SRPRB

SRPRB (SR beta) is the integral ER membrane GTPase subunit of the heterodimeric signal recognition particle (SRP) receptor, which mediates cotranslational targeting of secretory and membrane proteins to the ER (PMID:3021779, PMID:8247130, PMID:8299417). SRPRB anchors the peripherally associated SR alpha subunit to the ER membrane through a tight interaction with the amino-terminal membrane-binding domain of SR alpha, and is required for SR alpha membrane assembly and protein stability (PMID:7844142, PMID:7797564, PMID:37643813). As a member of the GTPase superfamily, SRPRB binds GTP specifically, and together with SR alpha stimulates GTP binding and hydrolysis by SRP54 in a three-GTPase cascade that drives protein targeting (PMID:8247130, PMID:8299417, PMID:7844142). A 1.7 Å crystal structure of SRPRB-GTP bound to the SR alpha interaction domain showed that the binding interface overlaps the switch 1 region, establishing eukaryotic SR as a conditional heterodimer whose dimerization is governed by the SRPRB GTP/GDP switch cycle (PMID:12654246). Beyond targeting, the GTP-binding activity of SRPRB is required to recruit the oligosaccharyltransferase (OST) complex and assemble an N-glycosylation-competent translocon, coordinating ER translation with N-glycosylation (PMID:36921042). In the heart, cardiac-specific overexpression of Srprb exacerbates ventricular remodeling and arrhythmia by activating ER stress and TLR4/CaMKII/NF-κB signaling (PMID:41568549).

1. 1986 High

   Established that the SRP receptor is not a single protein but a tight, stoichiometric heterodimer, defining SR beta/SRPRB as an obligate partner of SR alpha at the ER membrane.

   Evidence SRP-Sepharose affinity chromatography, reciprocal immunoprecipitation, and sucrose gradient fractionation of liver microsomes

   PMID:3021779

   Open questions at the time

   • Did not define which subunit carries enzymatic/GTPase activity
   • Membrane topology of each subunit unresolved
2. 1993 High

   Identified SR beta as a GTP-binding protein and placed it within a three-GTPase cascade, answering how the receptor regulates SRP54 during targeting.

   Evidence In vitro GTP-binding and GTPase assays with purified SRP and SRP receptor components

   PMID:8247130 PMID:8299417

   Open questions at the time

   • Structural basis of GTP-dependent regulation not resolved
   • Order and kinetics of the GTPase switches not fully defined
3. 1995 High

   Resolved the membrane architecture of the receptor by cloning SR beta as an integral transmembrane GTPase that anchors the peripheral SR alpha, defining the basis of receptor assembly.

   Evidence cDNA cloning and domain analysis, UV cross-linking GTP-binding assay, proteolytic protection topology, and biochemical fractionation; deletion mutagenesis localizing the SR alpha membrane-binding domain

   PMID:7797564 PMID:7844142

   Open questions at the time

   • Atomic-level interface between SR alpha and SR beta not yet defined
   • Regulatory role of the GTP/GDP cycle in dimerization not established
4. 2003 High

   Provided the structural mechanism for receptor dimerization, showing SR is a conditional heterodimer controlled by the SR beta nucleotide state.

   Evidence 1.7 Å X-ray crystal structure of SR beta-GTP bound to the SR alpha interaction domain with supporting biochemical binding assays

   PMID:12654246

   Open questions at the time

   • Did not capture the full assembled receptor with membrane context
   • GDP-bound and apo states not structurally compared in same study
5. 2023 High

   Extended SRPRB function beyond targeting by showing its GTP-binding activity recruits the OST complex to build an N-glycosylation-competent translocon.

   Evidence Guanine analog chemical probes, GTP-binding site mutagenesis, co-IP of SR beta with OST complex, N-glycosylation assays, and CRISPR/Cas9 knockout cells

   PMID:36921042

   Open questions at the time

   • Direct physical contacts between SRPRB and specific OST subunits not mapped
   • Whether OST recruitment is concurrent with or sequential to targeting unresolved
6. 2023 High

   Defined SRPRB as a determinant of SR alpha protein stability and tested the requirement of the receptor for ER mRNA localization.

   Evidence CRISPR/Cas9 SRPRB knockout plus siRNA double-knockout, cell fractionation, and 4SU-seq deep sequencing of ER-associated mRNAs

   PMID:37643813

   Open questions at the time

   • Mechanism of SR alpha destabilization upon SRPRB loss not defined
   • Viability of double-knockout implies redundant targeting pathways not identified
7. 2026 Medium

   Linked SRPRB dosage to cardiac pathology, showing overexpression drives arrhythmia and remodeling through ER stress and TLR4/CaMKII/NF-κB signaling.

   Evidence AAV cardiac-specific overexpression/knockdown in pressure-overload mice with echocardiography, ECG, in vivo electrophysiology, and pathway inhibitor experiments in primary cardiomyocytes and fibroblasts

   PMID:41568549

   Open questions at the time

   • How a canonical SRP receptor subunit engages TLR4/CaMKII/NF-κB is mechanistically unexplained
   • Whether the phenotype depends on SRPRB GTPase/targeting function not tested
8. 2026 Low

   Associated SRPRB with cancer cell proliferation and migration phenotypes in TNBC, implicating WNT signaling.

   Evidence siRNA/shRNA knockdown with proliferation, migration, invasion, apoptosis assays and Western blot of WNT pathway proteins

   PMID:41875940

   Open questions at the time

   • WNT pathway link inferred from protein levels without epistasis or rescue
   • Single-lab knockdown without orthogonal validation or in vivo support

• It remains unclear how the canonical ER-membrane SRP-receptor GTPase function of SRPRB mechanistically connects to its reported roles in cell proliferation, cardiac remodeling, and WNT/NF-κB signaling.
• No experiments distinguish targeting-dependent from targeting-independent disease functions
• Cytoplasmic localization reports conflict with established integral ER membrane topology
• Direct effectors connecting SRPRB to NF-κB or WNT not identified