Regular Article PLATELETS AND THROMBOPOIESIS ADAP interactions with talin and kindlin promote platelet integrin aIIbb3 activation and stable fibrinogen binding

Integrins engage in bidirectional signaling. Fibrinogen binding to integrins is regulated by inside-out signals initiated by agonist receptors, a process often called “integrin activation.” In turn, ligandbound integrins transduce outside-in signals to regulate cellular responses, among them cytoskeletal reorganization. In platelets, integrin aIIbb3 bidirectional signaling is required for efficient hemostatic platelet responses to vascular injury. Full inside-out signaling can promote increases in integrin affinity through conformational changes and increases in integrin avidity through receptor clustering; the relative contribution of each process can vary with the integrin and the cell studied. Recently, talin and kindlin, 2 adapter proteins that bind to integrin-b cytoplasmic tails, have emergedas essential regulators of integrin activation. Talin is a 280 kDa protein of the FERMT4 (4.1/ ezrin/radixin, moesin domain T4) family composed of head (amino acids 1-433) and rod (482-2541) domains that are normally clasped but upon relief of auto-inhibition can bind via the Ferm3 phospho tyrosinebinding subdomain to regions of integrinb tails that include a central b-NPXY motif and a membrane-proximal motif. Talin-membrane association is promoted in platelets by talin’s interaction with Rap1 and its effector, Rap1-interacting adapter molecule (RIAM). The kindlin family of proteins is composed of kindlin-1, kindlin-2, and kindlin-3, which interact with the membrane-distal portion of b cytoplasmic tails that includes an NXXY motif. Deficiency of kindlin-3, the predominant isoform in hematopoietic cells, causes defects in integrin activation, resulting in a profound immune and hemostatic disorder, leukocyte adhesion deficiency syndrome (LAD-III syndrome). Talin is necessary and sufficient to activate aIIbb3 in purified in vitro systems, but both talin and kindlin-3 are required for full integrin activation in platelets. A third adapter protein implicated in integrin regulation is the hematopoietic-restricted, alternatively-spliced 120/130 kDa adhesion and degranulation promoting adapter protein (ADAP). ADAP was previously implicated in aIIbb3 activation in human and mouse platelets downstream of both tyrosine-kinase–coupled and G-protein– coupled receptors. Recentwork has identified several ADAP binding partners (Figure 1A). In particular, ADAP stabilizes 2 Src kinase–associated phosphoproteins (SKAPs), SKAP1 and SKAP2, through ADAP residues 339 to 438, with only SKAP2 expressed in platelets. ADAP’s roles in signal transduction cascades have largely been explored in T lymphocytes, where ADAP and SKAP1 constitute a functional unit to enhanceb2 integrin–mediated adhesion upon T-cell receptor stimulation. In these cells, SKAP1 is the relevant effector operating through interactions with the Rap1-guanosine triphosphate–binding proteins RIAM or regulator of cell adhesion and polarization enriched in lymphoid tissues (RapL). When lymphocytes are stimulated through chemokine (C-C motif) receptor