inscuteable (insc, Q9W2R4, FBgn0011674) — curation notes

Identity / synonyms

• FlyBase FBgn0011674; CG11312; UniProt Q9W2R4 (859 aa).
• UniProt/FlyBase list nem ("not enough muscles") and l(2)05475 / l(2)k12405 among synonyms, i.e. the nem mutant (PMID:7615665) is an allele of insc. This matters: the somatic-muscle and PNS phenotypes annotated from PMID:7615665 are insc(nem) phenotypes.
• Domain architecture (UniProt): large N-terminal disordered/low-complexity region (1–~300), a central ARM-like / ARM-type fold (Gene3D 1.25.10.10, SUPFAM ARM repeat), and a Protein inscuteable homologue C-terminal (Insc_C, Pfam PF19427; InterPro IPR045789/IPR039921) domain (~464–743). No catalytic domain — consistent with a scaffold/adaptor, not an enzyme. ComplexPortal CPX-3174 = raps-insc complex.

Core molecular picture

Insc is a cytoskeletal adaptor / scaffold that couples apical cell polarity to mitotic-spindle orientation during asymmetric cell division (ACD) in Drosophila neuroblasts (NBs) and sensory-organ precursors (pI/SOPs).

• First characterized as a neural-precursor gene whose product "is localized to the apical submembranous surface of neuroblasts and other cell types and shows certain features common to a family of putative cytoskeletal associated proteins" PMID:8626022, suggesting "a possible role for the protein in cytoskeleton organization" PMID:8626022.
• Genetically required for spindle orientation and basal determinant localization: "orientation of the mitotic spindle and correct localization of Numb and Prospero in these cells require the inscuteable gene" and ectopic Insc reorients spindles PMID:8779714, PMID:8779714. Insc "localizes to the apical cell cortex before mitosis" PMID:8779714.
• Scaffold/nucleator: "Inscuteable nucleates an apical complex and is required for protein localization, spindle orientation, and RNA localization" PMID:9267024.

Molecular mechanism: the adaptor bridge (Par3 ↔ Pins/Gαi)

The molecular MF is best defined by the structural/biochemical work: Insc is the adaptor that "bridges between Par3 and the spindle tethering machinery." "the adaptor Inscuteable (Insc) bridges between Par3 and the spindle tethering machinery assembled on NuMA∶LGN∶Gαi(GDP), thus triggering apico-basal spindle orientation" PMID:22171003. The Insc–Pins(LGN) interface is defined by crystal structure of Pins TPR:Insc PEPT PMID:22171003; Insc competes with NuMA/Mud for LGN(Pins) with higher affinity PMID:22171003. Pins (raps/LGN) recruits Insc to the apical cortex: "recruitment of Insc to the apical cortex" is a Mud-independent Pins/Gαi function PMID:17726110.

This is why the raps-insc complex (GO:1990499, ComplexPortal CPX-3174) IDA and the cytoskeletal adaptor activity (GO:0008093) MF are the load-bearing annotations. "raps" = Pins = Drosophila LGN ortholog.

Localization

• Apical cortex (GO:0045179) is the canonical site — multiple independent IDA papers: interphase→metaphase apical crescent PMID:11707517; apically localized with Miranda/Pros/Stau PMID:9888987; part of the Insc/Par apical pathway PMID:16377571.
• Also annotated to broader cell cortex (GO:0005938) and cytoplasm (GO:0005737) — cytoplasm is a non-specific pool; cell cortex is the correct parent of apical cortex.

Biological processes

• Asymmetric (neuroblast) cell division / regulation thereof (GO:0055059, GO:0008356, GO:0009786) — core. Apical complex genes control spindle geometry and daughter-cell size PMID:12526793. Two parallel apical pathways (Baz/aPKC and Pins/Gαi) PMID:12526793.
• Establishment of mitotic spindle orientation — insc's precise spindle role is orientation (and, per Kaltschmidt, spindle rotation + anaphase asymmetry) PMID:10620800. The GOA term GO:0040001 "establishment of mitotic spindle localization" is better captured by GO:0000132 "establishment of mitotic spindle orientation" (verified via QuickGO).
• Apical protein localization (GO:0045176) — Insc directs apical localization of partners e.g. Cornetto PMID:11707517, PMID:11707517.
• RNA localization (GO:0006403) — indirect/scaffolding: insc + staufen localize prospero mRNA apically PMID:9267024. Non-core (downstream of the scaffold role).
• Neuroblast/sibling fate, CNS (GO:0007400, GO:0051960, GO:0055059) — insc required for sibling cell fate specification via correct ACD → differential Notch PMID:19782677; lineage-stage-dependent requirement PMID:19782677. Glia-neuron switch in NB6-4T needs insc PMID:10903176.
• Sensory organ development (GO:0007423) — real (pI/SOP asymmetric divisions) PMID:12526793. NOTE: the GOA reference for this TAS is PMID:10973066, a review titled "Genetics of heart development" whose abstract concerns cardiac development in zebrafish/Drosophila and does not mention insc or sensory organs — this looks like a wrong/mis-applied citation. Term kept (non-core); reference flagged.

Peripheral / pleiotropic phenotypes (non-core)

• PNS development + somatic muscle development (GO:0007422, GO:0007525) from the nem=insc P-element allele: "The phenotype of the P-element mutation of the nem gene suggests that it may be required for the development of the somatic \nmusculature and the chordotonal organs of the PNS" PMID:7615665. Likely reflects downstream consequences of defective ACD in precursor populations; kept non-core.
• Malpighian tubule tip cell differentiation (GO:0061382) IMP from PMID:10625542 (abstract is about wg/Notch/numb and does not name insc; full text presumably assays insc). Consistent with insc expression in Malpighian tubules PMID:8626022. Kept non-core; not removed (experimental IMP, full text not cached).
• Defense response to Gram-negative bacterium (GO:0050829) IMP from a genome-wide V. cholerae susceptibility screen: "Inscuteable and Cyp6a20, both of which increased susceptibility to V. cholerae infection" PMID:19046341, but the authors state "The role of these proteins in immunity to \ninfection has not been characterized" PMID:19046341. A single distal screen hit with uncharacterized mechanism (possibly indirect via epithelial/gut cell division). Kept non-core / weak.

Reference caveats

• PMID:19088087 (Gaziova & Bhat) — its abstract + cached full text are about mid (T-box) in the RP2/sib lineage and do not mention insc; yet it is the GOA IMP reference for insc "regulation of asymmetric cell division" and "regulation of nervous system development". These insc functions are correct and well supported by other papers, but the specific citation's relevance to insc could not be verified from cached text. Terms accepted/kept-non-core; supported instead by canonical primary refs; reference flagged UNVERIFIED.
• PMID:10973066 — heart-development review used for insc "sensory organ development" (see above); flagged WRONG_IDENTIFIER/MISCITED.

Core-function synthesis

• MF: cytoskeletal adaptor activity (GO:0008093) — bridges Par3/Bazooka to Pins(raps)/Gαi.
• Complex: raps-insc complex (GO:1990499).
• Location: apical cortex (GO:0045179).
• Directly involved in: asymmetric neuroblast division (GO:0055059), establishment of mitotic spindle orientation (GO:0000132), apical protein localization (GO:0045176).