Signal transduction. The hunt for Ras targets

The torso receptor tyrosine kinase can activate Raf in a Ras-independent pathway

Activation of the receptor tyrosine kinase (RTK) torso defines the spatial domains of expression of the transcription factors tailless and huckebein. Previous analyses have demonstrated that Ras1 (p21ras) operates upstream of the D-Raf (Raf1) serine/threonine kinase in this signaling pathway. By using a recently developed technique of germline mosaics, we find that D-Raf can be activated by torso in the complete absence of Ras1. This result is supported by analysis of D-Raf activation in the absence of either the exchange factor Son of sevenless (Sos) or the adaptor protein drk (Grb2), as well as by the phenotype of a D-Raf mutation that abolishes binding of Ras1 to D-Raf. Our study provides in vivo evidence that Raf can be activated by an RTK in a Ras-independent pathway.

Rho-related proteins: actin cytoskeleton and cell cycle

The past year has produced an abundance of data on the function and regulation of Rho-related GTP-binding proteins. In mammalian cells, it has been shown that Rho is required for contractile ring assembly at cell division, as well as for regulating extracellular factor induced actin reorganization. In addition, many new regulators and/or potential targets for Rho, Rac and Cdc42 have been characterized, including several oncogene products, protein kinases and signal transducing proteins in mammalian cells, and genes defined by cell cycle or bud emergence mutations in yeast. These provide further connections between Rho-related proteins, signal transduction pathways and changes in actin organization during cell cycle entry and progression.

Ha-Ras stimulates uptake and phosphorylation of ethanolamine: inhibition by wortmannin

Transformation of NIH 3T3 fibroblasts by Ha-Ras resulted in large increases in the phosphorylation of both [14C]ethanolamine (Etn) and [14C]choline (Cho) when these precursors were added to the medium. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), preferentially decreased phosphorylation of externally added Etn in the Ha-Ras transformed, but not in the untransformed, fibroblasts. However, wortmannin had no effect on the phosphorylation of Etn formed endogenously by phorbol ester-stimulated hydrolysis of phosphatidylethanolamine. Data suggest that interaction of mutated Ras with PI3K leads to specific stimulation of Etn uptake, followed by nearly quantitative phosphorylation of Etn by a Ras-activated Cho/Etn kinase.

Gene therapy and pharmacological treatment of inherited neurological disorders

The identification of mutant genes responsible for inherited neurological disorders gives an opportunity to consider new approaches to their treatment. Although replacement of defective genes in postmitotic neurons is unlikely to be possible in the near future, the identification of gene products, and definitive delineation of the cellular dysfunction and death that result from mutated gene products, may suggest new therapeutic options. This article reviews these advances and considers approaches that may provide hope for alleviation of these disorders.

Small G proteins and the neutrophil NADPH oxidase

The NADPH oxidase of phagocytic cells is a multimeric enzyme complex activated during phagocytosis. It catalyzes the production of the superoxide anion, precursor of many toxic oxygen metabolites involved in the defense against microorganisms. The enzyme becomes active after assembly on a membrane bound flavocytochrome b of cytosolic factors p47 phox, p67 phox and p40 phox and of low molecular mass GTP binding proteins. This paper reviews recent results concerning the role of two small G proteins, Rac and Rap 1A in oxidase activation. Native prenylated small G proteins are either in the form of a complex in which the GDP bound G protein is associated with a guanine nucleotide dissociation inhibitor, GDI, or in an active GTP bound form able to trigger the activity of its effector. Rac and Rho share a common GDI. As chemotaxis, under Rho control, and oxidase activation, under Rac control, show mutually exclusive signalling pathways, we propose a model where the GDI would switch from one pathway to the other by sequestering either Rac or Rho.

Polyoma virus middle T antigen: meddler or mimic?

Polyoma virus middle T antigen duplicates the actions of growth-factor receptors in binding the signalling molecules phosphatidylinositol 3'-OH kinase and Shc. These properties indicate that middle T is mitogenic and may be required to overcome inhibition of DNA replication during the lytic life cycle of the virus.

Mechanistic studies of a signaling pathway activated by the organic dimerizer FK1012

BACKGROUND

The T-cell receptor (TCR) signaling pathway is initiated by regulated association of TCR chains, including the zeta chain. A recently reported method for inducing the dimerization or oligomerization of targeted proteins in cells used the TCR pathway as a test system. In cells transfected with cDNA encoding MZF3E, a chimeric receptor comprising the intracellular domain of the zeta chain and three copies of FK506-binding protein (FKBP), low concentrations of a synthetic dimer of the natural product FK506 (FK1012) activated the expression of reporter genes. We set out to examine the signaling pathway initiated by FK1012.

RESULTS

We characterized the effect of FK1012 on MZF3E and a second chimeric receptor, MZF1E, which contains the zeta chain and one copy of FKBP. Only MZF3E gave FK1012-activated signaling, as shown by an increase in the kinase activity associating with MZF3E, and the appearance of specific phosphotyrosine-containing proteins. Signaling required localization of MZF3E to the inner plasma membrane, and activation of gene transcription in response to FK1012 was dependent on the protein phosphatase calcineurin and the transcriptional activator NF-AT. Some signaling events in the pathway had different kinetics when activated by MZF3E instead of the TCR, however. An unexpected requirement for the prolonged activation of calcineurin was observed.

CONCLUSIONS

Synthetic dimerizers can be used to gain control over cellular processes that require the association of specific intracellular proteins. The TCR signaling pathway was selected as an initial test system; we show here that one can indeed activate this signaling pathway by inducing the oligomerization of the cytoplasmic tail of the zeta chain with the cell-permeable reagent FK1012.