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The Rap GTPases coordinate actin and microtubule cytoskeleton reorganization, and promote antigen extraction at the B cell immune synapse Wang, Jia Chao

Abstract

B cells that bind antigens displayed on antigen-presenting cells (APCs) form an immune synapse (IS), a polarized cellular structure that optimizes the dual functions of the B cell receptor (BCR), signal transduction and antigen internalization. Immune synapse formation involves the polarization of the microtubule-organizing center (MTOC) towards the APC. I showed that BCR-induced MTOC polarization requires the Rap1 GTPases (which has two isoforms, Rap1a and Rap1b), an evolutionarily conserved regulator of cell polarity, as well as cofilin-1, an actin-severing protein that is regulated by Rap1. MTOC reorientation towards the antigen contact site correlated strongly with cofilin-1-dependent actin reorganization and cell spreading. I also showed that BCR-induced MTOC polarization requires the dynein motor protein as well as IQGAP1, a scaffolding protein that can link the actin and microtubule cytoskeletons. At the periphery of the immune synapse, IQGAP1 associates closely with F-actin structures and with the microtubule plus-end-binding protein CLIP-170. Moreover, the accumulation of IQGAP1 at the antigen contact site depends on F-actin reorganization that is controlled by Rap1 and cofilin-1. I also demonstrate that the hematopoietic-cell specific cortactin-homologue, HS1, is essential for regulating actin cytoskeletal remodeling during immune synapse formation and acts downstream of Rap to promote BCR-induced antigen gathering. Additionally, inhibiting the Rap1-cofilin-1 pathway, CLIP-170 expression, or cytoskeletal dynamics impairs the ability of B cells to acquire antigens from APCs. Thus, Rap1 coordinates actin and microtubule organization at the IS, facilitating antigen acquisition from APCs.

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