Adducin Regulation

Adducin promotes association of spectrin with actin and caps the fast growing end of actin filaments. Adducin contains N-terminal core, neck, and C-terminal tail domains, is a substrate for protein kinases A (PKA) and C (PKC), and binds to Ca²⁺/calmodulin. Ser-726 and Ser-713 in the C-terminal MARCKS-related domains of α- and β-adducin, respectively, were identified as the major phosphorylation sites common for PKA and PKC. PKA, in addition, phosphorylated α-adducin at Ser-408, -436, and -481 in the neck domain. Phosphorylation by PKA, but not PKC, reduced the affinity of adducin for spectrin-F-actin complexes as well as the activity of adducin in promoting binding of spectrin to F-actin. The myristoylated alanine-rich protein kinase C substrate-related domain of β-adducin was identified as the dominant Ca²⁺-dependent calmodulin-binding site. Calmodulin-binding was inhibited by phosphorylation of β-adducin and of a MARCKS-related domain peptide by PKA and PKC. Calmodulin in turn inhibited the rate, but not the extent, of phosphorylation of β-adducin, but not α-adducin, by PKA and that of each subunit by PKC. These findings suggest a complex reciprocal relationship between regulation of adducin function by calmodulin binding and phosphorylation by PKA and PKC.