Forward genetic screens with CRISPR-Cas9 genome editing enable high-resolution detection of genetic vulnerabilities in cancer cells. We conducted genome-wide CRISPR-Cas9 screens in RNF43-mutant pancreatic ductal adenocarcinoma (PDAC) cells, which rely on Wnt signaling for proliferation. Through these screens, we discovered a unique requirement for a Wnt signaling circuit: engaging FZD5, one of the ten Frizzled receptors encoded in the human genome. Our results uncover an underappreciated level of context-dependent specificity at the Wnt receptor level. We further derived a panel of recombinant antibodies that reports the expression of nine FZD proteins and confirms that FZD5 functional specificity cannot be explained by protein expression patterns. Additionally, antibodies that specifically bind FZD5 and FZD8 robustly inhibited the growth of RNF43-mutant PDAC cells grown in vitro and as xenografts in vivo, providing orthogonal support for the functional specificity observed genetically. Proliferation of a patient-derived PDAC cell line harboring an RNF43 variant was also selectively inhibited by the FZD5 antibodies, further demonstrating their use as a potential targeted therapy. Tumor organoid cultures from colorectal carcinoma patients that carried RNF43 mutations were also sensitive to the FZD5 antibodies, highlighting the potential generalizability of these findings beyond PDAC. Our results show that CRIPSR-based genetic screens can be leveraged to identify and validate cell surface targets for antibody development and therapy.
The role of Gβγ subunits in cellular signaling has become well established in the past 20 years. Not only do they regulate effectors once thought to be the sole targets of Gα subunits, but it has become clear that they also have a unique set of binding partners and regulate signaling pathways that are not always localized to the plasma membrane. However, this may be only the beginning of the story. Gβγ subunits interact with G protein-coupled receptors, Gα subunits, and several different effector molecules during assembly and trafficking of receptor-based signaling complexes and not simply in response to ligand stimulation at sites of receptor cellular activity. Gβγ assembly itself seems to be tightly regulated via the action of molecular chaperones and in turn may serve a similar role in the assembly of specific signaling complexes. We propose that specific Gβγ subunits have a broader role in controlling the architecture, assembly, and activity of cellular signaling pathways.
The identification of ubiquitin E3 ligase substrates has been challenging, due in part to low-affinity, transient interactions, the rapid degradation of targets and the inability to identify proteins from poorly soluble cellular compartments. SCF -TrCP1 and SCF -TrCP2 are well-studied ubiquitin E3 ligases that target substrates for proteasomal degradation, and play important roles in Wnt, Hippo, and NFB signaling. Combining 26S proteasome inhibitor (MG132) treatment with proximity-dependent biotin labeling (BioID) and semiquantitative mass spectrometry, here we identify SCF -TrCP1/2 interacting partners. Based on their enrichment in the presence of MG132, our data identify over 50 new putative SCF -TrCP1/2 substrates. We validate 12 of these new substrates and reveal previously unsuspected roles for -TrCP in the maintenance of nuclear membrane integrity, processing (P)-body turnover and translational control. Together, our data suggest that -TrCP is an important hub in the cellular stress response. The technique presented here represents a complementary approach to more standard IP-MS methods and should be broadly applicable for the identification of substrates for many ubiquitin E3 ligases. Molecular & Cellular
By analyzing public data sets of gene expression in human breast cancers we observed that increased levels of transcripts encoding the planar cell polarity (PCP) proteins SCRIB and VANGL1 correlate with increased risk of patient relapse. Experimentally, we found that reducing expression of SCRIB by short-hairpin RNAs (shRNAs) reduces the growth of human breast cancer cells in xenograft assays. To investigate SCRIB-associated proteins that might participate in the responses of breast cancer cells to altered levels of SCRIB, we used mass spectrometry and confocal microscopy. These studies reveal that SCRIB is present in at least two unique protein complexes: (1) a complex of SCRIB, ARHGEF, GIT and PAK (p21-activated kinase), and (2) a complex of SCRIB, NOS1AP and VANGL. Focusing on NOS1AP, we observed that NOS1AP colocalizes with both SCRIB and VANGL1 along cellular protrusions in metastatic breast cancer cells, but does not colocalize with either SCRIB or VANGL1 at cell junctions in normal breast cells. We investigated the effects of shRNA-mediated knockdown of NOS1AP and SCRIB in vitro, and found that reducing NOS1AP and SCRIB slows breast cancer cell migration and prevents the establishment of leading–trailing polarity. We also find that reduction of NOS1AP enhances anchorage-independent growth. Collectively these data point to the relevance of NOS1AP and SCRIB protein complexes in breast cancer.
Bioluminescence resonance energy transfer (BRET)and co-immunoprecipitation experiments revealed that heterotrimeric G proteins and their effectors were found in stable complexes that persisted during signal transduction. Adenylyl cyclase, Kir3.1 channel subunits and several Gprotein subunits (G␣ ␣ s , G␣ ␣ i , G  1 and G␥ ␥ 2 ) were tagged with luciferase (RLuc) or GFP, or the complementary fragments of YFP (specifically G  1 -YFP 1-158 and G␥ ␥ 2 -YFP 159-238 , which heterodimerize to produce fluorescent YFP-G  1 ␥ ␥ 2 ). BRET was observed between adenylyl-cyclase-RLuc or Kir3.1-RLuc and GFP-G␥ ␥ 2 , GFP-G  1 or YFP-G  1 ␥ ␥ 2 . G␣ ␣ subunits were also stably associated with both effectors regardless of whether or not signal transduction was initiated by a receptor agonist. Although BRET between effectors and G ␥ ␥ was increased by receptor stimulation, our data indicate that these changes are likely to be conformational in nature. Furthermore, receptor-sensitive G-protein-effector complexes could be detected before being transported to the plasma membrane, providing the first direct evidence for an intracellular site of assembly.
Much is known about  2 -adrenergic receptor trafficking and internalization following prolonged agonist stimulation. However, less is known about outward trafficking of the  2 -adrenergic receptor to the plasma membrane or the role that trafficking might play in the assembly of receptor signaling complexes, important for targeting, specificity, and rapidity of subsequent signaling events. Here, by using a combination of bioluminescence resonance energy transfer, bimolecular fluorescence complementation, and confocal microscopy, we evaluated the steps in the formation of the core receptor-G protein heterotrimer complex. By using dominant negative Rab and Sar GTPase constructs, we demonstrate that receptor dimers and receptor-G␥ complexes initially associate in the endoplasmic reticulum, whereas G␣ subunits are added to the complex during endoplasmic reticulum-Golgi transit. We also observed that G protein heterotrimers adopt different trafficking itineraries when expressed alone or with stoichiometric co-expression with receptor. Furthermore, deliberate mistargeting of specific components of these complexes leads to diversion of other members from their normal subcellular localization, confirming the role of these early interactions in targeting and formation of specific signaling complexes.
The intrinsic structural determinants for export trafficking of G protein-coupled receptors (GPCRs) have been mainly identified in the termini of the receptors. In this report, we determined the role of the first intracellular loop (ICL1) in the transport from the endoplasmic reticulum (ER) to the cell surface of GPCRs. The α 2B -adrenergic receptor (AR) mutant lacking the ICL1 is unable to traffic to the cell surface and to initiate signaling measured as ERK1/2 activation. Mutagenesis studies identify a single Leu48 residue in the ICL1 modulates α 2B -AR export from the ER. The ER export function of the Leu48 residue can be substituted by Phe, but not Ile, Val, Tyr and Trp, and is unlikely involved in correct folding or dimerization of α 2B -AR in the ER. Importantly, the isolated Leu residue is remarkably conserved in the center of the ICL1s among the family A GPCRs and is also required for the export to the cell surface of β 2 -AR, α 1B -AR and angiotensin II type 1 receptor. These data indicate a crucial role for a single Leu residue within the ICL1 in ER export of GPCRs.
Here we demonstrate using a variety of techniques that DRiP78, an endoplasmic reticulum resident protein known to regulate the trafficking of several seven transmembrane receptors, interacts specifically with the G␥ subunit but not G or G␣ subunits. Furthermore, we demonstrate that DRiP78 and the G subunit can compete for the G␥ subunit. DRiP78 also protects G␥ from degradation until a stable partner such as G is provided. Furthermore, DRiP78 interaction may represent a mechanism for assembly of specific G␥ heterodimers, as selectivity was observed among G␥ isoforms for interaction with DRiP78 depending on the presence of particular G subunits. Interestingly, we could detect an interaction between DRiP78 and PhLP, suggesting a role of DRiP78 in the assembly of G␥ by linking G␥ to PhLP⅐G complexes. Our results, therefore, suggest a role of DRiP78 as a chaperone in the assembly of G␥ subunits of the G protein.
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