Critical Roles for the IQ‐Motif and Actin‐Bnding Domain of Endothelial Cell IQGAP1 During Leukocyte Transendothelial Migration

Abstract

In order to mount an appropriate inflammatory response, leukocytes must cross the endothelium to gain access to the target tissue and carry out their essential roles. This process is referred to as transendothelial migration (TEM) and requires a novel endothelial cell membrane compartment, the Lateral Border Recycling Compartment (LBRC), which is delivered to the migrating leukocyte to supply it with several requisite membrane proteins. IQGAP1, a large soluble scaffolding protein, is associated with the LBRC and is required for its function during TEM. In a variety of other processes, IQGAP1 integrates signals from diverse pathways to regulate cytoskeletal remodeling, Rho family GTPase activity, and other downstream readouts, however its role in TEM remains unclear. Here we show that genetic ablation of endothelial IQGAP1 expression arrests leukocytes during TEM by preventing the targeted delivery of the LBRC. Expression of a full length GFP‐tagged IQGAP1 construct completely restores this defect and also shows that endothelial IQGAP1 is enriched around the leukocyte during TEM. Furthermore, using fluorescently tagged IQGAP1 truncation constructs, we show that the actin‐binding domain of IQGAP1 is required for its localization to the junction in resting endothelial cells and its recruitment and function during TEM. Interestingly, the IQ motif of IQGAP1, which facilitates its interaction with calmodulin, is not required for localization to the junction or recruitment during TEM, but is necessary for IQGAP1′s function during TEM. Together these data suggest that IQGAP1 integrates signals from various sources to facilitate the function and recruitment of the LBRC during TEM. Supported by R01 HL046849 and R37 HL064774.