Protein-protein interaction plays important roles in transducing signals elicited from receptors on the cell surface to the nucleus. The Src homology 2 (SH2) and SH3 domains have been shown to bind to tyrosine-phosphorylated proteins (33) and proline-rich motifs (54), respectively (reviewed in references 4, 27, 51, and 61). There are a number of signaling molecules involved in the tyrosine kinase cascade which have either the SH2 domain or the SH3 domain or both, including the GTPase-activating protein for Ras (Ras GAP), phospholipase C-␥, the p85 subunit of PI-3 kinase, and Src and related kinases. While these molecules have enzymatic activities, there is another group of molecules that consist mostly of SH domains without any enzymatic domains. Crk (34, 38), Grb2/Ash (30, 37), Shc (52), and Nck (28) belong to the latter group, the so-called adapter molecules. These multivalent adapter molecules may connect signaling molecules on them. Besides the SH2 and SH3 domains, recently the pleckstrin homology domain also has been identified in a number of signaling molecules (40).The stimulation of cells with various growth factors or cytokines activates their cognate receptor tyrosine kinases or nonreceptor tyrosine kinases associating with the receptors, which results in the tyrosine phosphorylation of various signaling molecules (60). Each tyrosine-phosphorylated molecule is recognized and bound by a specific SH2 domain (64, 65). Ras GAP, phospholipase C-␥, and the p85 subunit of PI-3 kinase have been shown to become tyrosine phosphorylated and bind to activated receptors through their SH2 domains (4,27,51,61). The interaction between SH3 domains and proline-rich motifs seems to be constitutive and not to depend on stimulation of the cells (54). These interactions by the SH2 and SH3 domains are responsible for the formation of multimolecular signaling complexes, some of which translocate to the plasma membrane.

The Ras-related small GTPases Rac, Rho, Cdc42, and RalA bind filamin, an actin filament-crosslinking protein that also links membrane and other intracellular proteins to actin. Of these GTPases only RalA binds filamin in a GTP-specific manner, and GTP-RalA elicits actin-rich filopods on surfaces of Swiss 3T3 cells and recruits filamin into the filopodial cytoskeleton. Either a dominant negative RalA construct or the RalA-binding domain of filamin 1 specifically block Cdc42-induced filopod formation, but a Cdc42 inhibitor does not impair RalA's effects, which, unlike Cdc42, are Rac independent. RalA does not generate filopodia in filamin-deficient human melanoma cells, whereas transfection of filamin 1 restores the functional response. RalA therefore is a downstream intermediate in Cdc42-mediated filopod production and uses filamin in this pathway.

CRK protein, together with GRB2/ASH and Nck proteins, belongs to the adaptor-type Src homology (SH)2-containing molecules, which transduce signals from tyrosine kinases. Here another guanine nucleotide-releasing protein (GNRP), C3G, has been identified as a CRK SH3-binding protein. The nucleotide sequence of a 4.1-kb C3G cDNA contains a 3.2-kb open reading frame encoding a 121-kDa protein, and antibodies against C3G have been shown to detect a protein of 130-140 kDa. The carboxyl terminus of C3G has a peptide sequence homologous to GNRPs for Ras, and the expression of this carboxyl terminus region suppresses the loss of CDC25 function in the yeast Saccharomyces cerevisiae. The C3G protein expressed in Escherichia coli binds to CRK and GRB2/ASH proteins. Mutational analysis of C3G assigns the SH3 binding region to a 50-amino acid region containing a proline-rich sequence. The mRNAs of both the C3G and CRK proteins are expressed ubiquitously in human adult and fetal tissues. The results of these studies suggest that the complex of CRK and C3G, or GRB2/ASH and C3G, may transduce the signals from tyrosine kinases to Ras in a number of different tissues.Growth factors elicit various responses through the activation of receptor-type and non-receptor-type tyrosine kinases (1). A group of cytoplasmic enzymes containing common amino acid sequences, designated Src homology (SH)2 and SH3 domains, play a pivotal role in transducing signals from the tyrosine kinases (2, 3). The SH2 domain responds to the signals from tyrosine kinases by binding to the tyrosinephosphorylated proteins, including the tyrosine kinases themselves. Some of the signals are also transmitted to proteins bound to the SH3 domains, but much less information is available on SH3-mediated signaling.The v-Crk protein was originally identified as an oncoprotein of a chicken retrovirus, CT10 (4). The protooncogene product v-Crk represents a newly emerging class of proteins consisting mostly of the SH2 and SH3 domains (5, 6). These proteins, now known as adaptorproteins, include Nck, GRB2/ ASH, Sem-5, and Drk (2, 7, 8). Sem-5 of Caenorhabditis elegans and Drk of Drosophila melanogaster appear to be homologues of mammalian GRB2/ASH. All of the adaptor proteins may be involved in the growth of fibroblasts. Overexpression or microinjection ofCRK, Nck, and GRB2 induces transformation of rat 3Y1 fibroblasts or DNA replication in mouse 3T3 fibroblasts (6,9,10). One common feature of the adaptor proteins may be signal transmission to Ras. The sem-5 gene of C. elegans has been mapped genetically downstream of let-23 tyrosine kinase and upstream of let-60 Ras-like protein (11). Similarly, drk of Drosophila is mapped between sevenless (sev) receptor tyrosine kinase and son of sevenless (sos), which encodes a guanine nucleotide-releasing protein (GNRP) for Rasl. Recently, the Sos protein ofDrosophila was shown to bind to the Drk protein, which contains SH2 and SH3 domains (7,8). Anti-Ras antibody also inhibited neuronal differentiation of PC12 cells induced by th...

Adult mammalian neurogenesis occurs in the hippocampus and the olfactory bulb, whereas neocortical adult neurogenesis remains controversial. Several occurrences of neocortical adult neurogenesis in injured neocortex were recently reported, suggesting that neural stem cells (NSCs) or neuronal progenitor cells (NPCs) that can be activated by injury are maintained in the adult brain. However, it is not clear whether or where neocortical NSCs/NPCs exist in the brain. We found NPCs in the neocortical layer 1 of adult rats and observed that their proliferation was highly activated by global forebrain ischemia. Using retrovirus-mediated labeling of layer 1 proliferating cells with membrane-targeted green fluorescent protein, we found that the newly generated neurons were GABAergic and that the neurons were functionally integrated into the neuronal circuitry. Our results suggest that layer 1 NPCs are a source of adult neurogenesis under ischemic conditions.