id: Q9V393 gene_symbol: krz product_type: PROTEIN status: DRAFT taxon: id: NCBITaxon:7227 label: Drosophila melanogaster description: >- Kurtz (Krz) is the single non-visual (beta-)arrestin of Drosophila melanogaster, a cytoplasmic member of the arrestin family with two arrestin domains. As a canonical arrestin, it binds activated/phosphorylated G-protein-coupled receptors and receptor-family proteins and acts as an adaptor that promotes their clathrin-mediated internalization, desensitization and downregulation; conserved residues required for GPCR targeting to clathrin-coated pits and for clathrin binding are functionally important. Krz is essential for viability and neural function and, because it is the only non-visual arrestin in the fly, has broad pleiotropic roles as an attenuator of multiple developmental and immune signalling pathways. It limits receptor-tyrosine-kinase/MAPK (ERK) signalling by directly binding and sequestering inactive ERK away from its activating kinase MEK, thereby restraining the Torso pathway in the early embryo; it limits Toll/NF-kappaB (Dorsal) signalling in part by binding and stabilizing the IkappaB orthologue Cactus and, in larvae, through a functional interaction with the SUMO protease Ulp1; it can promote internalization and proteasomal degradation of the GPCR-family receptor Smoothened (antagonizing Hedgehog signalling when over-expressed) and facilitates Smoothened desumoylation via Ulp1; and it forms a Notch-Deltex-Kurtz complex that drives ubiquitination-dependent degradation of the Notch receptor. In the adult nervous system Krz is required for exploratory locomotor behaviour. The protein is predominantly cytoplasmic/cytosolic, acting transiently at the plasma membrane and endocytic compartments during receptor internalization. existing_annotations: - term: id: GO:0007600 label: sensory perception evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: >- Phylogenetic (IBA) annotation reflecting the arrestin family's conserved role in sensory/GPCR signalling; Krz controls Drosophila olfaction and other sensory GPCR responses. A real but distal, pleiotropic role rather than the gene's core biochemical function. action: KEEP_AS_NON_CORE reason: >- Consistent with Krz functioning as a GPCR regulator that controls sensory responses such as olfaction, but this is a downstream physiological outcome of the core arrestin activity rather than the molecular function itself; kept as non-core. supported_by: - reference_id: PMID:20802461 supporting_text: controls Drosophila olfaction, behaviour, sensitivity to osmotic stress - term: id: GO:0001664 label: G protein-coupled receptor binding evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: enables review: summary: >- Core, evolutionarily conserved molecular function of a non-visual arrestin: binding activated/phosphorylated GPCRs (and GPCR-family receptors such as Smoothened) to desensitize them and drive their internalization. action: ACCEPT reason: >- Well supported both by the canonical arrestin mechanism and by direct experimental demonstration in Drosophila that Krz binds the GPCR-family receptor Smoothened. This is a central, defining molecular function of Krz and is retained as core. supported_by: - reference_id: PMID:21437272 supporting_text: Krz binds to Smo and internalizes it via clathrin-coated vesicles - reference_id: PMID:20802461 supporting_text: promote internalization of GPCRs on receptor activation - term: id: GO:0005737 label: cytoplasm evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: is_active_in review: summary: >- Krz is predominantly a cytoplasmic protein, consistent with the non-visual arrestins being cytosolic proteins that act on receptors at/near the plasma membrane. action: ACCEPT reason: >- Directly observed by antibody staining in both embryos and imaginal discs, matching the IBA phylogenetic assignment. supported_by: - reference_id: PMID:20802461 supporting_text: the Krz protein is expressed throughout the blastoderm embryo and is predominantly cytoplasmic - reference_id: PMID:21437272 supporting_text: the protein is localized in the cytoplasm of imaginal cells - term: id: GO:0002031 label: G protein-coupled receptor internalization evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: >- Core arrestin function: Krz promotes clathrin-mediated internalization of activated GPCRs/GPCR-family receptors, shown directly for Smoothened and reported for other GPCRs on activation. action: ACCEPT reason: >- Canonical, conserved arrestin activity supported experimentally in Drosophila (Smo internalization via clathrin-coated vesicles; promotion of GPCR internalization on receptor activation). Retained as core. supported_by: - reference_id: PMID:21437272 supporting_text: promote Smoothened internalization and degradation in a clathrin- and proteosomal-dependent manner - reference_id: PMID:20802461 supporting_text: promote internalization of GPCRs on receptor activation - term: id: GO:0007165 label: signal transduction evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: involved_in review: summary: >- Very general InterPro-based (IEA) annotation reflecting that arrestins participate in signal transduction. Not wrong but uninformative relative to the specific regulatory roles captured elsewhere. action: ACCEPT reason: >- A broad IEA that is biologically correct (Krz is a regulator of multiple signalling pathways); acceptable as a high-level term even though more specific process terms are also annotated. supported_by: - reference_id: PMID:21437272 supporting_text: participate in a variety of signalling events - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:20802461 qualifier: enables review: summary: >- Bare 'protein binding' from the interaction between Krz and ERK (rolled), the WITH/FROM = UniProtKB:P40417 row of this IPI pair. Uninformative as annotated; the specific, experimentally supported activity is binding of the MAP kinase ERK, which Krz sequesters in its inactive form. The second partner from the same reference (UniProtKB:Q03017, Cactus) is adjudicated in the entry below. action: MODIFY reason: >- 'protein binding' conveys no functional information. The IPI evidence (Krz directly binds ERK/rolled; IntAct NbExp=12) supports the more informative molecular function 'mitogen-activated protein kinase binding' (GO:0051019). proposed_replacement_terms: - id: GO:0051019 label: mitogen-activated protein kinase binding supported_by: - reference_id: PMID:20802461 supporting_text: Krz can directly bind and sequester an inactive form of ERK, thus preventing its activation by the upstream kinase, MEK - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:20802461 qualifier: enables review: summary: >- The second IPI row from the same reference, WITH/FROM = UniProtKB:Q03017 (Cactus, the Drosophila IkappaB orthologue). Co-immunoprecipitation from S2 cells and an in vitro translated-protein assay support a direct Krz-Cactus interaction; Krz stabilizes Cactus and thereby limits Toll/Dorsal signalling. Unlike the ERK interaction, Cactus binding is insensitive to the 'pre-activated' Krz-R209E conformation. action: KEEP_AS_NON_CORE reason: >- The interaction itself is well supported - co-immunoprecipitation from S2 cells plus an in vitro translated-protein assay - so the term is not an over-annotation; it is simply uninformative, and the Cactus interaction is peripheral to Krz's core GPCR-binding adaptor function, so it is retained as non-core. Unlike the ERK partner, there is no specific GO molecular-function term for binding an IkappaB/Cactus-family inhibitor (GO has no 'I-kappaB binding' MF; GO:0051059 NF-kappaB binding is the wrong target, since Cactus is the inhibitor, not the Rel factor), so no replacement term is proposed rather than forcing an inaccurate one. The biology this interaction supports is already captured by the GO:0045751 negative regulation of Toll signaling pathway annotation, which cites the same experiments. supported_by: - reference_id: PMID:20802461 supporting_text: We detected an interaction using co-immunoprecipitation from S2 cells ( Figure 7J ), as well as using in vitro translated proteins ( Supplementary Figure S7 ), suggesting a direct binding. - reference_id: PMID:20802461 supporting_text: We observed that Krz can directly bind to the Drosophila orthologue of IκBα, Cactus - term: id: GO:0045751 label: negative regulation of Toll signaling pathway evidence_type: IMP original_reference_id: PMID:24077307 qualifier: involved_in review: summary: >- Krz limits Toll/NF-kappaB signalling in larvae/immunity, acting together with the SUMO protease Ulp1; loss of krz produces an inflammatory phenotype with nuclear accumulation of Toll effectors Dorsal and Dif. action: KEEP_AS_NON_CORE reason: >- Experimentally supported inhibitory role in Toll signalling and inflammation, but a pleiotropic downstream regulatory role rather than the core arrestin biochemical function. supported_by: - reference_id: PMID:24077307 supporting_text: Loss of function of krz or Ulp1 in Drosophila larvae results in a similar inflammatory phenotype - term: id: GO:0120177 label: negative regulation of torso signaling pathway evidence_type: IMP original_reference_id: PMID:20802461 qualifier: involved_in review: summary: >- Krz restrains the Torso RTK pathway in the early embryo by limiting ERK activity; maternal loss of krz expands and intensifies dpERK and Torso target-gene expression. action: KEEP_AS_NON_CORE reason: >- Directly supported by maternal krz mutant analysis (increased/expanded dpERK; upregulated tll/hkb). A specific developmental-signalling role downstream of the core ERK-binding activity; kept as non-core. supported_by: - reference_id: PMID:20802461 supporting_text: the normal function of Krz is to limit the activity of ERK, and hence the Torso pathway, in the early embryo - term: id: GO:0005829 label: cytosol evidence_type: IDA original_reference_id: PMID:21437272 qualifier: located_in review: summary: >- Direct antibody localization of Krz to the cytoplasm/cytosol of imaginal cells. action: ACCEPT reason: >- IDA-quality localization data; consistent with the cytosolic nature of non-visual arrestins and with the cytoplasm (IBA) and Reactome (TAS) cytosol annotations. supported_by: - reference_id: PMID:21437272 supporting_text: the protein is localized in the cytoplasm of imaginal cells - term: id: GO:0031648 label: protein destabilization evidence_type: IMP original_reference_id: PMID:21437272 qualifier: involved_in review: summary: >- Over-expressed Krz reduces steady-state Smoothened levels by promoting its proteasomal degradation (destabilization), an outcome of Krz-driven Smo internalization. action: KEEP_AS_NON_CORE reason: >- Supported by S2-cell and imaginal-disc data showing Krz enhances Smo turnover via the proteasome. This destabilizing activity is a consequence of the core internalization/adaptor function acting on a specific receptor; retained as non-core. supported_by: - reference_id: PMID:21437272 supporting_text: Krz enhances Smo degradation via the proteosomal pathway - term: id: GO:0045879 label: negative regulation of smoothened signaling pathway evidence_type: IMP original_reference_id: PMID:21437272 qualifier: involved_in review: summary: >- Over-expression of Krz phenocopies loss of Hedgehog signalling and reduces Hh target-gene expression (Ptc, En) by promoting Smoothened internalization and degradation. Note the effect is seen only on over-expression; krz loss-of-function has no Smo phenotype. action: KEEP_AS_NON_CORE reason: >- Experimentally supported antagonism of Smoothened/Hh signalling, but manifested only upon Krz over-expression and dispensable in loss-of-function, so a conditional, non-core regulatory role. supported_by: - reference_id: PMID:21437272 supporting_text: Krz has the potential to antagonize Hh signalling, although this antagonism is only observed upon its over-expression - term: id: GO:0045879 label: negative regulation of smoothened signaling pathway evidence_type: IMP original_reference_id: PMID:28195188 qualifier: involved_in review: summary: >- Excess Krz inhibits Smoothened sumoylation and prevents Smoothened accumulation, thereby down-regulating Hedgehog/Smoothened signalling; the effect maps to the Krz regulatory domain. action: KEEP_AS_NON_CORE reason: >- Independent experimental support (a second lab) for Krz antagonizing Smoothened signalling, here via the SUMO axis. A specific, non-core regulatory role. supported_by: - reference_id: PMID:28195188 supporting_text: inhibits Smo sumoylation and prevents Smo accumulation through Krz regulatory domain - term: id: GO:0060189 label: positive regulation of protein desumoylation evidence_type: IMP original_reference_id: PMID:28195188 qualifier: involved_in review: summary: >- Krz bridges Smoothened to the SUMO protease Ulp1, promoting Smo desumoylation; RNAi knockdown of Krz weakens the Smo-Ulp1 interaction. action: KEEP_AS_NON_CORE reason: >- Supported by the demonstration that Krz facilitates the Smo-Ulp1 interaction that drives desumoylation. A specialized adaptor/scaffolding role in the SUMO pathway; non-core. supported_by: - reference_id: PMID:28195188 supporting_text: Krz likely facilitates the interaction between Smo and Ulp1 because knockdown of Krz by RNAi attenuates Smo-Ulp1 interaction - term: id: GO:0045746 label: negative regulation of Notch signaling pathway evidence_type: IGI original_reference_id: PMID:16284625 qualifier: involved_in review: summary: >- Krz forms a trimeric Notch-Deltex-Kurtz complex that promotes ubiquitination-dependent degradation of the Notch receptor, thereby down-regulating Notch signalling. GOA carries this term as two IGI rows from the same reference, one per genetic-interaction partner (WITH/FROM = FB:FBgn0000524 and FB:FBgn0004647, the other two members of that complex); both rows are adjudicated together by this single entry. action: KEEP_AS_NON_CORE reason: >- Supported by the paper's own abstract (identification of the Notch-Deltex-Krz complex and ubiquitination-dependent Notch degradation), consistent with the arrestin adaptor role in receptor downregulation; a specific developmental-signalling role kept as non-core. supported_by: - reference_id: PMID:16284625 supporting_text: This complex mediates the degradation of the Notch receptor through a ubiquitination-dependent pathway. - term: id: GO:0045751 label: negative regulation of Toll signaling pathway evidence_type: IMP original_reference_id: PMID:20802461 qualifier: involved_in review: summary: >- In the early embryo Krz limits Toll signalling: maternal loss of krz expands the Dorsal nuclear gradient and Toll target-gene (twi, rho) domains; Krz binds and stabilizes the IkappaB orthologue Cactus. action: KEEP_AS_NON_CORE reason: >- Directly supported by maternal krz mutant analysis and Krz-Cactus interaction. A specific developmental-signalling role (distinct paper/context from the larval Toll annotation); non-core. supported_by: - reference_id: PMID:20802461 supporting_text: Krz functions during normal development to limit the activity of Toll - term: id: GO:0043409 label: negative regulation of MAPK cascade evidence_type: IMP original_reference_id: PMID:20802461 qualifier: involved_in review: summary: >- Krz negatively regulates the ERK/MAPK cascade by directly binding and sequestering inactive ERK (rolled), preventing its activation by MEK; loss of krz increases overall ERK activity. action: KEEP_AS_NON_CORE reason: >- Strongly supported mechanistically (direct ERK binding plus increased dpERK in krz mutants). The underlying molecular activity (MAP kinase binding) is captured in core_functions; this process-level inhibitory role across multiple RTKs is a pleiotropic regulatory function kept as non-core. supported_by: - reference_id: PMID:20802461 supporting_text: loss of krz function results in an overall increase in ERK activity - term: id: GO:0048260 label: positive regulation of receptor-mediated endocytosis evidence_type: IMP original_reference_id: PMID:19850026 qualifier: involved_in review: summary: >- Over-expression of the beta-arrestin Krz reduces Smoothened levels, the first evidence that arrestins promote receptor (Smo) internalization/endocytosis in flies - a manifestation of the canonical arrestin adaptor-for-endocytosis function. action: ACCEPT reason: >- Captures the core, conserved arrestin activity of promoting clathrin/receptor-mediated endocytosis; supported by Krz over-expression reducing Smo. Retained as core (canonical arrestin adaptor role). supported_by: - reference_id: PMID:19850026 supporting_text: This is the first evidence that arrestins participate in Smo regulation in flies - reference_id: PMID:21437272 supporting_text: Krz binds to Smo and internalizes it via clathrin-coated vesicles - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-DME-2071846 qualifier: located_in review: summary: >- Reactome (TAS) cytosol localization for the "Deltex recruits Kurtz arrestin to Notch" step, consistent with the cytoplasmic/cytosolic localization of Krz. action: ACCEPT reason: >- Agrees with direct antibody localization of Krz to the cytoplasm/cytosol; a correct and well-supported cellular-component annotation. supported_by: - reference_id: PMID:21437272 supporting_text: the protein is localized in the cytoplasm of imaginal cells - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-NUL-2071858 qualifier: located_in review: summary: >- Reactome (TAS) cytosol localization for the "Kurtz promotes ubiquitination of Notch" step, consistent with the cytoplasmic/cytosolic localization of Krz. action: ACCEPT reason: >- Duplicate cytosol localization from a related Reactome reaction; agrees with the IDA/IBA cytoplasm/cytosol evidence. Retained. supported_by: - reference_id: PMID:21437272 supporting_text: the protein is localized in the cytoplasm of imaginal cells - term: id: GO:0035641 label: locomotory exploration behavior evidence_type: IMP original_reference_id: PMID:17151232 qualifier: involved_in review: summary: >- Krz is specifically required in the nervous system for exploratory locomotor activity evoked by a novel arena and for mechanically stimulated activity; it is not required for spontaneous activity. action: KEEP_AS_NON_CORE reason: >- Experimentally supported organismal behavioural phenotype, a distal physiological consequence of Krz's neuronal GPCR-regulatory activity rather than its core molecular function; non-core. supported_by: - reference_id: PMID:17151232 supporting_text: There is a specific requirement for the kurtz nonvisual arrestin in the nervous system for both the exploration stimulated by the novel arena and the mechanically stimulated activity. core_functions: - description: >- Non-visual arrestin adaptor that binds activated GPCRs / GPCR-family receptors and promotes their clathrin-mediated internalization and endocytic downregulation, acting from the cytoplasm. molecular_function: id: GO:0001664 label: G protein-coupled receptor binding directly_involved_in: - id: GO:0002031 label: G protein-coupled receptor internalization - id: GO:0048260 label: positive regulation of receptor-mediated endocytosis locations: - id: GO:0005737 label: cytoplasm - id: GO:0005829 label: cytosol supported_by: - reference_id: PMID:21437272 supporting_text: promote Smoothened internalization and degradation in a clathrin- and proteosomal-dependent manner - reference_id: PMID:21437272 supporting_text: Krz binds to Smo and internalizes it via clathrin-coated vesicles - reference_id: PMID:20802461 supporting_text: promote internalization of GPCRs on receptor activation - description: >- Directly binds the MAP kinase ERK (rolled), sequestering its inactive form and thereby limiting ERK/MAPK-cascade activation downstream of receptor tyrosine kinases. molecular_function: id: GO:0051019 label: mitogen-activated protein kinase binding directly_involved_in: - id: GO:0043409 label: negative regulation of MAPK cascade locations: - id: GO:0005737 label: cytoplasm supported_by: - reference_id: PMID:20802461 supporting_text: Krz can directly bind and sequester an inactive form of ERK, thus preventing its activation by the upstream kinase, MEK - reference_id: PMID:20802461 supporting_text: loss of krz function results in an overall increase in ERK activity suggested_questions: - question: >- Does Krz bind clathrin and/or the AP-2 adaptor directly in Drosophila (as predicted by the conserved Leu440/IsoLeu441/Leu443 clathrin-binding motif), and can a specific clathrin-binding or clathrin-adaptor molecular function be experimentally assigned? - question: >- Which endogenous GPCRs (beyond Smoothened) are physiological Krz substrates in vivo, and is Krz's ERK-sequestration mechanism used downstream of GPCRs as well as RTKs? suggested_experiments: - description: >- Endogenous co-immunoprecipitation / proximity-labeling of Krz to identify its receptor and adaptor (clathrin, AP-2) interactome under basal versus receptor-activated conditions. - description: >- Live imaging of tagged endogenous Krz to test transient recruitment to the plasma membrane and endosomes during agonist-induced receptor internalization. references: - id: GO_REF:0000002 title: Gene Ontology annotation through association of InterPro records with GO terms findings: [] - id: GO_REF:0000033 title: Annotation inferences using phylogenetic trees findings: [] - id: PMID:16284625 title: Regulation of Notch signalling by non-visual beta-arrestin. findings: [] reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- PubMed-verified; abstract-only in cache. Directly supports the Notch-Deltex-Kurtz complex and ubiquitination-dependent Notch degradation (basis of GO:0045746 IGI and the two Reactome Notch reactions). - id: PMID:17151232 title: Exploratory activity in Drosophila requires the kurtz nonvisual arrestin. findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Full text available; directly supports the neuronal requirement of krz for exploratory locomotor behavior (GO:0035641 IMP). - id: PMID:19850026 title: Regulation of smoothened by Drosophila G-protein-coupled receptor kinases. findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Full text available; primarily about Gprk2, but explicitly shows Krz over-expression reduces Smo (first evidence arrestins regulate Smo in flies), supporting GO:0048260. Notes krz LOF has no Smo phenotype. - id: PMID:20802461 title: β-arrestin Kurtz inhibits MAPK and Toll signalling in Drosophila development. findings: [] reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Full text available; primary source for Krz-ERK binding/sequestration (MAPK, GO:0043409, GO:0120177, GO:0051019), Krz-Cactus binding and Toll inhibition (GO:0045751), and cytoplasmic localization. - id: PMID:21437272 title: Role of the Drosophila non-visual ß-arrestin kurtz in hedgehog signalling. findings: [] reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Full text available; primary source for Krz-Smo binding, clathrin/proteasome-dependent Smo internalization and degradation (GO:0002031, GO:0031648, GO:0045879) and cytoplasmic localization. Antagonism of Hh only on over-expression. - id: PMID:24077307 title: Regulation of Toll signaling and inflammation by β-arrestin and the SUMO protease Ulp1. findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Full text available; supports Krz limiting Toll signalling/inflammation in larvae via the SUMO protease Ulp1 (GO:0045751 IMP). - id: PMID:28195188 title: SUMO regulates the activity of Smoothened and Costal-2 in Drosophila Hedgehog signaling. findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Full text available; supports excess Krz inhibiting Smo sumoylation/accumulation (GO:0045879) and Krz facilitating the Smo-Ulp1 interaction that promotes desumoylation (GO:0060189). - id: Reactome:R-DME-2071846 title: Deltex recruits Kurtz arrestin to Notch findings: [] - id: Reactome:R-NUL-2071858 title: Kurtz promotes ubiquitination of Notch findings: []