Members of the plexin family are unique transmembrane receptors in that they interact directly with Rho family small GTPases; moreover, they contain a GTPase-activating protein (GAP) domain for R-Ras, which is crucial for plexin-mediated regulation of cell motility. However, the functional role and structural basis of the interactions between the different intracellular domains of plexins remained unclear. Here we present the 2.4 Å crystal structure of the complete intracellular region of human plexin-B1. The structure is monomeric and reveals that the GAP domain is folded into one structure from two segments, separated by the Rho GTPase binding domain (RBD). The RBD is not dimerized, as observed previously. Instead, binding of a conserved loop region appears to compete with dimerization and anchors the RBD to the GAP domain. Cell-based assays on mutant proteins confirm the functional importance of this coupling loop. Molecular modeling based on structural homology to p120 GAP ⅐H-Ras suggests that Ras GTPases can bind to the plexin GAP region. Experimentally, we show that the monomeric intracellular plexin-B1 binds R-Ras but not H-Ras. These findings suggest that the monomeric form of the intracellular region is primed for GAP activity and extend a model for plexin activation.Plexins are single transmembrane receptors for guidance cues, called semaphorins, which regulate the motility and positional maintenance of certain cells. With this function, the receptors play critical roles in many developmental processes, including axon guidance, angiogenesis, and bone formation (1, 2). Moreover, plexins and their ligands are also involved in the regulation of the immune response, in cancer progression, and are thought to restrain tissue regeneration after injury (3, 4).Plexins are unusual receptors in that they interact directly with Rho and Ras family small GTPases (5-7). An intracellular region that has high homology to Ras GTPase-activating proteins (GAPs) 7 facilitates the hydrolysis of R-Ras-bound GTP. This deactivation of R-Ras leads to functional inhibition of integrins and to a loss of cell adhesion in response to semaphorins (5-8). Interestingly, no GAP activity of plexin-B1 was detected toward the R-Ras-homologous H-Ras (5), suggesting greater substrate specificity compared with the GAP protein p120 GAP (9). How the plexin receptor is activated and specifically how the GAP function is regulated have been questions of considerable interest (10 -12). A number of studies have pointed to a sequence segment that interrupts the GAP-homologous region and is capable of binding small Rho family GTPases. In the case of plexin-B1, this Rho GTPase binding domain (RBD) can associate with Rnd1, Rac1, and RhoD, which are thought to regulate plexin function. Specifically, in vitro studies in a number of laboratories have used the intracellular region of plexins expressed as two fragments, named C1 (containing the RBD and an N-terminal GAP-homologous segment) and C2 (C-terminal GAP segment). The studies suggest that such fragm...

EEG source imaging (ESI) is a model-based imaging technique that integrates temporal and spatial components of EEG to identify the generating source of electrical potentials recorded on the scalp. Recent advances in computer technologies have made the analysis of ESI data less time-consuming, and have rekindled interest in this technique as a clinical diagnostic tool. On the basis of the available body of evidence, ESI seems to be a promising tool for epilepsy evaluation; however, the precise clinical value of ESI in presurgical evaluation of epilepsy and in localization of eloquent cortex remains to be investigated. In this Review, we describe two fundamental issues in ESI; namely, the forward and inverse problems, and their solutions. The clinical application of ESI in surgical planning for patients with medically refractory focal epilepsy, and its use in source reconstruction together with invasive recordings, is also discussed. As ESI can be used to map evoked responses, we discuss the clinical utility of this technique in cortical mapping-an essential process when planning resective surgery for brain regions that are in close proximity to eloquent cortex.

The plexin family of transmembrane receptors are important for axon guidance, angiogenesis, but also in cancer. Recently, plexin-B1 somatic missense mutations were found in both primary tumors and metastases of breast and prostate cancers, with several mutations mapping to the Rho GTPase binding domain (RBD) in the cytoplasmic region of the receptor. Here we present the NMR solution structure of this domain, confirming that the protein has both a ubiquitin-like fold and surface features. Oncogenic mutations T1795A and T1802A are located in a loop region, perturb the average structure locally, and have no effect on Rho GTPase binding affinity. Mutations L1815F and L1815P are located at the Rho GTPase binding site and are associated with a complete loss of binding for Rac1 and Rnd1. Both are found to disturb the conformation of the beta3-beta4 sheet and the orientation of surrounding side chains. Our study suggests that the oncogenic behavior of the mutants can be rationalized with reference to the structure of the RBD of plexin-B1.