This defect in adhesion is accompanied by reduced T-cell proliferation and interleukin-2 production.51–53 Defects in T-cell selection have also been documented in certain ADAP-deficient transgenic models expressing a single TCR.54 ADAP binds directly to Src kinase-associated protein of molecular weight 55 000 (SKAP) by the interaction of the SKAP-55 SH3 domain to a proline-rich region in ADAP or the interaction of the ADAP SH3c domain to a tyrosine-based RKXXYXXY motif in SKAP-55 (Fig. 1).55–58 SKAP-55 is expressed in a restricted manner in T cells as a positive regulator for integrin activation, T-cell adhesion and T–APC www.selleckchem.com/products/pci-32765.html conjugate formation.51,59,60 The role of SKAP-55 in the

regulation of integrin activation could not be replaced by its homologue protein SKAP-55-related (SKAP-55R, also termed SKAP-55 Hom).59,61 Disruption of the ADAP–SKAP-55 module by deletion of the SKAP-55 SH3 domain or the ADAP proline-rich domain impairs formation of T–APC conjugates, LFA-1 adhesion and may prevent the membrane translocation of small G protein Rap1, a key player of integrin activation.51,62 Although important this website for integrin activation, SLP-76, ADAP and SKAP-55 do not

interact with integrin directly. Recently, we have identified that the ADAP–SKAP-55 module comprises a complex with the Rap1–RapL module after TCR stimulation. It has been demonstrated that RapL binds activated Rap1 after TCR or chemokine stimulation, and this interaction brings RapL close to the cell membrane FER to allow direct binding of the RapL to the cytoplasmic domain of the αL chain of LFA-1 (Fig. 1). RapL-deficient T or B cells are defective in cell adhesion and trafficking. We found that the N-terminal domain of SKAP-55 binds to the C-terminal SARAH domain of RapL, resulting in the formation of an SKAP-55–RapL–Rap1 complex that binds to LFA-1 and increases adhesion to ICAM-1. The Rap1–RapL complex formation and LFA-1 binding fail to occur in SKAP-55-deficient T cells. By contrast, chemokines SDF1

and CCL21 induce normal migration of SKAP-55-deficient T cells.63 Hence, SKAP-55 appears to serve as a specific adaptor to couple the TCR with the activation of the Rap1–RapL module for integrin adhesion. Another Rap1–GTP binding partner is Rap1–GTP-interacting adapter molecule (RIAM). Over-expression of RIAM increases cell spreading, lamellipod formation, integrin activation and adhesion.64 It has been shown that RIAM constitutively interacts with SKAP-55, and that the ADAP–SKAP-55 module promotes the membrane location of the RIAM–Rap1 module following TCR activation to facilitate integrin activation.65 In addition, the ability of RIAM to bind to profilin, Ena/VASP proteins and talin suggests that RIAM promotes integrin activation through effects on the actin cytoskeleton, particularly the interaction of talin with integrin cytoplasmic tails (Fig. 1).