Actin Monomer-Binding Proteins and the Regulation of Actin Dynamics in Plants

Abstract

Plants are uniquely adapted to respond to environmental and developmental signals for survival. Many signals result in dramatic changes of cell shape or cytoplasmic organization that are dependent on the actin cytoskeleton. The dynamic nature of the actin cytoskeleton is conferred by a wide variety of actin-binding proteins. One class of these proteins is capable of binding to free actin monomers and thereby regulates the polymerization of actin filaments. Two such proteins have been identified in plants: profilin and actin-depolymerizing factor (ADF). These proteins comprise multigene families in plants and the isoforms of each protein have unique developmentally and spatially regulated expression patterns. Biochemical analysis of the plant monomer-binding proteins indicates that they are able to both stimulate and inhibit actin polymerization in vitro. Furthermore, microinjection of these proteins into cells reveals that simple models for the interaction of monomer-binding proteins with actin are inadequate. The complex effects on actin in vitro and in vivo are due to the ability of profilin and ADF to interact with a number of other ligands, such as regulatory proteins and polyphosphoinositide lipids. The monomer-binding proteins also respond to changes in cytosolic Ca 2+ and pH. Regulation of these proteins by phosphorylation adds an additional level of complexity for the study of their role in coordinating actin reorganization in plant cells. A model of actin filament assembly in tip-growing cells that incorporates the activities of profilin and ADF is presented.