RIAM Activates Integrins by Linking Talin to Ras GTPase Membrane-targeting Sequences

Rap1 small GTPases interact with Rap1-GTP-interacting adaptor molecule (RIAM), a member of the MRL (Mig-10/RIAM/Lamellipodin) protein family, to promote talin-dependent integrin activation. Here, we show that MRL proteins function as scaffolds that connect the membrane targeting sequences in Ras GTPases to talin, thereby recruiting talin to the plasma membrane and activating integrins. The MRL proteins bound directly to talin via short, N-terminal sequences predicted to form amphipathic helices. RIAM-induced integrin activation required both its capacity to bind to Rap1 and to talin. Moreover, we constructed a minimized 50-residue Rap-RIAM module containing the talin binding site of RIAM joined to the membrane-targeting sequence of Rap1A. This minimized Rap-RIAM module was sufficient to target talin to the plasma membrane and to mediate integrin activation, even in the absence of Rap1 activity. We identified a short talin binding sequence in Lamellipodin (Lpd), another MRL protein; talin binding Lpd sequence joined to a Rap1 membrane-targeting sequence is sufficient to recruit talin and activate integrins. These data establish the mechanism whereby MRL proteins interact with both talin and Ras GTPases to activate integrins.Increased affinity (“activation”) of cellular integrins is central to physiological events such as cell migration, assembly of the extracellular matrix, the immune response, and hemostasis (1). Each integrin comprises a type I transmembrane α and β subunit, each of which has a large extracellular domain, a single transmembrane domain, and a cytoplasmic domain (tail). Talin binds to most integrin β cytoplasmic domains and the binding of talin to the integrin β tail initiates integrin activation (2–4). A small, PTB-like domain of talin mediates activation via a two-site interaction with integrin β tails (5), and this PTB domain is functionally masked in the intact talin molecule (6). A central question in integrin biology is how the talin-integrin interaction is regulated to control integrin activation; recent work has implicated Ras GTPases as critical signaling modules in this process (7).Ras proteins are small monomeric GTPases that cycle between the GTP-bound active form and the GDP-bound inactive form. Guanine nucleotide exchange factors (GEFs) promote Ras activity by exchanging bound GDP for GTP, whereas GTPase-activating proteins (GAPs)³ enhance the hydrolysis of Ras-bound GTP to GDP (for review, see Ref. 8). The Ras subfamily members Rap1A and Rap1B stimulate integrin activation (9, 10). For example, expression of constitutively active Rap1 activates integrin αMβ2 in macrophage, and inhibition of Rap1 abrogated integrin activation induced by inflammatory agonists (11–13). Murine T-cells expressing constitutively active Rap1 manifest enhanced integrin dependent cell adhesion (14). In platelets, Rap1 is rapidly activated by platelet agonists (15, 16). A knock-out of Rap1B (17) or of the Rap1GEF, RasGRP2 (18), resulted in impairment of αIIbβ3-dependent platelet aggregation, highlighting the importance of Rap1 in platelet aggregation in vivo. Thus, Rap1 GTPases play important roles in the activation of several integrins in multiple biological contexts.Several Rap1 effectors have been implicated in integrin activation (19–21). Rap1-GTP-interacting adaptor molecule (RIAM) is a Rap1 effector that is a member of the MRL (Mig-10/RIAM/Lamellipodin) family of adaptor proteins (20). RIAM contains Ras association (RA) and pleckstrin homology (PH) domains and proline-rich regions, which are defining features of the MRL protein family. In Jurkat cells, RIAM overexpression induces β1 and β2 integrin-mediated cell adhesion, and RIAM knockdown abolishes Rap1-dependent cell adhesion (20), indicating RIAM is a downstream regulator of Rap1-dependent signaling. RIAM regulates actin dynamics as RIAM expression induces cell spreading; conversely, its depletion reduces cellular F-actin content (20). Whereas RIAM is greatly enriched in hematopoietic cells, Lamellipodin (Lpd) is a paralogue present in fibroblasts and other somatic cells (22).Recently we used forward, reverse, and synthetic genetics to engineer and order an integrin activation pathway in Chinese hamster ovary cells expressing a prototype activable integrin, platelet αIIbβ3. We found that Rap1 induced formation of an “integrin activation complex” containing RIAM and talin (23). Here, we have established the mechanism whereby Ras GTPases cooperate with MRL family proteins, RIAM and Lpd, to regulate integrin activation. We find that MRL proteins function as scaffolds that connect the membrane targeting sequences in Ras GTPases to talin, thereby recruiting talin to integrins at the plasma membrane.