Structural, biochemical, and functional properties of the Rap1-Interacting Adaptor Molecule (RIAM)

Abstract

Leukocytes, the leading players of immune system, are involved in innate and adaptive immune responses. Leukocyte adhesion to endothelial cells during transmigration or to antigen presenting cells during T cell activation, requires integrin activation through a process termed inside-out integrin signaling. In hematopoietic cells, Rap1 and its downstream effector RIAM (Rap1-interacting adaptor molecule) form a cornerstone for inside-out integrin activation. The Rap1/RIAM pathway is involved in signal integration for activation, actin remodeling and cytoskeletal reorganization in T cells, as well as in myeloid cell differentiation and function. RIAM is instrumental for phagocytosis, a process requiring particle recognition, cytoskeletal remodeling and membrane protrusion for engulfment and digestion. In the present review, we discuss the structural and molecular properties of RIAM and the recent discoveries regarding the functional role of the Rap1/RIAM module in hematopoietic cells.

Keywords: Adhesion; Integrins; Leukocytes; Phagocytosis; RIAM; Rap1.

Fig. 1

Structural characterization of RIAM and its homologs. (A) Subunits of human RIAM and homology with MRL family proteins are schematically presented. The proline-rich (PRR), RA, and PH domains are shown. The coiled-coil regions are indicated with an asterisk. (B) Human RIAM is a proline-rich protein and has six putative profilin-binding motifs (highlighted in gray) and six EVH1-binding motifs (underlined).

Fig. 2

RIAM implication in the integrin activation. (A) The composition of talin subunits. F0, F1, F2, and F3 subdomains are located at the N-terminus. The C-terminus talin-R includes 13 helical bundles (R1 to R13) followed by a dimerization domain (DD). (B) Although R8 is the strongest binding talin domain, the N-terminus talin binding region of RIAM also binds on talin F3 next to the integrin site, and competes with the talin autoinhibitory intramolecular interaction maintained between its F3 and R9 domains, thereby unmasking the integrin binding site on talin F3 and allowing talin F3-integrin binding. (C) RIAM itself is also subsect to autoinhibition induced by binding of its N-terminus to the RA domain. Phosphorylation of Tyr45 in RIAM N-terminus, mediated by focal adhesion kinase (FAK), releases this intramolecular interaction thereby alleviating RIAM autoinhibition and allowing the binding of Rap1. In addition, RIAM autoinhibition is induced by oligomerization of two RIAM molecules via their PH domains masking the PI(4,5)P₂-binding site. Phosphorylation of Tyr267 and Tyr427 in the RIAM PH domain by Src family kinases releases this intermolecular interaction and promotes PH domain interaction with PI(4,5)P₂, localization of RIAM at the plasma membrane and integrin activation.

Fig. 3

RIAM is involved in inside-out signaling dynamics in T cells and myeloid cells. Upon stimulation via TCR or chemokine receptors, activated talin is recruited to the membrane where it mediates conformational change and activation of integrins. This process couples receptor signaling to donwstream cellular events such as migration and chemotaxis. (A) In conventional T cells this process requires the Rap1/RIAM module. (B) In Treg cells, Lamellopodin (Lpd), a RIAM paralogue, might contribute to integrin activation, rendering RIAM partially redundant for such function. (C) RIAM is localized in the phagocytic cup and enhances pathogen clearance through complement-mediated phagocytosis and production of ROS.