J Biol Chem. 2008 Oct 21. [Epub ahead of print]
Role of intermonomer ionic bridges in the stabilization of the actin filament
Stokasimov E, McKane M, Rubenstein PA.
Biochemistry, University of Iowa Carver College of Medicine, Iowa City, IA 52242.
Filament formation is required for most of actin's functions. However, the intermonomer interactions that stabilize F-actin have not been elucidated because of a lack of an F-actin crystal structure. The Holmes muscle actin model suggests that an ionic interaction between R39 of one monomer and E167 of an adjacent monomer in the same strand contributes to this stabilization. Yeast actin has an A167 instead. F-actin molecular dynamics modeling predicts another interaction between R39 of one monomer and D275 of an opposing strand monomer. In Toxoplasma gondii actin, which forms short stubby filaments, the D275 equivalent is replaced by R leading to a potential filament-destabilizing charge-charge repulsion. Using yeast actin, we tested the effect of A167E as a potential stabilizer and A167R and D275R as potential filament disruptors. All mutations caused abnormal growth and mitochondrial malfunction. A167E and D275R actins polymerize normally and form relatively normal appearing filaments. A167R nucleates filaments more slowly and forms filament bundles. The R39D/A167R double mutant, which reestablishes an ionic bond in the opposite orientation reverses this polymerization and bundling defect. Stoichiometric amounts of yeast cofilin have little effect on WT and A167E filaments. However, D275R and A167R actin depolymerization is profound with cofilin. While our results suggest that disruption of an interaction between R39 and D275 is not sufficient to cause fragmentation, it suggests that it changes filament stability thereby disposing it for enhanced cofilin depolymerizing effects. A167 results demonstrate the in vivo and in vitro importance of another potential R39 ionic interaction.
PMID: 18945676 [PubMed - as supplied by publisher]