The active-site cysteine residue of Ca ²⁺ /calmodulin-dependent protein kinase I is protected from irreversible modification via generation of polysulfidation

Ca ²⁺ /calmodulin (CaM)-dependent protein kinase (CaMK) I is activated by the phosphorylation of a crucial activation loop Thr ¹⁷⁷ by upstream kinases, CaMK kinase (CaMKK), and regulates axonal or dendritic extension and branching. Reactive sulfur species (RSS) modulate protein functions via polysulfidation of the reactive Cys residues. Here, we report that the activity of CaMKI was reversibly inhibited via its polysulfidation at Cys ¹⁷⁹ by RSS. In vitro incubation of CaMKI with the exogenous RSS donor Na ₂ S ₃ resulted in a dose-dependent inhibition of the phosphorylation at Thr ¹⁷⁷ by CaMKK and inactivation of the enzymatic activity. Dithiothreitol (DTT), a small molecule reducing reagent, rescued these inhibitions. Conversely, mutated CaMKI (C179V) was resistant to the Na ₂ S ₃ -induced inactivation. In transfected cells expressing CaMKI, ionomycin-induced CaMKI activity was decreased upon treatment with Na ₂ S ₄ , whereas cells expressing mutant CaMKI (C179V) proved resistant to this treatment. A biotin-polyethylene glycol-conjugated maleimide capture assay revealed that CaMKI was a target for polysulfidation in cells. Furthermore, the polysulfidation of CaMKI protected Cys ¹⁷⁹ from its irreversible modification, known as protein succination. Thus, we propose that CaMKI was reversibly inhibited via polysulfidation of Cys ¹⁷⁹ by RSS, thereby protecting it from irreversible modification.