Biological properties of the BCL-2 family protein BCL-RAMBO, which regulates apoptosis, mitochondrial fragmentation, and mitophagy

Abstract

Mitochondria play an essential role in the regulation of cellular stress responses, including cell death. Damaged mitochondria are removed by fission and fusion cycles and mitophagy, which counteract cell death. BCL-2 family proteins possess one to four BCL-2 homology domains and regulate apoptosis signaling at mitochondria. BCL-RAMBO, also known as BCL2-like 13 (BCL2L13), was initially identified as one of the BCL-2 family proteins inducing apoptosis. Mitophagy receptors recruit the ATG8 family proteins MAP1LC3/GABARAP via the MAP1LC3-interacting region (LIR) motif to initiate mitophagy. In addition to apoptosis, BCL-RAMBO has recently been identified as a mitophagy receptor that possesses the LIR motif and regulates mitochondrial fragmentation and mitophagy. In the 20 years since its discovery, many important findings on BCL-RAMBO have been increasingly reported. The biological properties of BCL-RAMBO are reviewed herein.

Keywords: BCL-RAMBO; BCL2L13; apoptosis; cell death; miRNAs; mitochondrial fragmentation; mitophagy; phosphorylation.

FIGURE 1

Extrinsic and intrinsic cell death signaling pathways. In the extrinsic pathway, death receptors, upon engagement with ligands, recruit adaptor proteins to their death domain (DD) and procaspases 8 and 10, leading to the formation of a death-inducing signaling complex (DISC). In the DISC, procaspases 8 and 10 are activated into initiator active caspases 8 and 10, which cleave procaspases 3, 6, and 7 to effector active caspases 3, 6, and 7, leading to apoptosis. Mitochondria harbor BCL-2 family proteins and VDAC on mitochondrial outer membranes, ANT on mitochondrial inner membranes, and CYCS and SMAC at the intermembrane space. In the intrinsic pathway, mitochondrial outer membrane permeability (MOMP) causes the release of CYCS and SMAC into the cytosol. CYCS induces complex formation consisting of APAF1 and procaspase 9 (apoptosome). In the apoptosome, procaspase 9 is activated to initiator active caspase 9, which cleaves procaspases 3, 6, and 7 to active caspases 3, 6, and 7, leading to apoptosis. SMAC antagonizes IAP proteins, which target caspases. Mitochondrial permeability transition (MPT) releases <1,500-Da solutes, which cause mitophagy and cell death. BH3-interacting domain death agonist (BID) is cleaved into truncated BID (tBID) by active caspases 8 and 10. tBID translocates to mitochondria and interacts with BCL-2 family proteins, triggering apoptosis signaling.

FIGURE 2

Ubiquitin-dependent and-independent mitophagy pathways. (A) In the ubiquitin-dependent pathway, PINK1 is stabilized on mitochondrial outer membranes (MOM) and phosphorylates PINK1 itself, the E3 ubiquitin (Ub) ligase Parkin, MOM-associated substrates, and free and polymerized Ub (black arrows). Parkin ubiquitinates MOM-associated substrates (red arrows). Poly-Ub chains of MOM-associated substrates interact with the ubiquitin-binding domain (UB) of adaptor proteins, which also possess the MAP1LC3-interacting region (LIR) motif. The LIR motif interacts with MAP1LC3 proteins anchored to phagophores. (B) Mitophagy receptors harbor the LIR motif and transmembrane (TM) domain anchored to MOM. In the ubiquitin-independent pathway, mitophagy receptors interact with MAP1LC3 proteins anchored to phagophores.

FIGURE 3

Structures of human and mouse BCL-RAMBO. (A) Human BCL-RAMBO consists of 485 amino acids, while mouse BCL-RAMBO consists of 434 amino acids. The BCL-2 homology (BH) domain, No BH motif (BHNo) domain, MAP1LC3-interacting region (LIR) motif, and transmembrane (TM) domain are shown. (B) The amino acid sequences of human BCL-RAMBO and mouse BCL-RAMBO are aligned using Clustal Omega (https://www.ebi.ac.uk/Tools/msa/clustalo/).

FIGURE 4

Structures of mitophagy receptors anchored to mitochondrial outer membranes (MOM). (A) Mitophagy receptors possess the MAP1LC3-interacting region (LIR) motif and transmembrane (TM) domain. Abbreviations: BCL-2 homology (BH); peptidyl prolyl cis-trans isomerase (PPI); tetratricopeptide repeats (TPR). (B) Structures of the TM domain of human mitophagy receptors. The TM domain (magenta) is shown with numbers referring to positions in the amino acid sequence. Basic amino acids are shown in green. Asterisks indicate the stop codon. (C) Structures of the LIR motif. The LIR motifs (blue) of human mitophagy receptors are shown with numbers referring to positions in the amino acid sequence.

FIGURE 5

Interaction between BCL-RAMBO and BCL-RAMBO-binding proteins. (A) BCL-RAMBO is expressed on mitochondrial outer membranes (MOM). BCL-RAMBO interacts with PGAM5 expressed on the mitochondrial inner membranes (MIM), which is dependent on a transmembrane domain of BCL-RAMBO, but not its C-terminal intermembrane tail. BCL-RAMBO interacts with ANT expressed on the MIM and inhibits ANT activity, which exchanges ATP and ADP. BCL-RAMBO interacts with VDAC expressed on MOM. (B) BCL-RAMBO interacts with CERS2 and CERS6, and inhibits their catalytic activities, thereby inhibiting apoptosis induced by dihydroceramide and ceramide. (C) BCL-RAMBO interacts with MAP1LC3 proteins via its LIR motif. The phosphorylation of BCL-RAMBO at Ser272 is necessary for the interaction with MAP1LC3. BCL-RAMBO interacts with the ULK1 complex, which promotes mitophagy. (D) The BCL-RAMBO-dependent activation of caspases is promoted by PGAM5, ANT, and VDAC. BCL-RAMBO induces the mitochondrial release of CYCS via mitochondrial outer membrane permeability (MOMP). CYCS initiates the APAF1-dependent activation of caspase-9, and subsequent activation of caspases 3, 6, and 7, leading to apoptosis. BCL-RAMBO induces mitochondrial permeability transition (MPT), which causes mitophagy and cell death.