Cell entry of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is mediated by its surface glycoprotein, Spike. The S1 subunit of Spike contains the N-terminal domain (NTD) and the receptor-binding domain (RBD), which mediates recognition of the host cell receptor angiotensinconverting enzyme 2 (ACE2). The S2 subunit drives fusion

Members of the Wnt/Wingless (Wg) family of signalling proteins organize many aspects of animal development by regulating the expression of particular target genes in responding cells. Recent biochemical studies indicate that the vertebrate HMG-domain proteins Lef-1 and XTcf-3 can physically interact with beta-catenin, a homologue of Drosophila Armadillo (Arm), the most downstream component known in the Wnt signal transduction pathway. However, these studies do not address whether the endogenous Lef/Tcf family members are required in vivo to transduce Wnt signals. Using genetic methods in Drosophila, we define a new segment polarity gene, pangolin (pan), and show that its product is required in vivo for Wg signal transduction in embryos and in developing adult tissues. In addition, we show that pan encodes a Lef/Tcf homologue and provide evidence that its protein product binds to the beta-catenin homologue Armadillo in vivo. Finally, we demonstrate that Pan functions downstream of Arm to transduce the Wg signal. Thus, our results indicate that Pan is an essential component of the Wg transduction pathway and suggest that it acts directly to regulate gene transcription in response to Wg signalling.

Wnt-induced signaling via beta-catenin plays crucial roles in animal development and tumorigenesis. Both a seven-transmembrane protein in the Frizzled family and a single transmembrane protein in the LRP family (LDL-receptor-related protein 5/6 or Arrow) are essential for efficiently transducing a signal from Wnt, an extracellular ligand, to an intracellular pathway that stabilizes beta-catenin by interfering with its rate of destruction. However, the molecular mechanism by which these two types of membrane receptors synergize to transmit the Wnt signal is not known. We have used mutant and chimeric forms of Frizzled, LRP and Wnt proteins, small inhibitory RNAs, and assays for beta-catenin-mediated signaling and protein localization in Drosophila S2 cells and mammalian 293 cells to study transmission of a Wnt signal across the plasma membrane. Our findings are consistent with a mechanism by which Wnt protein binds to the extracellular domains of both LRP and Frizzled receptors, forming membrane-associated hetero-oligomers that interact with both Disheveled (via the intracellular portions of Frizzled) and Axin (via the intracellular domain of LRP). This model takes into account several observations reported here: the identification of intracellular residues of Frizzled required for beta-catenin signaling and for recruitment of Dvl to the plasma membrane; evidence that Wnt3A binds to the ectodomains of LRP and Frizzled; and demonstrations that a requirement for Wnt ligand can be abrogated by chimeric receptors that allow formation of Frizzled-LRP hetero-oligomers. In addition, the beta-catenin signaling mediated by ectopic expression of LRP is not dependent on Disheveled or Wnt, but can also be augmented by oligomerization of LRP receptors.

Although extensively studied biochemically, members of the Protein 4. 1 superfamily have not been as well characterized genetically. Studies of coracle, a Drosophila Protein 4.1 homologue, provide an opportunity to examine the genetic functions of this gene family. coracle was originally identified as a dominant suppressor of EgfrElp, a hypermorphic form of the Drosophila Epidermal growth factor receptor gene. In this article, we present a phenotypic analysis of coracle, one of the first for a member of the Protein 4. 1 superfamily. Screens for new coracle alleles confirm the null coracle phenotype of embryonic lethality and failure in dorsal closure, and they identify additional defects in the embryonic epidermis and salivary glands. Hypomorphic coracle alleles reveal functions in many imaginal tissues. Analysis of coracle mutant cells indicates that Coracle is a necessary structural component of the septate junction required for the maintenance of the transepithelial barrier but is not necessary for apical-basal polarity, epithelial integrity, or cytoskeletal integrity. In addition, coracle phenotypes suggest a specific role in cell signaling events. Finally, complementation analysis provides information regarding the functional organization of Coracle and possibly other Protein 4.1 superfamily members. These studies provide insights into a range of in vivo functions for coracle in developing embryos and adults.

Wnt signaling is critical to many aspects of development, and aberrant activation of the Wnt signaling pathway can cause cancer. Dishevelled (Dvl) protein plays a central role in this pathway by transducing the signal from the Wnt receptor complex to the ␤-catenin destruction complex. Dvl also plays a pivotal role in the planar cell polarity pathway that involves the c-Jun N-terminal kinase (JNK). How functions of Dvl are regulated in these two distinct pathways is not clear. We show that deleting the C-terminal two-thirds of Dvl, which includes the PDZ and DEP domains and is essential for Dvl-induced JNK activation, rendered the molecule a much more potent activator of the ␤-catenin pathway. We also found that casein kinase I (CKI), a previously identified positive regulator of Wnt signaling, stimulated Dvl activity in the Wnt pathway, but dramatically inhibited Dvl activity in the JNK pathway. Consistent with this, overexpression of CKI in Drosophila melanogaster stimulated Wnt signaling and disrupted planar cell polarity. We also observed a correlation between the localization and the signaling activity of Dvl in the ␤-catenin pathway and the JNK pathway. Furthermore, by using RNA interference, we demonstrate that the Drosophila CKI homologue Double time positively regulates the ␤-catenin pathway through Dvl and negatively regulates the Dvl-induced JNK pathway. We suggest that CKI functions as a molecular switch to direct Dvl from the JNK pathway to the ␤-catenin pathway, possibly by altering the conformation of the C terminus of Dvl.The Wnt family of glycoproteins regulate cell growth and cell fate determination in a wide range of metazoan organisms, including Drosophila melanogaster and mammals. Wnts play pivotal roles in diverse developmental processes, such as segmentation in D. melanogaster, control of asymmetric division in Caenorhabditis elegans, and axis formation and patterning of the central nervous system in vertebrates (10,56).Genetic experiments in D. melanogaster and biochemical studies in Xenopus and mammalian cells have established a framework for the Wnt signaling pathway (15, 39). In the absence of a Wnt signal, cytoplasmic ␤-catenin is bound to Axin and constitutively phosphorylated at several N-terminal Ser and Thr residues by glycogen synthase kinase 3 (GSK3); phosphorylated ␤-catenin is then recognized by ␤-TrCP and degraded by the ubiquitin-proteasome system. Wnt signaling is initiated by the binding of secreted Wnt protein to its receptors, Frizzled and LRP5/6, activating Dishevelled (Dvl) by an unknown mechanism. Dvl induces the dissociation of the Axin-GSK3-␤-catenin complex and the stabilization of ␤-catenin, presumably through the inactivation of GSK3 and failure to phosphorylate ␤-catenin. Accumulated ␤-catenin enters the nucleus, binds to members of LEF/TCF family of transcription factors, and alters the transcription of Wnt target genes.Deregulated Wnt signaling contributes to tumorigenesis. Wnt-1, the founding member of the Wnt family, was first identified as a gene activated b...