Members of the LEF-1/TCF family of transcription factors have been implicated in the transduction of Wnt signals. However, targeted gene inactivations of Lef1, Tcf1, or Tcf4 in the mouse do not produce phenotypes that mimic any known Wnt mutation. Here we show that null mutations in both Lef1 and Tcf1, which are expressed in an overlapping pattern in the early mouse embryo, cause a severe defect in the differentiation of paraxial mesoderm and lead to the formation of additional neural tubes, phenotypes identical to those reported for Wnt3a-deficient mice. In addition, Lef1 −/− Tcf1 −/− embryos have defects in the formation of the placenta and in the development of limb buds, which fail both to express Fgf8 and to form an apical ectodermal ridge. Together, these data provide evidence for a redundant role of LEF-1 and TCF-1 in Wnt signaling during mouse development.

Wnt signaling is thought to be mediated via interactions between ␤-catenin and members of the LEF-1/TCF family of transcription factors. Here we study the mechanism of transcriptional regulation by LEF-1 in response to a Wnt-1 signal under conditions of endogenous ␤-catenin in NIH 3T3 cells, and we examine whether association with ␤-catenin is obligatory for the function of LEF-1. We find that Wnt-1 signaling confers transcriptional activation potential upon LEF-1 by association with ␤-catenin in the nucleus. By mutagenesis, we identified specific residues in LEF-1 important for interaction with ␤-catenin, and we delineated two transcriptional activation domains in ␤-catenin whose function is augmented in specific association with LEF-1. Finally, we show that a Wnt-1 signal and ␤-catenin association are not required for the architectural function of LEF-1 in the regulation of the T-cell receptor ␣ enhancer, which involves association of LEF-1 with a different cofactor, ALY. Thus, LEF-1 can assume diverse regulatory functions by association with different proteins.

Targeted inactivation of the murine gene encoding the transcription factor LEF-1 abrogates the formation of organs that depend on epithelial-mesenchymal tissue interactions. In this study we have recombined epithelial and mesenchymal tissues from normal and LEF-1-deficient embryos at different stages of development to define the LEF-1-dependent steps in tooth and whisker organogenesis. At the initiation of organ development, formation of the epithelial primordium of the whisker but not tooth is dependent on mesenchymal Left gene expression. Subsequent formation of a whisker and tooth mesenchymal papilla and completion of organogenesis require transient expression of Lefl in the epithelium. These experiments indicate that the effect of Lefl expression is transmitted from one tissue to the other. In addition, the finding that the expression of Lefl can be activated by bone morphogenetic protein 4 (BMP-4) suggests a regulatory role of this transcription factor in BMP-mediated inductive tissue interactions.[Key Words: Lefl; BMP-4; epithelial-mesenchymal interactions; tooth and whisker development]

Previously, we characterized cDNAs encoding polypyrimidine tract-binding protein {PTB) and showed that a complex between PTB and a 100-kD protein was necessary for pre-mRNA splicing. In this paper we have used two different in vitro-binding assays to confirm and extend the interaction between these two proteins. Peptide sequence information was used to clone and sequence cDNAs encoding alternatively spliced forms of the 100-kD protein. It contains two consensus RNA-binding domains and an unusual amino terminus rich in proline and glutamine residues. The protein is highly basic and migrates anomalously on SDS gels. Owing to its interaction with PTB and its role in pre-mRNA splicing, we have termed the 100-kD protein PTB-associated splicing factor (PSF). The RNA-binding properties of PSF are apparently identical to those of PTB. Both proteins, together and independently, bind the polypyrimidine tract of mammalian introns. Biochemical complementation, antibody inhibition, and immunodepletion experiments demonstrate that PSF is an essential pre-mRNA splicing factor required early in spliceosome formation. Bacterially synthesized PSF is able to complement immunodepleted extracts and restore splicing activity. Despite association with PSF, complementary experiments with antibodies against PTB do not suggest an essential role for PTB in pre-mRNA splicing.[Key Words: Pre-mRNA splicing; polypyrimidine tract-binding protein; PTB-associated splicing factor; cloning] Received December 9, 1992; revised version accepted January 5, 1993.The removal of intervening sequences from pre-mRNAs requires the formation of splicing complexes in an ordered, ATP-dependent pathway (for reviews, see Sharp 1987; Krainer and Maniatis 1988;Steitz et al. 1988;Smith et al. 1989a;Green 1991;Guthrie 1991). The assembly of multiple factors into these spliceosomes leads to intron excision through a two-step cleavage-ligation reaction. The development of mammalian in vitro splicing systems has allowed the identification of several cisand trans-acting components necessary for pre-mRNA splicing. Chief among the required cis-acting elements are a consensus sequence at the 5' splice site, a branchpoint sequence and adjacent polypyrimidine tract, and the 3' splice site AG dinucleotide (Mount 1982;Ohshima and Gotoh 1987). Among the best characterized of the trans-acting factors are the family of small nuclear ribonucleoprotein particles, the U snRNPs (Steitz et al. 1988). U1 and U2 snRNP initially base-pair with the 5' splice site and the branchpoint sequence, respectively, followed by the joining of U4/U6 and U5 as part of a preformed triple snRNP (Behrens and Lfihrmann 1991). Before catalysis, multiple base-pairing interactions between the pre-mRNA and small nuclear RNAs (snRNAs) Steitz 1992), multiple protein components, both snRNP and non-snRNP, are also required for pre-mRNA splicing. Each snRNP particle consists of one or two small RNAs complexed with common and unique proteins (Liihrmann 1988). Additional snRNPs are found associated with multi-snRN...