Catalytic subunit of the phosphorylase b kinase (PHK), which mediates the neural and hormonal regulation of glycogen breakdown (glycogenolysis) by phosphorylating and thereby activating glycogen phosphorylase. In vitro, phosphorylates PYGM, TNNI3, MAPT/TAU, GAP43 and NRGN/RC3. 2 ATP + phosphorylase b = 2 ADP + phosphorylase a. ATP + L-seryl-[tau protein] = ADP + H(+) + O-phospho-L-seryl-[tau protein] ATP + L-threonyl-[tau protein] = ADP + H(+) + O-phospho-L-threonyl-[tau protein] ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Hexadecamer of 4 heterotetramers, each composed of alpha, beta, gamma, and delta subunits. Alpha (PHKA1 or PHKA2) and beta (PHKB) are regulatory subunits, gamma (PHKG1 or PHKG2) is the catalytic subunit, and delta is calmodulin. The two calmodulin-binding domains appear to act in concert to bind a single molecule of calmodulin and are pseudosubstrate/autoinhibitory domains. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family.