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.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0007600 sensory perception | IBA GO_REF:0000033 | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:20802461 controls Drosophila olfaction, behaviour, sensitivity to osmotic stress |
| GO:0001664 G protein-coupled receptor binding | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: PMID:21437272 Krz binds to Smo and internalizes it via clathrin-coated vesicles PMID:20802461 promote internalization of GPCRs on receptor activation |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | 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. Reason: Directly observed by antibody staining in both embryos and imaginal discs, matching the IBA phylogenetic assignment. Supporting Evidence: PMID:20802461 the Krz protein is expressed throughout the blastoderm embryo and is predominantly cytoplasmic PMID:21437272 the protein is localized in the cytoplasm of imaginal cells |
| GO:0002031 G protein-coupled receptor internalization | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: PMID:21437272 promote Smoothened internalization and degradation in a clathrin- and proteosomal-dependent manner PMID:20802461 promote internalization of GPCRs on receptor activation |
| GO:0007165 signal transduction | IEA GO_REF:0000002 | ACCEPT | 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. 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. Supporting Evidence: PMID:21437272 participate in a variety of signalling events |
| GO:0005515 protein binding | IPI PMID:20802461 Ξ²-arrestin Kurtz inhibits MAPK and Toll signalling in Drosop... | MODIFY | 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. 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 replacements: mitogen-activated protein kinase binding Supporting Evidence: PMID:20802461 Krz can directly bind and sequester an inactive form of ERK, thus preventing its activation by the upstream kinase, MEK |
| GO:0005515 protein binding | IPI PMID:20802461 Ξ²-arrestin Kurtz inhibits MAPK and Toll signalling in Drosop... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:20802461 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. PMID:20802461 We observed that Krz can directly bind to the Drosophila orthologue of IΞΊBΞ±, Cactus |
| GO:0045751 negative regulation of Toll signaling pathway | IMP PMID:24077307 Regulation of Toll signaling and inflammation by Ξ²-arrestin ... | KEEP AS NON CORE | 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. Reason: Experimentally supported inhibitory role in Toll signalling and inflammation, but a pleiotropic downstream regulatory role rather than the core arrestin biochemical function. Supporting Evidence: PMID:24077307 Loss of function of krz or Ulp1 in Drosophila larvae results in a similar inflammatory phenotype |
| GO:0120177 negative regulation of torso signaling pathway | IMP PMID:20802461 Ξ²-arrestin Kurtz inhibits MAPK and Toll signalling in Drosop... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:20802461 the normal function of Krz is to limit the activity of ERK, and hence the Torso pathway, in the early embryo |
| GO:0005829 cytosol | IDA PMID:21437272 Role of the Drosophila non-visual Γ-arrestin kurtz in hedgeh... | ACCEPT | Summary: Direct antibody localization of Krz to the cytoplasm/cytosol of imaginal cells. Reason: IDA-quality localization data; consistent with the cytosolic nature of non-visual arrestins and with the cytoplasm (IBA) and Reactome (TAS) cytosol annotations. Supporting Evidence: PMID:21437272 the protein is localized in the cytoplasm of imaginal cells |
| GO:0031648 protein destabilization | IMP PMID:21437272 Role of the Drosophila non-visual Γ-arrestin kurtz in hedgeh... | KEEP AS NON CORE | Summary: Over-expressed Krz reduces steady-state Smoothened levels by promoting its proteasomal degradation (destabilization), an outcome of Krz-driven Smo internalization. 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. Supporting Evidence: PMID:21437272 Krz enhances Smo degradation via the proteosomal pathway |
| GO:0045879 negative regulation of smoothened signaling pathway | IMP PMID:21437272 Role of the Drosophila non-visual Γ-arrestin kurtz in hedgeh... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:21437272 Krz has the potential to antagonize Hh signalling, although this antagonism is only observed upon its over-expression |
| GO:0045879 negative regulation of smoothened signaling pathway | IMP PMID:28195188 SUMO regulates the activity of Smoothened and Costal-2 in Dr... | KEEP AS NON CORE | Summary: Excess Krz inhibits Smoothened sumoylation and prevents Smoothened accumulation, thereby down-regulating Hedgehog/Smoothened signalling; the effect maps to the Krz regulatory domain. Reason: Independent experimental support (a second lab) for Krz antagonizing Smoothened signalling, here via the SUMO axis. A specific, non-core regulatory role. Supporting Evidence: PMID:28195188 inhibits Smo sumoylation and prevents Smo accumulation through Krz regulatory domain |
| GO:0060189 positive regulation of protein desumoylation | IMP PMID:28195188 SUMO regulates the activity of Smoothened and Costal-2 in Dr... | KEEP AS NON CORE | Summary: Krz bridges Smoothened to the SUMO protease Ulp1, promoting Smo desumoylation; RNAi knockdown of Krz weakens the Smo-Ulp1 interaction. 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. Supporting Evidence: PMID:28195188 Krz likely facilitates the interaction between Smo and Ulp1 because knockdown of Krz by RNAi attenuates Smo-Ulp1 interaction |
| GO:0045746 negative regulation of Notch signaling pathway | IGI PMID:16284625 Regulation of Notch signalling by non-visual beta-arrestin. | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:16284625 This complex mediates the degradation of the Notch receptor through a ubiquitination-dependent pathway. |
| GO:0045751 negative regulation of Toll signaling pathway | IMP PMID:20802461 Ξ²-arrestin Kurtz inhibits MAPK and Toll signalling in Drosop... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:20802461 Krz functions during normal development to limit the activity of Toll |
| GO:0043409 negative regulation of MAPK cascade | IMP PMID:20802461 Ξ²-arrestin Kurtz inhibits MAPK and Toll signalling in Drosop... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:20802461 loss of krz function results in an overall increase in ERK activity |
| GO:0048260 positive regulation of receptor-mediated endocytosis | IMP PMID:19850026 Regulation of smoothened by Drosophila G-protein-coupled rec... | ACCEPT | 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. 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). Supporting Evidence: PMID:19850026 This is the first evidence that arrestins participate in Smo regulation in flies PMID:21437272 Krz binds to Smo and internalizes it via clathrin-coated vesicles |
| GO:0005829 cytosol | TAS Reactome:R-DME-2071846 | ACCEPT | Summary: Reactome (TAS) cytosol localization for the "Deltex recruits Kurtz arrestin to Notch" step, consistent with the cytoplasmic/cytosolic localization of Krz. Reason: Agrees with direct antibody localization of Krz to the cytoplasm/cytosol; a correct and well-supported cellular-component annotation. Supporting Evidence: PMID:21437272 the protein is localized in the cytoplasm of imaginal cells |
| GO:0005829 cytosol | TAS Reactome:R-NUL-2071858 | ACCEPT | Summary: Reactome (TAS) cytosol localization for the "Kurtz promotes ubiquitination of Notch" step, consistent with the cytoplasmic/cytosolic localization of Krz. Reason: Duplicate cytosol localization from a related Reactome reaction; agrees with the IDA/IBA cytoplasm/cytosol evidence. Retained. Supporting Evidence: PMID:21437272 the protein is localized in the cytoplasm of imaginal cells |
| GO:0035641 locomotory exploration behavior | IMP PMID:17151232 Exploratory activity in Drosophila requires the kurtz nonvis... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:17151232 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. |
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Download this section (compressed HTML)Q: 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?
Q: 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?
Experiment: Endogenous co-immunoprecipitation / proximity-labeling of Krz to identify its receptor and adaptor (clathrin, AP-2) interactome under basal versus receptor-activated conditions.
Experiment: Live imaging of tagged endogenous Krz to test transient recruitment to the plasma membrane and endosomes during agonist-induced receptor internalization.
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