ACTIN FILAMENT CROSS-LINKING BY CHICKEN GIZZARD FILAMIN IS REGULATED BY PHOSPHORYLATION IN-VITRO

Filamin is a dimeric muscle phosphoprotein that cross-links actin filaments. We have found that purified chicken gizzard filamin is phosphorylated in vitro at serine residues by the Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Up to 0.9 mol of phosphate can be incorporated into 1 mol of filamin dimer. Phosphorylation by CaM kinase II increases filamin's critical actin filament gelling concentration and diminishes the amount of actin sedimented by filamin at low G-force. The modulation of filamin function by CaM kinase II requires ATP, Ca2+, and calmodulin, and it is abolished when CaM kinase II is inactivated with heat, Protein phosphatase 2A removed the phosphate added by CaM kinase II and restored filamin's actin filament cross-linking activity to the untreated basal level. In cosedimentation experiments, phosphorylation reduces the binding of filamin to actin filaments. The K-d for binding of filamin to actin filaments increases similar to 2-fold, from 3.2 to 6.9 mu M, following CaM kinase II-mediated phosphorylation. Phosphorylation by CaM kinase II, therefore, regulates the binding of filamin to actin filaments.