RNAi is proving to be a powerful experimental tool for the functional annotation of mammalian genomes. The full potential of this technology will be realized through development of approaches permitting regulated manipulation of endogenous gene expression with coordinated reexpression of exogenous transgenes. We describe the development of a lentiviral vector platform, pSLIK (single lentivector for inducible knockdown), which permits tetracycline-regulated expression of microRNA-like short hairpin RNAs from a single viral infection of any naïve cell system. In mouse embryonic fibroblasts, the pSLIK platform was used to conditionally deplete the expression of the heterotrimeric G proteins G␣12 and G␣13 both singly and in combination, demonstrating the G␣13 dependence of serum response element-mediated transcription. In RAW264.7 macrophages, regulated knockdown of G␤2 correlated with a reduced Ca 2؉ response to C5a. Insertion of a GFP transgene upstream of the G␤2 microRNA-like short hairpin RNA allowed concomitant reexpression of a heterologous mRNA during tetracycline-dependent target gene knockdown, significantly enhancing the experimental applicability of the pSLIK system. G protein ͉ tetracycline

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental pollutant with many toxic effects, including endocrine disruption, reproductive dysfunction, immunotoxicity, liver damage, and cancer. These are mediated by TCDD binding to and activating the aryl hydrocarbon receptor (AhR), a basic helix-loop-helix transcription factor. In this regard, targeting the AhR using novel small molecule inhibitors is an attractive strategy for the development of potential preventive agents. In this study, by screening a chemical library composed of approximately 10,000 compounds, we identified a novel compound, 2-methyl-

Mammalian phospholipase D (PLD) plays a key role in several signal transduction pathways and is involved in many diverse functions. To elucidate the complex molecular regulation of PLD, we investigated PLD-binding proteins obtained from rat brain extract. Here we report that a 43-kDa protein in the rat brain, ␤-actin, acts as a major PLD2 direct-binding protein as revealed by peptide mass fingerprinting in combination with matrixassisted laser desorption ionization/time-of-flight mass spectrometry. We also determined that the region between amino acids 613 and 723 of PLD2 is required for the direct binding of ␤-actin, using bacterially expressed glutathione S-transferase fusion proteins of PLD2 fragments. Intriguingly, purified ␤-actin potently inhibited both phosphatidylinositol-4,5-bisphosphateand oleate-dependent PLD2 activities in a concentrationdependent manner (IC 50 ‫؍‬ 5 nM). In a previous paper, we reported that ␣-actinin inhibited PLD2 activity in an interaction-dependent and an ADP-ribosylation factor 1 In vitro binding analyses showed that ␤-actin could displace ␣-actinin binding to PLD2, demonstrating independent interaction between cytoskeletal proteins and PLD2. Furthermore, ARF1 could steer the PLD2 activity in a positive direction regardless of the inhibitory effect of ␤-actin on PLD2. We also observed that ␤-actin regulates PLD1 and PLD2 with similar binding and inhibitory potencies. Immunocytochemical and co-immunoprecipitation studies demonstrated the in vivo interaction between the two PLD isozymes and actin in cells. Taken together, these results suggest that the regulation of PLD by cytoskeletal proteins, ␤-actin and ␣-actinin, and ARF1 may play an important role in cytoskeleton-related PLD functions. Mammalian phospholipase D (PLD)1 hydrolyzes phosphatidylcholine (PC) to generate phosphatidic acid and choline in response to a variety of signals, which can include hormones, neurotransmitters, and growth factors (1). phosphatidic acid itself has been shown to be an intracellular lipid second messenger and to be involved in multiple physiological events such as the promotion of mitogenesis, stimulation of respiratory bursts, secretory processes, actin cytoskeletal reorganization, and the activation of Raf-1 kinase and phosphatidylinositol 4-phosphate (PtdIns4P) 5-kinase isoforms in a large number of cells. These relationships suggest that agonist-induced PLD activation may play roles in multiple signaling events (2-7). The mammalian PLD isozymes identified thus far, PLD1 and PLD2, share a sequence homology of ϳ50%, but they have very different regulatory properties. PLD1 has low basal activity in the presence of phosphatidylinositol-4,5-bisphosphate (PIP 2 ) and can be activated by several cytosolic factors including protein kinase C ␣ and small GTP-binding proteins such as Rho A, Rac-1, ARF1, RalA, and CDC42 (8 -15). PLD2 also depends on PIP 2 but has a higher basal activity than PLD1 (16), and it has been proposed that PLD2 may be closely associated with different cellular inhibitors. Alth...

PLC (phospholipase C) plays an important role in intracellular signal transduction by hydrolysing phosphatidylinositol 4,5-bisphosphate, a membrane phospholipid. To date, 12 members of the mammalian PLC isoforms have been identified and classified into five isotypes beta, gamma, delta, epsilon and zeta, which are regulated by distinct mechanisms. In the present study, we describe the identification of a novel PLC isoform in the brains of human and mouse, named PLC-eta, which contains the conserved pleckstrin homology domain, X and Y domains for catalytic activity and the C2 domain. The first identified gene encoded 1002 (human) or 1003 (mouse) amino acids with an estimated molecular mass of 115 kDa. The purified recombinant PLC-eta exhibited Ca2+-dependent catalytic activity on phosphatidylinositol 4,5-bisphosphate. Furthermore, molecular biological analysis revealed that the PLC-eta gene was transcribed to several splicing variants. Although some transcripts were detected in most of the tissues we examined, the transcript encoding 115 kDa was restricted to the brain and lung. In addition, the expression of the 115 kDa protein was defined in only nerve tissues such as the brain and spinal cord. In situ hybridization analysis with brain revealed that PLC-eta was abundantly expressed in various regions including cerebral cortex, hippocampus, zona incerta and cerebellar Purkinje cell layer, which are neuronal cell-enriched regions. These results suggest that PLC-eta may perform fundamental roles in the brain.

Among the phospholipase C that catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate, four mammalian phospholipase C-␤ (PLC-␤) isotypes (isotypes 1-4) are activated through G protein-coupled receptors (GPCRs). Although the regulation of the PLC-␤s by GPCRs and heterotrimeric G proteins has been extensively studied, little is known about the molecular determinants that regulate their activity. The PLC-␤ isozymes carry a putative PSD-95/Dlg/ZO-1 (PDZ) binding motif (X(S/T)X(V/L)COOH) at their carboxyl terminus, which is implicated in specific interactions with anchor proteins. Using the yeast two-hybrid system, we identified Na ؉ /H ؉ exchanger regulatory factor 2 (NHERF2) as a protein that interacted with a C-terminal heptapeptide of PLC-␤3. Immunoprecipitation studies revealed that NHERF2 interacts specifically with PLC-␤3, but not with other PLC-␤ isotypes. Furthermore, PLC-␤3 interacted with NHERF2 rather than with other PDZ-containing proteins. This interaction required the COOH-terminal NTQL sequence of PLC-␤3 and the second PDZ domain of NHERF2. Interestingly, NHERF2 potentiated the PLC-␤ activation by carbachol in COS7 and HeLa cells, while mutant NHERF2, lacking the second PDZ domain, had no such effect. Taken together, the data suggest that NHERF2 may act as a modulator underlying the process of PLC-␤3-mediated signaling.