BIT2 (systematic name YBR270C) is an accessory subunit of TOR complex 2 (TORC2), the rapamycin-insensitive TOR-kinase complex of budding yeast. TORC2 is built around the Tor2 Ser/Thr protein kinase together with Lst8, Avo1, Avo2, Avo3/Tsc11, and one of the two paralogous accessory subunits Bit61 or Bit2, and it signals to the AGC-family kinases Ypk1/Ypk2 to control plasma-membrane homeostasis, sphingolipid metabolism, and actin cytoskeleton organization during polarized growth. BIT2 belongs to the Bit61/PRR5 family (Pfam HbrB), whose members are the fungal Bit61/Bit2 and the metazoan PRR5/PRR5L (Protor-1/Protor-2) accessory subunits of TORC2/mTORC2; these proteins have no known catalytic activity and act as regulatory/scaffolding subunits. BIT2 physically associates with the TORC2 subunit Tsc11 and with the TORC2 downstream effectors Slm1 and Slm2. It arose from the whole-genome duplication as the paralog of BIT61, with which it is largely interchangeable within TORC2; bit2 null mutants are viable, consistent with functional redundancy between the two paralogs. The specific, BIT2-distinct molecular contribution to TORC2 has not been experimentally defined.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0007163 establishment or maintenance of cell polarity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetically inferred (IBA) participation in establishment/maintenance of cell polarity. TORC2 signaling, via Slm1/Slm2 and the Ypk1/Ypk2 kinases, does regulate the actin cytoskeleton and polarized growth, so a family-level link to cell polarity is biologically reasonable. However, this is a downstream, process-level inference and there is no BIT2-specific experimental evidence for a direct role in cell polarity. Reason: Retain as a plausible IBA process annotation propagated across the Bit61/PRR5 family, but treat it as non-core: it reflects the broad downstream output of TORC2 signaling rather than a demonstrated, BIT2-specific function. The core, better-supported process annotation for BIT2 is TORC2 signaling (GO:0038203). |
| GO:0019887 protein kinase regulator activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetically inferred (IBA) molecular function. As an accessory subunit of the Tor2-kinase-containing TORC2, BIT2 plausibly contributes to modulating the complex's protein-kinase activity, and this is the least uninformative molecular-function term available for the family. However, there is no experimental demonstration that BIT2 itself enables (independently has) protein kinase regulator activity; the Bit61/PRR5 (HbrB) domain has no catalytic motifs and the paralog family members are non-catalytic accessory subunits. Reason: Keep as an IBA family molecular-function annotation but mark non-core: BIT2 does not independently "enable" kinase regulation; any such activity is a property it contributes to as part of TORC2. This is captured more accurately in core_functions as contributes_to_molecular_function (protein kinase regulator activity) alongside TORC2 membership. This annotation is the locus of the gene's molecular-function knowledge gap (see knowledge_gaps). |
| GO:0031932 TORC2 complex | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically inferred (IBA) membership of the TORC2 complex, consistent with the whole Bit61/Bit2/PRR5/Protor family being TORC2/mTORC2 accessory subunits. This is corroborated by the experimental IDA row (below) and by the TORC2 subunit inventory in the cryo-EM literature. Reason: TORC2 complex membership is the best-supported, core annotation for BIT2, supported by both phylogenetic inference and direct experimental data. Supporting Evidence: PMID:29170376 TORC2 (mTORC2) is composed of Tor2 (mTOR), Lst8 (mLst8/GΞ²L), Avo1 (hSin1), Avo2 (no ortholog), Avo3 (Rictor) and Bit61 or its paralog Bit2 (Protor1 or Protor2). |
| GO:0038203 TORC2 signaling | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically inferred (IBA) participation in TORC2 signaling, following directly from TORC2 complex membership. TORC2 signaling to the Ypk1/Ypk2 AGC kinases and the Slm1/Slm2 effectors is the pathway in which BIT2 acts. Reason: Core biological-process annotation for BIT2: as a TORC2 subunit it is involved in TORC2 signaling. Well-supported both phylogenetically and by the experimental IC row. |
| GO:0031932 TORC2 complex | IDA PMID:15689497 The pleckstrin homology domain proteins Slm1 and Slm2 are re... | ACCEPT | Summary: Direct experimental (IDA) evidence, assigned by SGD, for BIT2 as part of the TORC2 complex. The cached copy of PMID:15689497 is abstract-only and its abstract foregrounds the paralog Bit61 binding Slm1/Slm2/Avo2; the BIT2-specific complex-membership data are in the full text/figures that the SGD curator read. Per curation policy, the experimental annotation is retained and not downgraded on the basis of the abstract naming the paralog. Reason: This is the experimental anchor for BIT2's core function (TORC2 complex membership). Defer to the SGD curator, who had access to the full text; do not REMOVE or weaken an IDA annotation merely because the abstract-only cache emphasizes the paralog Bit61. Supporting Evidence: PMID:15689497 Slm1 and Slm2 physically interact with Avo2 and Bit61, two components of the TORC2 signaling complex, which mediates Tor2 signaling to the actin cytoskeleton. |
| GO:0038203 TORC2 signaling | IC PMID:15689497 The pleckstrin homology domain proteins Slm1 and Slm2 are re... | ACCEPT | Summary: Curator-inferred (IC) participation in TORC2 signaling, derived from the experimental TORC2 complex membership (GO:0031932). A subunit of TORC2 necessarily participates in TORC2 signaling. Reason: Sound IC annotation logically entailed by the experimental complex-membership evidence; a core biological-process annotation for BIT2. |
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: Root molecular_function term with the No-Data (ND) evidence code, correctly reflecting that no specific molecular function has been experimentally determined for BIT2. The Bit61/PRR5 (HbrB) domain has no catalytic activity, and BIT2 acts as an accessory subunit whose distinct biochemical contribution to TORC2 is undefined. Reason: The ND annotation is an honest and appropriate reflection of BIT2's molecular-function knowledge gap (see knowledge_gaps), not a curation defect. It should be retained until a specific BIT2 molecular activity is demonstrated. |
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Download this section (compressed HTML)Q: Does BIT2 make any non-redundant contribution to TORC2 relative to its paralog BIT61 (e.g. condition-specific incorporation, differential expression, or a distinct effector bias), or are the two paralogs fully interchangeable within TORC2?
Q: What is the biochemical role of the BIT2 accessory subunit within TORC2 β does it modulate Tor2 kinase activity, substrate (Ypk1/Ypk2) or effector (Slm1/Slm2) selection, complex assembly/stability, or TORC2 membrane localization?
Experiment: Assay Tor2 kinase activity toward Ypk1/Ypk2 and TORC2 complex integrity in wild-type, bit2-delta, bit61-delta, and bit2-delta bit61-delta strains, to determine whether either paralog is required for TORC2 activity/assembly and whether they are functionally redundant.
Hypothesis: BIT2 and BIT61 are largely redundant accessory subunits; loss of both, but not either alone, measurably impairs TORC2 kinase output or complex stability.
Type: genetic and biochemical (in vitro kinase assay; co-purification)
Experiment: Purify TORC2 via tagged BIT2 (rather than Bit61) and determine its structure and effector (Slm1/Slm2) contacts, to test whether BIT2-containing TORC2 differs architecturally or in effector engagement from Bit61-containing TORC2.
Hypothesis: BIT2-containing TORC2 occupies the same accessory-subunit position (Avo3-associated edge) as Bit61-containing TORC2, with comparable effector contacts.
Type: affinity purification and cryo-EM / crosslinking-MS
Experiment: Proximity labeling (BioID/TurboID) with tagged BIT2 in yeast to map its proximal interactome within and beyond TORC2, distinguishing an accessory-scaffold role from a dedicated effector-recruiting role.
Hypothesis: The BIT2 proximal interactome is dominated by TORC2 subunits and Slm1/Slm2, consistent with an accessory/scaffolding function rather than an independent enzymatic activity.
Type: proximity-labeling proteomics
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: BIT2's specific molecular function within TORC2 is unknown. No experiment defines a BIT2-specific biochemical activity, and its molecular_function is annotated ND. It is undetermined whether BIT2 regulates Tor2 kinase activity, effector/substrate selection, complex assembly, or localization.
OPEN BIOLOGYONTOLOGY MF_DARK
What is known: BIT2 is an experimentally confirmed subunit of TORC2 (IDA) and thereby participates in TORC2 signaling; it associates with Tsc11 and the effectors Slm1/Slm2 (UniProt SUBUNIT, from PMID:11283351 and PMID:15689497). It belongs to the Bit61/PRR5 (HbrB) family, whose members (fungal Bit61/Bit2; metazoan PRR5/PRR5L = Protor-1/Protor-2) are non-catalytic accessory subunits of TORC2/mTORC2. The sequence has no catalytic motif.
Significance: Assigning the molecular role of each TORC2 accessory subunit is required to understand how this master regulator of membrane and actin homeostasis is tuned. The absence of a GO term for an "accessory/regulatory subunit of a kinase complex" is why the well-defined complex membership still leaves BIT2 MF-dark.
What would resolve it: Biochemical dissection of BIT2's contribution to TORC2 (kinase assays, effector-engagement and assembly assays with bit2 alleles); development of an ontology term for accessory kinase-complex subunit activity.
Provenance (the field's own admissions):
Gap: BIT2's non-redundant function relative to its whole-genome-duplication paralog BIT61 is unknown. TORC2 contains "Bit61 or its paralog Bit2", but whether the two are fully interchangeable or have distinct, condition-specific roles has not been established, and no bit2-delta bit61-delta double-mutant phenotype is reported.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: BIT2 and BIT61 are paralogs arising from the yeast whole-genome duplication; both are accessory subunits of TORC2 and both single-gene deletions are viable. The cryo-EM TORC2 structure was solved from Bit61-containing complexes purified from a bit2-delta strain, so the two paralogs can substitute for one another in the complex.
Significance: Distinguishing redundant from specialized paralog function determines whether BIT2 merits a distinct functional annotation and whether TORC2 output can be differentially tuned via subunit choice.
What would resolve it: Comparative phenotyping and TORC2 activity assays across bit2-delta, bit61-delta and the double mutant under diverse stresses; measurement of paralog-specific expression and condition-dependent incorporation into TORC2.
Provenance (the field's own admissions):
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