The barbed end capping activity resides in the C-terminal half of IQGAP1. It is possible that the capping activity of IQGAP1 accounts for its stimulation of cell migration. We further find that bacterially expressed recombinant IQGAP1 fragments easily co-purify with nucleic acids that turn out to activate N-WASP protein to branch filaments with Arp2/3 complex. The present results open perspectives for tackling the function of IQGAP1 in more complex reconstituted systems.

Rac1 is deactivated at integrin activation sites via an IQGAP1/filamin-A/RacGAP1 pathway

“…4B and supplementary material Table S3). In addition to Rac1 and RhoA, there is evidence that IQGAP1 can recruit and regulate other small GTPases including Cdc42 (Bashour et al, 1997;Kuroda et al, 1996), Arf6 (Hu et al, 2009) and Rap1 (Awasthi et al, 2010;Jeong et al, 2007) and their downstream effectors (supplementary material Table S3) (Noritake et al, 2004;Pelikan-Conchaudron et al, 2011;Sakurai-Yageta et al, 2008). A recurring theme from these studies, and from ours, is that modulation of GTPase activity by IQGAP1 is due to the recruitment of other proteins.…”

The Cdc42 Guanine Nucleotide Exchange Factor FGD6 Coordinates Cell Polarity and Endosomal Membrane Recycling in Osteoclasts

“…Thus, FGD6 may regulate the interactions of integrins with different F-actin meshworks. Another FGD6 interactor is IQGAP1, which binds Cdc42 (37) and has barbed end capping activity of actin filaments (38). IQGAP1 contains an atypical phosphoinositide binding domain at its C terminus allowing its binding to PI(3,4,5)P 3 (39,40), consistent with its localization to podosomes/sealing zones of osteoclasts.…”

IQGAPs as Key Regulators of Actin-cytoskeleton Dynamics

“…4) although this particular function of IQGAP1 in cells has not been addressed. Carlier and colleagues found that IQGAP1 purified from bacteria, caps the barbed ends with a higher affinity for the ADP-bound terminal subunits in actin filaments (Pelikan-Conchaudron et al, 2011). In addition, this activity depends on the C-terminal half of IQGAP1 and the capping activity is inhibited by calmodulin irrespective of the Ca 2+ concentration.…”