Calcineurin B-mediated Ca 2+ sensing translates stress signal intensity into the assembly of phase-separated condensates at PERK complexes.

Endoplasmic reticulum (ER) stress activates protein kinase RNA-like ER kinase (PERK), which initially promotes adaptive responses but remains the only active UPR branch during prolonged stress, mediating both early cytoprotective and chronic pro-apoptotic signaling. Recently, we identified translocon-generated Ca 2+ microdomains that promote PERK phosphorylation during early UPR, revealing a mechanism by which local Ca 2+ signals regulate UPR activation. However, the molecular mechanism linking these Ca 2+ microdomains to PERK activation remains elusive. Previously, we showed that calcineurin (CN), a Ca 2+ -dependent heterodimer composed of catalytic (CNA) and regulatory (CNB) subunits, exerts a non-canonical pro-survival function by promoting PERK autophosphorylation. Here, using super-resolution microscopy, CRISPR-Cas9 editing, in silico analyses, and optogenetic droplet assays, we identify CNB as a local Ca 2+ sensor that couples translocon-generated Ca 2+ signals to liquid condensate assembly, thereby promoting adaptive PERK phosphorylation. These findings establish CNB-mediated condensate assembly as a mechanism that translates local Ca 2+ signals into spatially organized early adaptive PERK signaling.