Boyenle, I. D. | Search bioRxiv

bioRxiv2026

A Conserved Flexible N-Terminal Domain Tunes the Calcium Sensitivity of Sorcin by Stabilizing Its Active Conformation

Sorcin is a penta-EF-hand Ca2+-binding protein that acts as a Ca2+ sensor and regulator of Ca2+ homeostasis. Although the structure and Ca2+-dependent activation of Sorcin is well characterized, the function of its flexible N-terminal domain (NTD) remains unclear. We combined sequence analysis, Ca2+-induced aggregation assays, multidimensional NMR spectroscopy, and long-timescale molecular dynamics (MD) simulations to define the NTDs role in Sorcin activation. Sequence comparisons showed that the NTD is conserved across vertebrates despite its intrinsic disorder, indicating functional importance. NTD deletion markedly reduced Ca2+ responsiveness. Relative to full-length Sorcin, the construct with NTD truncation required more than twofold higher Ca2+ concentrations and over tenfold higher protein concentrations to initiate aggregation, while aggregation kinetics slowed by nearly three orders of magnitude. Temperature-dependent measurements yielded an apparent activation energy of 36.7 kJ{middle dot}mol-1, consistent with aggregation driven by Ca2+-induced conformational activation rather than denaturation. NMR chemical shift perturbations localized the effects of NTD removal to the EF-hand Ca2+-binding loops and adjacent D-helix. Apo-state MD simulations revealed transient intra- and intermolecular NTD-SCBD contacts that explain some perturbations and support direct and allosteric regulation. Ca2+-bound simulations further showed that SCBD departs more readily from the crystallographic active conformation than full-length Sorcin, indicating that the NTD stabilizes the active state through dynamic contacts. Together, these findings establish the NTD as a critical regulator that enhances Ca2+ responsiveness by shifting Sorcins conformational equilibrium toward the active state and suggest that flexible N-terminal extensions regulate signaling within the penta-EF-hand protein family.

biophysics↗

bioRxiv2026

Long-Timescale Molecular Dynamics Reveal a Coordination-Biased Conformational Selection Mechanism for Sorcin Activation

Sorcin is a dimeric penta-EF-hand Ca2+-binding protein that regulates intracellular Ca2+ homeostasis through Ca2+-dependent conformational activation and target recognition, and it has also been implicated in multidrug resistance in cancer. Although crystal structures have defined the apo inactive and Ca2+-bound active states of Sorcin, the transition pathways connecting these states and the conformational ensembles populated under each condition remain poorly understood. Here, we used long-timescale all-atom molecular dynamics simulations on Anton 3, totaling [~]90 s, to define the Ca2+-coupled conformational landscape of dimeric human Sorcin at atomic resolution. Starting from the Ca2+-bound structure, we directly observed the transition from the active to the inactive state following Ca2+ removal, demonstrating that loss of Ca2+ coordination is sufficient to drive inactivation on the microsecond timescale. Simulations initiated from the Ca2+-bound crystal structure with retained ions unexpectedly revealed ultrafast Ca2+ dissociation and rebinding at all EF-hand sites, indicating weak intrinsic Ca2+ affinity and highly dynamic ion exchange. In complementary simulations initiated from the apo structure, Sorcin spontaneously sampled active-like conformations even in the absence of stable Ca2+ binding, supporting a conformational selection mechanism in which Ca2+ shifts the population toward pre-existing active states rather than inducing the transition de novo. Across all conditions, we also observed pronounced and persistent structural asymmetry between the two protomers, revealing that the Sorcin homodimer is dynamically heterogeneous despite its symmetric crystal structures. Together, these results support a coordination-biased conformational selection model for Sorcin activation, in which weak and rapidly exchanging Ca2+ binding stabilizes, rather than induces, the active state. This work provides a dynamic framework for understanding Sorcin function as a fast Ca2+ sensor and offers broader mechanistic insight into activation principles of EF-hand Ca2+-binding proteins.

biophysics↗