ADDUCIN REGULATION - DEFINITION OF THE CALMODULIN-BINDING DOMAIN AND SITES OF PHOSPHORYLATION BY PROTEIN-KINASE-A AND PROTEIN-KINASE-C

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 (P KA) and C (PKC), and binds to Ca2+/calmodulin. Ser-726 and Ser-713 in the C-terminal MARCKS-related domains of alpha- and beta-adducin, resp ectively, were identified as the major phosphorylation sites common fo r PKA and PKC, PKA, in addition, phosphorylated alpha-adducin at Ser-4 08, -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-rela ted domain of beta-adducin was identified as the dominant Ca2+-depende nt calmodulin-binding site. Calmodulin-binding was inhibited by phosph orylation of beta-adducin and of a MARCKS-related domain peptide by PR A and PRC. Calmodulin in turn inhibited the rate, but not the extent, of phosphorylation of beta-adducin, but not alpha-adducin, by PKA and that of each subunit by PKC. These findings suggest a complex reciproc al relationship between regulation of adducin function by calmodulin b inding and phosphorylation by PKA and PKC.