Brain adducin: a protein kinase C substrate that may mediate site-directed assembly at the spectrin-actin junction

Erythrocyte adducin is a membrane skeletal protein that binds to calmodulin, is a major substrate for protein kinase C, and associates preferentially with spectrin-actin complexes. Erythrocyte adducin also promotes association of spectrin with actin, and this activity is inhibited by calmodulin. This study describes the isolation and characterization of a brain peripheral membrane protein closely related to erythrocyte adducin. Brain and erythrocyte adducin have at least 50% antigenic sites in common, each contains a protease-resistant core of Mr = 48,000-48,500, and both proteins are comprised of two partially homologous polypeptides of Mr = 103,000 and 97,000 (erythrocytes) and Mr = 104,000 and 107,000-110,000 (brain). Brain and erythrocyte adducin associate preferentially with spectrin-actin complexes as compared to spectrin or actin alone, and both proteins also promote binding of spectrin to actin. Brain adducin binds calmodulin in a calcium-dependent manner, although the Kd of 1.3 microM is weaker by 5-6-fold than the Kd of erythrocyte adducin for calmodulin. Brain adducin is a substrate for protein kinase C in vitro and can accept up to 2 mol of phosphate/mol of protein. Adducin provides a potential mechanism in cells for mediating site-directed assembly of additional spectrin molecules and possibly other proteins at the spectrin-actin junction. Brain tissue contains 12 pmol of adducin/mg of membrane protein, which is the most of any tissue examined other than erythrocytes, which have 50 pmol/mg. The presence of high amounts of adducin in brain suggests some role for this protein in specialized activities of nerve cells.