Distinct Biochemical Properties of Arabidopsis thaliana Actin Isoforms

Plants and animals express multiple actin isoforms in a manner that is dependent on tissues, organs and the stage of development. Previous genetic analyses suggested that individual actin isoforms have specific roles in cells, but there is little biochemical evidence to support this hypothesis. In this study, we purified four recombinant Arabidopsis actin isoforms, two major vegetative actin isoforms, ACT2 and ACT7, and two major reproductive isoforms, ACT1 and ACT11, and characterized them biochemically. Phalloidin bound normally to the filaments of the two reproductive actins as well as to the filaments of skeletal muscle actin. However, phalloidin bound only weakly to ACT7 filaments and hardly at all to ACT2 filaments, despite the conserved sequence of the phalloidin-binding site. Polymerization and phosphate release rates among these four actin isoforms were also significantly different. Moreover, interactions with profilin (PRF) were also different among the four Arabidopsis actin isoforms. PRF1 and PRF2 inhibited the polymerization of ACT1, ACT11 and ACT7, while ACT2 was only weakly affected. Plant actin isoforms have different biochemical properties. This result supports the idea that actin isoforms play specific roles to achieve multiple cell functions.