Regulated Membrane Localization of Tiam1, Mediated by the NH2-terminal Pleckstrin Homology Domain, Is Required for Rac-dependent Membrane Ruffling and C-Jun NH2-terminal Kinase Activation

Rho-like GTPases, including Cdc42, Rac, and Rho, regulate signaling pathways that control actin cytoskeletal structures and transcriptional activation. The Tiam1 gene encodes an activator of Rac1, and similarly to constitutively activated (V12)Rac1, overexpression of Tiam1 in fibroblasts induces the formation of membrane ruffles. Tiam1 contains a Dbl homology (DH) domain and adjacent pleckstrin homology (PH) domain, hallmarks for activators of Rho-like GTPases. Unique for Tiam1 are an additional PH domain and a Discs-large homology region in the NH2-terminal part of the protein. Here we show that both in fibroblasts and COS cells, membrane localization of Tiam1 is required for the induction of membrane ruffling. A detailed mutational analysis, in combination with confocal laser scanning microscopy and immunoelectron microscopy, demonstrates that the NH2-terminal PH domain of Tiam1, but not the DH-adjacent PH domain, is essential for membrane association. This NH2-terminal PH domain of Tiam1 can be functionally replaced by the myristoylated membrane localization domain of c-Src, indicating that the primary function of this PH domain is to localize the protein at the membrane. After serum starvation, both membrane association of Tiam1 and ruffling can be induced by serum, suggesting that receptor stimulation induces membrane translocation of Tiam1. Similar to V12Rac1, Tiam1 stimulates the activity of the c-Jun NH2-terminal kinase (JNK). This Rac-dependent stimulation of JNK also requires membrane association of Tiam1. We conclude that the regulated membrane localization of Tiam1 through its NH2-terminal PH domain determines the activation of distinct Rac-mediated signaling pathways

Ecology and morphological characters of different populations of Poecilia sphenops vandepolli (Cyprinidontidae)

In the Netherlands Antilles, opposite the Venezuelan coast, the cyprinodont fish Poecilia sphenops vandepolli is found in fresh water, in brackish water, in the sea and in supersaline water. When comparing the populations from fresh water, sea water and supersaline water some significant morphological differences were found, e.g. in size, in depth of the body and of the caudal peduncle, in length of the head, and in the number of rays in the pectoral and caudal fins and the number of lateral scales. In raising experiments, however, it could be shown that these differences are phenotypic. The characteristics of the subspecies or varietas arubensis as described by VAN LIDTH DE JEUDE (1887) proved also to be phenotypic. Optimum growth was found in seawater. The adaptation to fresh water after transfer from sea water or supersaline water is quicker than in the opposite direction; this concerns specific weight adaptation, growth resumption and the change of preference for the new salinity after transfer. With respect to these characteristics fresh water is more favourable than sea water or brine. The inland migration after rainfall is not caused by the fresh water itself, but by an organic compound that is found in inland water, whether fresh or saline, and also in rain water after it has been in contact with the soil. From the fact that mollies also are attracted by IJsselmeer water, just as elvers are, it seems likely that mollies and elvers are attracted by the same organic compound. This behaviour of the molly causes irregular migrations from sea to inland waters which prevent the inland populations from developing into separate forms, races or subspecies