Molecular basis for the regulation of human phosphorylase kinase by phosphorylation and Ca2+

Phosphorylase kinase (PhK) regulates the degradation of glycogen by integrating diverse signals, providing energy to the organism. Dysfunctional mutations may directly lead to Glycogen Storage Disease type IX (GSD IX), whereas the abnormal expression of PhK is also associated with tumors. Here, we use cryo-electron microscopy (cryo-EM) to resolve its near-atomic structures in the inactive and active states. These structures reveal the interactions and relative locations of the four subunits (αβγδ) within the PhK complex. Phosphorylated α and β subunits induce PhK to present a more compact state, while Ca2+ causes sliding of the δ subunit along the helix of the γ subunit. Both actions synergistically activate PhK by enabling the de-inhibition of the γ subunit. We also identified different binding modes between PhK and its substrate, glycogen phosphorylase (GP), in two distinct states, using cross-linking mass spectrometry (XL-MS). This study provides valuable insights into the regulatory mechanisms of PhK, thereby enhancing our understanding of GSD IX and its implications in tumorigenesis. Glycogen phosphorylase kinase (PhK) initiates glycogen degradation by phosphorylating glycogen phosphorylase (GP), positioning itself at the core of glycogenolysis. Here, the authors utilize cryo-EM to elucidate the mechanisms by which human PhK is regulated by phosphorylation and calcium ions, and reveal its binding mode with the substrate GP.