A change in actin conformation associated with filament instability after P-i release

The ability of actin to both polymerize into filaments and to depolymerize permits the rapid rearrangements of actin structures that are essential for actin's function in most cellular processes. Filament polarity and dynamic properties are conferred by the hydrolysis of ATP on actin filaments. Rele ase of inorganic phosphate (P-i) from filaments after ATP hydrolysis promot es depolymerization. We identify a yeast actin mutation, Val-159 to Asn, wh ich uncouples P-i release from the conformational change that results in fi lament destabilization. Three-dimensional reconstructions of electron micro graphs reveal a conformational difference between ADP-P-i filaments and ADP filaments and show that ADP V159N filaments resemble ADP-P-i wild-type fil aments. Crystal structures of mammalian beta-actin in which the nucleotide binding cleft is in the "open" and "closed" states can be used to model act in filaments in the ADP and ADP-P-i conformations, respectively. We propose that these two conformations of G-actin may be related to two functional s tates of F-actin.