Orb (oo18 RNA-binding protein) is the Drosophila member of the CPEB1 subfamily of cytoplasmic polyadenylation element-binding proteins. It is a germline-enriched cytoplasmic RNA-binding protein whose C-terminal region of two RNA recognition motifs and a ZZ-type zinc-binding domain recognizes linear cytoplasmic polyadenylation element-like sequences in the 3-prime untranslated regions of target mRNAs. In the ovary Orb accumulates in the oocyte, where it promotes cytoplasmic poly(A)-tail elongation (in an ovarian complex with canonical poly(A) polymerase and the GLD-2-type poly(A) polymerase Wispy) and thereby activates localized translation of maternal mRNAs, including oskar, gurken and its own orb mRNA. Through this localized translational activation Orb is required for 16-cell cyst formation and oocyte differentiation, oskar protein accumulation at the posterior pole, gurken-dependent dorsoventral patterning and microtubule repolarization of the oocyte. Q8IMZ2 is the 559-residue product of orb transcripts C and E; it lacks the N-terminal 356 residues of the 915-residue canonical Orb product (Q9VCN7) but retains the complete C-terminal RNA-binding region.
Summary: Correct but uninformative InterPro2GO mapping from the RNA-binding-domain superfamily signature. Orb binds RNA, specifically CPE-like sequences in target mRNA 3-prime UTRs.
Reason: The generic term is true but less specific than the evidence. CLIP shows Orb binds CPE-like sequences in 3-prime UTRs through its RRMs, and the oskar 3-prime UTR cross-links to Orb in ovarian extracts. Q8IMZ2 retains the entire C-terminal RRM/ZZ region (residues 8-559 identical to canonical Orb 364-915). Refine to mRNA 3'-UTR binding, which the gene already carries.
Summary: Correct but general; the RRM signature supports RNA binding, and experimental data define the bound RNA as CPE-like sequences in target mRNA 3-prime UTRs.
Reason: Same claim as the nucleic acid binding row at a slightly finer level. The specific supported term, already carried by the gene, is mRNA 3'-UTR binding.
TAS PMID:11700288 Translational regulation and RNA localization in Drosophila ...
MODIFY
Summary: Traceable author statement from a review of translational control and RNA localization in Drosophila oocytes. mRNA binding is correct; the experimental literature places the binding in target 3-prime UTRs.
Reason: The review abstract does not itself name Orb (abstract-only cache), but the claim is well supported by primary data (oskar 3-prime UTR cross-linking and immunoprecipitation; transcriptome-wide CLIP). mRNA 3'-UTR binding is the is_a descendant supported by that evidence and already carried by the gene.
Summary: Core molecular function. CPEB-family InterPro mapping agrees with direct evidence that Orb binds CPE-like elements in target 3-prime UTRs, including the oskar 3-prime UTR.
Reason: Q8IMZ2 is in PANTHER subfamily PTHR12566:SF9 (CPEB1), and PAINT curators placed mRNA 3'-UTR binding at the CPEB family node (PTN000286163) from which the canonical Orb product inherits it. Direct Orb CLIP and cross-linking data confirm the function. The shorter isoform retains the complete RRM1, RRM2 and ZZ region.
IDA PMID:10101171 The posterior determinant gene nanos is required for the mai...
ACCEPT
Summary: Orb is a cytoplasmic protein of the germline cyst that becomes enriched in the oocyte.
Reason: The cited nanos study (abstract-only cache; its abstract does not mention Orb) probably used anti-Orb staining as an oocyte marker; the curator saw the full text and the claim is independently and directly confirmed by anti-ORB immunostaining in PMID:11809824. Cytoplasm is as precise as this row warrants; finer locations (cell cortex, P-body) are annotated on the canonical product.
Summary: Electronic subcellular-location mapping that agrees with the experimental cytoplasmic localization of Orb.
Reason: The UniProt subcellular location is itself ARBA-derived, so it is not independent evidence, but the claim is the same as the accepted IDA row and is directly supported by immunostaining.
Summary: Correct but general. In Drosophila the experimental evidence shows Orb activates translation of its targets by promoting cytoplasmic polyadenylation.
Reason: Orb-mediated poly(A) elongation stimulates oskar translation, Osk protein is undetectable in strong orb mutants, and Orb drives positive autoregulatory translation of orb mRNA and translation of gurken. Orb itself does part of the work, recognizing the target 3-prime UTR and recruiting poly(A) polymerases (it is in an ovarian complex with PAP and Wispy). Positive regulation of translation (an is_a descendant) is therefore supported. The family-level repressor activity inferred by PAINT for CPEB proteins has not been demonstrated for fly Orb and is not transferred here.
We conclude that Orb-mediated cytoplasmic polyadenylation stimulates oskar translation to achieve the high levels of Oskar protein necessary for posterior patterning and germline differentiation.
IMP PMID:11809824 Cytoplasmic localization and evolutionary conservation of ME...
KEEP AS NON CORE
Summary: orb mutant ovaries fail to form 16-cell cysts (strong allele) or show defective oocyte differentiation; Orb is required for germline cyst and oocyte development.
Reason: The MEI-218 paper directly analyses orb F303 and F343 mutant ovaries (full text cached). Orb acts in oocyte development through its translational-activator function, so the developmental process is retained as non-core.
Orb-mediated cytoplasmic polyadenylation stimulates oskar translation to achieve the high levels of Oskar protein necessary for posterior patterning and germline differentiation.
IMP PMID:15737929 The Drosophila fragile X protein functions as a negative reg...
KEEP AS NON CORE
Summary: Orb is required for localized gurken mRNA distribution and translation, which specifies the dorsal/ventral axis of the oocyte.
Reason: Abstract-only cache; the curator's IMP is accepted. Orb acts in this process at the level of its direct targets (gurken mRNA localization and translation), so it is part of oocyte axis specification, but this is a developmental deployment of the core translational-activation function and is kept as non-core.
orb is required during mid-oogenesis for the translation of the posterior/germline determinant oskar mRNA and the dorsal-ventral determinant gurken mRNA.
IMP PMID:7926753 orb is required for anteroposterior and dorsoventral pattern...
KEEP AS NON CORE
Summary: A maternal-effect orb allele produces ventralized eggshells, orb is needed for asymmetric gurken mRNA distribution, and orb interacts genetically with grk and top/DER.
Reason: Same claim as the PMID:15737929 row; consistent action. Orb acts on gurken mRNA directly, so the annotation is retained but as non-core.
IMP PMID:7926753 orb is required for anteroposterior and dorsoventral pattern...
KEEP AS NON CORE
Summary: Orb is required for posterior oskar mRNA distribution and for Oskar protein accumulation at the posterior pole; orb mutant embryos show posterior patterning defects.
Reason: Orb directly binds and activates translation of oskar mRNA, the posterior determinant, so it participates in A/P axis specification of the oocyte. Retained as a non-core developmental role.
Summary: Core molecular function: Orb is a sequence-specific mRNA-binding translational regulator of the CPEB family.
Reason: CPEB-family proteins act as both translational activators and repressors, and PAINT places a repressor-type activity at the family node, while fly Orb is experimentally an activator acting through recruitment of poly(A) polymerases rather than a direct effect on the ribosome. The broad translation regulator activity is therefore as precise as the combined evidence warrants for the molecular function; the direction is captured in the process term.
IMP PMID:7926753 orb is required for anteroposterior and dorsoventral pattern...
KEEP AS NON CORE
Summary: Strong orb mutants fail to localize oskar mRNA asymmetrically within the oocyte; Orb is found in oskar mRNA complexes.
Reason: Orb is physically associated with oskar mRNA and its mutant phenotype includes mislocalized oskar mRNA, so its involvement in oskar localization is accepted from the curator's IMP. Part of the phenotype may reflect the earlier requirement of orb for oocyte polarity (microtubule repolarization), so this is retained as non-core rather than core.
We show that repolarization of the oocyte also requires orb and that orb is needed for the proper functioning of the Par proteins.
Core Functions
Binds CPE-like elements in the 3-prime UTRs of target maternal mRNAs (oskar, gurken, orb) in the oocyte cytoplasm and activates their localized translation by promoting cytoplasmic poly(A)-tail elongation.
Q: Do the N-terminally truncated Orb isoforms (C/E, 559 aa; F, 617 aa) differ from the 915-residue isoform in oocyte localization, phosphorylation-dependent activation, or translational repression versus activation?
Q: PAINT infers a repressor-type translation regulator activity at the CPEB family node; is there direct evidence that fly Orb represses translation of any target, or is that function restricted to vertebrate CPEB1 in this subfamily?
Suggested Experiments
Experiment: Isoform-specific rescue of orb-null germlines with transgenes encoding only the 559-residue C/E product versus the 915-residue product, assaying 16-cell cyst formation, oskar poly(A) length and Osk protein accumulation.
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
Review rationale: Orb is an experimentally established CPEB-family RNA-binding translation regulator, and the same GO term already occurs in the selected record. Direct RNA-target profiling and germline translational studies support correctness independently of that electronic annotation. Q8IMZ2 is shorter at the N-terminus but retains the complete C-terminal RNA-binding machinery, making transfer of this broad function reasonable. The prediction adds no novel activity.
Supporting Evidence:
PMID:27791065: "Both Orb and Orb2 bind linear cytoplasmic polyadenylation element-like sequences in the 3' UTRs of largely overlapping target mRNAs, with Orb2 potentially having a broader specificity."
PMID:15737929: "Orb is responsible for mediating the translational activation of mRNAs localized within the developing oocyte."
PMID:12538512: "Orb-mediated cytoplasmic polyadenylation stimulates oskar translation"
oo18 RNA-binding protein (orb) encodes a RNA-binding protein required for the formation of the egg chamber and establishment of polarity during oogenesis. It is involved in mRNA polyadenylation and the regulation of osk mRNA translation.