It has been proposed that the local segregation of kinases and the tyrosine phosphatase CD45 underpins T cell receptor (TCR) triggering, but how segregation would occur and whether it can initiate signaling is unclear. Using structural and biophysical analysis we show that the extracellular region of CD45 is rigid and extends beyond the distance spanned by TCR-ligand complexes, implying that sites of TCR-ligand engagement would sterically exclude CD45. We also show that the formation of new structures characterized by spontaneous sub-micron scale CD45 and kinase segregation, called ‘close-contacts’, initiates signaling even when TCR ligands are absent. Our work reveals the structural basis for, and the unexpectedly potent signaling effects of local CD45 and kinase segregation. TCR ligands could heighten signaling simply by holding receptors in close-contacts.

Cerebral cavernous malformations (CCMs) are congenital vascular anomalies of the central nervous system that can result in hemorrhagic stroke, seizures, recurrent headaches, and focal neurologic deficits. Mutations in the gene KRIT1 are responsible for type 1 CCM (CCM1). We report that a novel gene, MGC4607, exhibits eight different mutations in nine families with type 2 CCM (CCM2). MGC4607, similar to the KRIT1 binding partner ICAP1alpha, encodes a protein with a phosphotyrosine-binding domain. This protein may be part of the complex pathway of integrin signaling that, when perturbed, causes abnormal vascular morphogenesis in the brain, leading to CCM formation.