Phosphorylation of Acanthamoeba actophorin (ADF/cofilin) blocks interaction with actin without a change in atomic structure

LIM-kinase activated by GST-Pak1 phosphorylates Acanthamoeba actophorin sto ichiometrically and specifically on serine 1. The atomic structure of phosp horylated actophorin determined by X-ray crystallography is essentially ide ntical with the structure of unphosphorylated actophorin. We compared bioch emical properties of phosphorylated actophorin, unphosphorylated actophorin and mutants of actophorin with serine 1 replaced by aspartic acid or alani ne. Phosphorylation strongly inhibits interaction of actophorin with Mg-ADP - or Mg-ATP-actin monomers and Mg-ADP-actin filaments, so Ser1 phosphorylat ion directly blocks interaction of actin-depolymerizing factor (ADF)/cofili n proteins with actin. About 30% of actophorin is phosphorylated in live am oebas grown in suspension culture. Phosphorylation of ADF/cofilin proteins by LIM-kinase or other enzymes will tend to stabilize actin filaments by in hibiting the ability of these proteins to sever and depolymerize older acti n filaments that have hydrolyzed their bound ATP and dissociated the phosph ate. (C) 2000 Academic Press.