UniProt: Q9BXK5 (B2L13_HUMAN). Synonyms: Bcl-rambo, Mil1, BCL2L13, ORFNames=CD003.
485 aa (canonical isoform 2 displayed, Q9BXK5-1). Single-pass tail-anchored membrane protein
(FT TRANSMEM 460..480). Contains BH4 (14..30), BH3 (100..116), BH1 (147..157), BH2 (193..206)
motifs, plus a unique ~250 aa insertion with two tandem repeats (A, B) preceding the C-terminal
transmembrane anchor.
BCL2L13/Bcl-rambo is a mitochondrial outer-membrane (tail-anchored) BCL-2-family protein. The two
best-established, distinct activities reported in the literature are:
Mitophagy receptor — BCL2L13 is the mammalian functional homolog of yeast Atg32. It carries a
WXXL-type LIR (LC3-interacting region) and binds LC3/GABARAP-family proteins to recruit the
autophagy machinery to mitochondria, inducing mitochondrial fragmentation and mitophagy. The
foundational study (Murakawa et al., Nat Commun 2015, PMID:26471991) is NOT among the GOA-cited
references and is NOT cached in this repo, so it cannot be used for verbatim supporting_text;
however, the UniProt IntAct interaction table directly records BCL2L13 binding to the human Atg8
ortholog GABARAPL2 (P60520), consistent with the LIR-dependent receptor model.
[file:human/BCL2L13/BCL2L13-uniprot.txt "Q9BXK5; P60520: GABARAPL2; NbExp=4; IntAct=EBI-747430, EBI-720116;"]
UniProt/Ensembl GOA also carries GO:0000423 mitophagy (IEA:Ensembl) and GO:0007005 mitochondrion
organization (IEA:Ensembl), reflecting this role, though these specific terms are not in the
seeded existing_annotations set.
[file:human/BCL2L13/BCL2L13-uniprot.txt "GO:0000423; P:mitophagy; IEA:Ensembl."]
Apoptosis (original characterization) — Bcl-rambo was first described as a pro-apoptotic
BCL-2 homolog whose cell-death activity is induced by its membrane-anchored C-terminal domain
(the unique insertion + TM), NOT by its BH motifs, and is blocked by caspase inhibitors/IAPs but
not by Bcl-xL. Notably it does NOT heterodimerize with other BCL-2-family members.
PMID:11262395
PMID:11262395
PMID:11262395
The apoptosis function is the basis of the GOA NAS annotations (PMID:11262395): apoptotic process
(GO:0006915), mitochondrion (GO:0005739), cysteine-type endopeptidase activator activity involved
in apoptotic process (GO:0008656, i.e. caspase-3 activation), and membrane (GO:0016020, IDA).
UniProt FUNCTION: "May promote the activation of caspase-3 and apoptosis."
[file:human/BCL2L13/BCL2L13-uniprot.txt "FUNCTION: May promote the activation of caspase-3 and apoptosis."]
The original overexpression-driven apoptosis phenotype is the historical view; the more recent and
mechanistically distinct mitophagy-receptor role (Atg32 homolog) is now regarded as a central,
conserved function. Distinguishing CORE (mitophagy receptor / mitochondrial fragmentation) from the
context-dependent apoptosis annotations is the main curation task.
Legionella pneumophila effector SidF specifically interacts with and neutralizes BNIP3 and Bcl-rambo
to inhibit host macrophage apoptosis, confirming Bcl-rambo as a genuine pro-death host protein and a
pathogen target. UniProt IntAct records Q9BXK5-sidF (Q5ZSD5, Xeno).
PMID:17360363
PMID:17360363
The seeded GO:0005515 protein binding IPI from PMID:17360363 corresponds to this SidF interaction;
as bare protein binding it is uninformative and is marked over-annotated.
UniProt: Isoform 2 (canonical, Q9BXK5-1): Mitochondrion membrane; Single-pass membrane protein;
Nucleus. Isoform 1 (Q9BXK5-2, a short 201-aa form): Nucleus.
[file:human/BCL2L13/BCL2L13-uniprot.txt "SUBCELLULAR LOCATION: [Isoform 2]: Mitochondrion membrane"]
Mitochondrial (outer) membrane is the core functional location. The Nucleus localization is reported
(largely for the truncated isoform 1, which lacks the TM anchor) and is secondary; isoform 1
(Q9BXK5-2) ends at residue 201 (VSP_000526/VSP_000527) and is nuclear.
Many GO:0005515 protein binding IPI annotations derive from high-throughput interactome papers
(16189514, 25416956, 25910212, 26871637, 28514442, 32296183, 32814053, 33961781) plus the specific
SidF study (17360363). The UniProt IntAct table is dominated by membrane-protein partners (many
single-pass TM proteins, e.g. PLN, MAL, PLP1, VAMP3/4, syntaxins), consistent with a
membrane-embedded tail-anchored protein; most are not functionally informative. "protein binding"
is uninformative and marked over-annotated. The functionally meaningful interaction is with the
Atg8-family protein GABARAPL2 (mitophagy receptor activity), but that is recorded as an IntAct hit,
not among the seeded protein-binding annotations with a cached PMID.
CORE (mitophagy receptor / mitochondrial fragmentation): mitochondrion (GO:0005739) localization,
membrane (better as mitochondrial outer membrane). NOTE: the defining mitophagy MF/BP terms are not
in the seeded set (they exist in UniProt GOA as IEA: GO:0000423 mitophagy, GO:0007005 mitochondrion
organization) — proposed as new/expanded annotation.
SECONDARY / NON-CORE: apoptotic process, regulation of apoptotic process, caspase-3 activator
activity (GO:0008656), nucleus localization.
OVER-ANNOTATED / UNINFORMATIVE: protein binding (GO:0005515) — all instances.
The strongest evidence for the mitophagy-receptor / Atg32-homolog function (Murakawa et al. 2015)
is not in the cached publications, so verbatim supporting_text for that function is drawn only from
the UniProt record (GABARAPL2 IntAct interaction; GO:0000423 mitophagy IEA) and is reflected in the
description, core_functions, and proposed_new_terms rather than asserted from an inaccessible PMID.
PMID correction: the foundational Murakawa et al. Nat Commun 2015 mitophagy/Atg32 paper is
PMID:26146385 (DOI 10.1038/ncomms8527), not PMID:26471991 as guessed earlier in these notes.
Verified via PubMed ID conversion (DOI->PMID) and metadata lookup. None of the new papers below are
cached in publications/, so they are added to the review as statement-only findings (no
verbatim supporting_text).
CONFIRMS (now with a citable primary PMID): BCL2L13 is the mammalian Atg32 homolog; it binds LC3
via a WXXI/LIR motif and induces both mitochondrial fragmentation and mitophagy. BH domains drive
fragmentation; the WXXI/LIR motif drives mitophagy. Fragmentation occurs without DRP1/DNM1L, and
mitophagy proceeds in Parkin-deficient cells (ubiquitin-independent, Parkin-independent).
PMID:26146385 (Murakawa 2015, Nat Commun). This grounds the existing
description/core_functions that were previously inferred only from UniProt.
NEW (mechanism): Phosphorylation regulates the receptor. A Ser site adjacent to the LIR
(Ser272, mouse numbering; human LIR is WQQI at ~276-279) tunes LC3 binding and mitophagic activity;
PGAM5 acts as a negative regulator by dephosphorylating BCL2L13, and BCL2L13 has been reported to
recruit the ULK1 complex to the mitochondrial outer membrane to initiate autophagosome formation.
Additional reported interactors/modulators include ULK1, PGAM5, ANT (SLC25A4), VDAC, and CERS2/CERS6.
Source: Kataoka 2022 review PMID:36589739 (review-level; mechanistic detail, treat as provisional
where based only on review synthesis).
NEW (localization/process): Beyond the mitochondrial outer membrane, BCL2L13 also localizes at
ER-mitochondria contact sites / mitochondria-associated membranes (MAMs) and regulates
ER-mitochondria Ca2+ homeostasis in skeletal muscle. Knockdown alters cytosolic Ca2+ release and
mitochondrial Ca2+ uptake without changing the NUMBER of ER-mito contacts (a functional, not
structural, contact-site effect); Bcl2l13-KO zebrafish show impaired muscle structure/function and
decreased mitochondrial complex activity. PMID:39175772 (Grepper 2024, iScience). This is a genuinely new functional context
(Ca2+ homeostasis at MAMs) not in the existing review.
NEW (disease / context-dependent fission wiring): In glioblastoma, BCL2L13 is upregulated and
promotes mitochondrial fission and high mitophagy flux by targeting DNM1L at Ser616, supporting tumor
proliferation/invasion. This is notable because it is DNM1L(DRP1)-DEPENDENT, in apparent contrast to
the DRP1-independent fragmentation seen by Murakawa 2015 - i.e., context-dependent fission wiring.
PMID:37660127 (Wang 2023, Cell Death Dis). Do NOT use this to override the DRP1-independent core
claim; both are valid in their respective contexts.
NEW (interactor/pathway, disease): In diabetic kidney disease, BCL2L13 was identified by LC-MS/MS as
an interacting partner of the inner-membrane protease YME1L; YME1L promotes BCL2L13 phosphorylation,
strengthening BCL2L13-LC3 binding and mitophagy to restrain renal tubular senescence.
PMID:38494498 (Luo 2024, Biol Res). New YME1L interactor.
NEW (disease, prognosis): In clear-cell and papillary renal cell carcinoma, BCL2L13 is downregulated
and lower expression correlates with poorer prognosis (independent of grade); BCL2L13 is positively
correlated with and acts through SLC25A4/ANT in its pro-apoptotic pathway. PMID:34193180 (Meng 2021, Cancer Cell Int). Contrasts with the pro-survival/oncogenic role in GBM,
reinforcing strong context/tissue dependence.
PROVISIONAL / not used to change annotations: a 2024 study reports miR-449b-5p directly targets the
BCL2L13 3'UTR in hypoxic AC16 cardiomyocytes, with BCL2L13 suppression being protective via PI3K/AKT
(Jiang 2024, Appl Biochem Biotechnol, DOI 10.1007/s12010-024-04931-5; 0 citations at report time).
Low-confidence/peripheral to GO core function; kept in notes only, no PMID added to the review.
Curation impact: existing annotation actions are unchanged. The new primary references (Murakawa 2015,
Kataoka 2022, Grepper 2024, Wang 2023, Luo 2024, Meng 2021) are added to references: as statement-only
findings. Murakawa 2015 now provides a citable primary basis for the mitophagy-receptor core function.
Grepper 2024 (ER-mito contact / Ca2+) and the YME1L/SLC25A4 interactors are the main genuinely-new
biology; added suggested questions/experiments accordingly.