RIMS2

RIMS2 (Rim2) is a multidomain active-zone scaffolding protein that couples small-GTPase signaling to regulated exocytosis at both neuronal and endocrine release sites (PMID:12578829, PMID:25343783). Through an N-terminal Rab-binding domain it acts as a Rab3A effector, recognizing Rab3A/B/C/D and Rab8A via an acidic cluster (Glu-50/51/52) in its α1 region, and alternative splicing of this domain tunes Rab3A affinity—the short-form binds Rab3A with high affinity and is recruited to dense-core vesicles to support stimulated secretion, while the low-affinity long form does not (PMID:12578829, PMID:16473611). In pancreatic beta-cells RIMS2 functions in a ternary complex with Epac2/cAMP-GEFII and the Ca2+-sensing scaffold Piccolo to mediate cAMP-dependent, PKA-independent insulin secretion, and the same Epac2–Rim2–Rab3A module is required for GLP-1 potentiation of glucokinase activity and glucose sensing (PMID:11598134, PMID:12401793, PMID:22147008). At central synapses RIMS2 acts redundantly with RIM1 to maintain presynaptic Ca2+ channel density and the readily releasable pool, with double knockout but not single knockout producing strong deficits (PMID:25343783). At photoreceptor ribbon synapses RIMS2 localizes interior to the RIBEYE-containing ribbon, and ribbon integrity is required to stabilize and enrich Cav1.4/RIM2 clusters at the active zone (PMID:22832038, PMID:32249787). Its C2A domain adopts a β-sandwich fold that does not bind Ca2+ and shows little binding to SNAP-25 or synaptotagmin-1, indicating it acts through Ca2+-independent protein interactions (PMID:16216076). Biallelic loss-of-function RIMS2 variants cause a syndromic congenital cone-rod synaptic disorder, consistent with its localization to the retinal outer plexiform layer, Purkinje cells, and pancreatic islets (PMID:32470375).

1. 2001 Medium

   Established that Rim2 operates downstream of cAMP-GEFII (Epac2) in a PKA-independent branch of incretin-potentiated insulin secretion, placing the protein in a defined cAMP signaling pathway rather than as a generic scaffold.

   Evidence Antisense knockdown plus PKA inhibition with insulin secretion assays in pancreatic islets

   PMID:11598134

   Open questions at the time

   • Does not define the direct molecular interactions linking Epac2 to Rim2
   • Does not resolve which exocytic step Rim2 controls
2. 2002 Medium

   Identified Piccolo as a Ca2+-sensing partner that dimerizes with Rim2 within the Epac2·Rim2·Piccolo complex, supplying the Ca2+-dependence of cAMP-induced insulin secretion.

   Evidence Co-IP and pulldown dimerization assays plus antisense Piccolo knockdown in islets

   PMID:12401793

   Open questions at the time

   • Stoichiometry and architecture of the ternary complex not resolved
   • Single-lab functional knockdown
3. 2003 High

   Defined the molecular basis of Rim2 as a Rab3A/Rab8A effector by mapping an acidic-cluster determinant in the Rab-binding domain, distinguishing its Rab selectivity from Rab27.

   Evidence Cotransfection binding screen across 42 Rab proteins with site-directed mutagenesis and chimeras

   PMID:12578829

   Open questions at the time

   • Functional consequence of Rab8A binding not characterized
   • Binding measured in cotransfection rather than reconstituted system
4. 2005 High

   Showed that the C2A domain, despite a canonical β-sandwich fold, lacks Ca2+-binding and weak binding to SNAP-25/synaptotagmin-1, redefining it as a Ca2+-independent protein-interaction module.

   Evidence 1.4 Å X-ray crystallography, NMR, and biochemical binding assays

   PMID:16216076

   Open questions at the time

   • The Ca2+-independent binding partner engaging the bottom face is unidentified
   • No in-cell validation of the proposed interaction surface
5. 2005 Medium

   Demonstrated that alternative splicing of the Rab-binding domain is a regulatory switch controlling Rab3A affinity, vesicle recruitment, and secretory competence.

   Evidence In vitro Rab3A binding, PC12 localization, NPY secretion, and E36A/R37S mutagenesis

   PMID:16473611

   Open questions at the time

   • Physiological regulation of splice-form choice not addressed
   • Single cell model
6. 2011 Medium

   Extended the Epac2–Rim2–Rab3A module to metabolic sensing by showing it is required for GLP-1 enhancement of glucokinase activity and downstream glucose handling.

   Evidence siRNA silencing of Epac2/Rim2/Rab3A with glucokinase, glucose uptake, and ATP assays in INS-1 and native beta-cells

   PMID:22147008

   Open questions at the time

   • Mechanistic link between Rim2 and glucokinase regulation unresolved
   • Single lab
7. 2014 High

   Resolved the functional relationship between RIM1 and RIM2 at a central synapse, showing they redundantly set presynaptic Ca2+ channel density and readily releasable pool size.

   Evidence Conditional single and double knockouts with direct presynaptic patch-clamp at the calyx of Held

   PMID:25343783

   Open questions at the time

   • Does not isolate RIM2-specific molecular targets
   • Channel-clustering mechanism not directly visualized
8. 2020 Medium

   Linked RIM2 to ribbon-synapse organization by showing the synaptic ribbon is required to stabilize and enrich Cav1.4/RIM2 clusters at photoreceptor active zones.

   Evidence STED imaging, RIBEYE morpholino/knockout in zebrafish and mouse with cluster quantification

   PMID:22832038 PMID:32249787

   Open questions at the time

   • Whether RIM2 directly tethers Cav1.4 versus depends on ribbon scaffold is unresolved
   • Mechanism of darkness-induced enrichment unknown
9. 2020 Medium

   Established RIMS2 as a human disease gene, with biallelic loss-of-function causing a syndromic cone-rod synaptic disorder and impaired insulin secretion, tying its retinal and endocrine roles to pathology.

   Evidence Whole-exome sequencing, immunolocalization, and truncating-variant expression with insulin secretion assays

   PMID:32470375

   Open questions at the time

   • Genotype-phenotype correlations across tissues not fully defined
   • Cerebellar Purkinje-cell function of RIMS2 not assayed

• The identity of the Ca2+-independent C2A binding partner, the regulation of RIM2 splicing in vivo, and how Rab3 phosphorylation gates RIM2 recruitment at disease-vulnerable synapses remain open.
• C2A-domain interacting protein unidentified
• In vivo control of RBM5-dependent RIMS2 splicing not mechanistically established
• LRRK2/Rab3-phosphorylation effect on RIM2 reported only in preprint