RHBDL3

Model summary

A human multi-pass membrane enzyme that couples a cytosolic calcium-sensing module to an intramembrane serine protease core to regulate signaling at the cell surface. Calcium binding to the N-terminal EF-hand module modulates activation of the membrane-embedded catalytic center, enabling controlled cleavage of membrane-tethered signaling precursors and receptors. By gating proteolysis in response to calcium, it tunes pathway initiation and propagation at the plasma membrane through transient protein interactions and regulated intramembrane proteolysis.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The N-terminal half is dominated by calcium-sensing modules: IPR011992 (EF-hand domain pair, residues 22–160) encompasses IPR002048 (EF-hand domain, residues 34–100), indicating a tandem EF-hand scaffold that binds Ca2+ and transduces conformational changes. This EF-hand region precedes and partially overlaps a membrane-embedded protease core: IPR035952 (Rhomboid-like superfamily, residues 159–368) and IPR022764 (Peptidase S54, rhomboid domain, residues 209–359) together define the intramembrane serine protease fold that houses the catalytic serine and histidine within transmembrane helices. Two family-level signatures, IPR051739 (Rhomboid-related Intramembrane Serine Proteases, residues 1–394) and IPR017213 (Peptidase S54, rhomboid, metazoan, residues 63–403), span nearly the full length, anchoring the identity as a metazoan rhomboid protease. The ordered layout—EF-hand pair followed by a rhomboid protease core—creates a Ca2+-regulated intramembrane serine endopeptidase whose activity and membrane residency are modulated by the N-terminal cytosolic sensor.

2. 02

   This architecture dictates molecular function.

   The rhomboid S54 protease domain (IPR022764/IPR035952) confers serine-type endopeptidase chemistry within the lipid bilayer, while the EF-hand pair tunes activity in response to Ca2+. Such enzymes typically require transient substrate capture and assembly with accessory factors, which is consistent with GO:0005515 protein binding as an intrinsic property of regulated intramembrane proteolysis.

3. 03

   From function to process, intramembrane serine proteolysis is a canonical driver of signaling cascades.

   By releasing or activating membrane-tethered signaling precursors and receptors, this protease initiates and modulates downstream pathways, aligning with GO:0007165 signal transduction. The EF-hand control layer provides a mechanism to couple Ca2+ fluxes to pathway gating, ensuring signal fidelity and timing.

4. 04

   Cellular location follows directly from the fold and topology.

   Rhomboid proteases are multi-pass membrane enzymes; the presence of the rhomboid intramembrane core and the experimentally supported multi-pass nature place the protein as an integral membrane component. The provided cellular label GO:0005886 plasma membrane is consistent with a role at the cell surface or closely apposed endomembranes that interface with signaling hubs. The EF-hand-to-rhomboid transition near residues ~159–368 suggests that the N-terminal Ca2+-sensing module remains cytosolic and regulates a membrane-embedded catalytic center at the plasma membrane.

5. 05

   Mechanistically, I hypothesize that Ca2+ binding to the EF-hand pair induces conformational changes that gate access to the rhomboid active site, switching the protease between inactive and substrate-engaged states.

   Substrates are likely membrane-tethered signaling proteins or precursors whose cleavage propagates signaling. The enzyme probably forms transient complexes with cytosolic adaptors and chaperones that stabilize substrate encounters and coordinate trafficking. Likely partners include membrane receptor precursors and scaffolds at the plasma membrane that present substrates to the rhomboid core for regulated intramembrane cleavage.