SRI

Sorcin (SRI) is a 22-kDa calcium-binding protein that functions in calcium signaling regulation, mitochondrial cytoprotection, and multidrug resistance. The SRI gene is genomically co-amplified with MDR1 (ABCB1) on chromosome 7 in multidrug-resistant human cancer cells, establishing its consistent overexpression in the MDR genomic domain (PMID:2901906). Sorcin physically associates with the cardiac ryanodine receptor (RyR2) at the sarcoplasmic reticulum, where it stabilizes RyR2 protein levels and reconstitutes functional caffeine-sensitive calcium release channels (PMID:7592856). A distinct mitochondrial isoform of sorcin interacts with the chaperone TRAP1, and reciprocal depletion experiments demonstrate mutual stabilization of sorcin and TRAP1, coupling sorcin to mitochondrial anti-apoptotic signaling and chemoresistance (PMID:20647321).

1. 1988 High

   The genomic basis of sorcin's association with multidrug resistance was established by demonstrating that SRI is co-amplified with MDR1 on chromosome 7 in human MDR cell lines, raising the question of whether sorcin has a functional role in drug resistance or is merely a bystander of gene amplification.

   Evidence Southern and Northern blotting across multiple human MDR cell lines

   PMID:2901906

   Open questions at the time

   • No evidence for a direct functional contribution of sorcin to drug efflux or resistance
   • Mechanism of sorcin's normal cellular function was unknown
   • Whether sorcin overexpression alone is sufficient to confer resistance was untested
2. 1995 High

   Sorcin's normal physiological function was revealed as a modulator of intracellular calcium release through direct physical association with the ryanodine receptor RyR2 at the sarcoplasmic reticulum, where it stabilizes RyR2 and enables caffeine-sensitive Ca²⁺ channel activity.

   Evidence Co-immunoprecipitation from cardiac myocytes, immunoelectron microscopy at the sarcoplasmic reticulum, and functional reconstitution of Ca²⁺ release in RyR2-negative fibroblasts

   PMID:7592856

   Open questions at the time

   • Structural basis of the sorcin–RyR2 interaction is undefined
   • Whether calcium binding to sorcin's EF-hands is required for RyR2 modulation was not tested
   • Relevance of sorcin–RyR2 interaction to MDR phenotype was not addressed
3. 2010 High

   A mechanistic link between sorcin and mitochondrial anti-apoptotic signaling was established by identifying a mitochondrial sorcin isoform that interacts with the chaperone TRAP1; reciprocal knockdown showed mutual stabilization, and the complex protects cells from chemotherapy-induced apoptosis.

   Evidence TRAP1 co-IP proteomics (mass spectrometry), mitochondrial subfractionation, shRNA/siRNA knockdown with apoptosis assays in colorectal carcinoma cells

   PMID:20647321

   Open questions at the time

   • Direct binding interface between sorcin and TRAP1 is uncharacterized
   • Whether the mitochondrial isoform arises from alternative translation or processing is unknown
   • Relative contributions of RyR2 modulation versus TRAP1-mediated cytoprotection to drug resistance are unresolved

• It remains unknown how sorcin's calcium-sensing EF-hand domains differentially regulate its interactions with RyR2 and TRAP1, and whether sorcin's MDR contribution is mechanistically distinct from MDR1/P-glycoprotein-mediated drug efflux.
• No structure of sorcin in complex with RyR2 or TRAP1
• No genetic loss-of-function models (knockout animal or patient mutations) for sorcin
• Calcium-dependence of the sorcin–TRAP1 interaction has not been tested