Structure and physiological investigation of arginylated actin

Actin is an evolutionarily conserved cytoskeletal protein with crucial roles in cell polarity, division, migration, and muscle contraction. Actin function is regulated in part by posttranslational modifications. One such modification in non-muscle cells is arginylation, in which an arginine residue is added to the N-terminus of {beta}-actin. What is the structure of arginylated {beta}-actin (R-{beta}-actin), are its interactions with other proteins altered and what phenotypes result when R-{beta}-actin is the sole actin isoform present in the cell? Here we report the 4.2 [A] structure of ADP-bound human R-{beta}-actin filaments, the overall structure of which is nearly identical to the filaments made of non-arginylated actin. In vitro functional assays using isoform-pure actins with defined post-translational modifications reveal that the interaction between myosin-II and actin is altered upon actin arginylation, due to frequent detachment of myosin-II from R-actin filaments. In vivo, we find that replacement of the only actin gene in Schizosaccharomyces pombe with a synthetic gene encoding R-Sp-actin reduces Arp2/3-based actin patches while thickening the formin-induced actin. Furthermore, consistent with altered interactions between myosin-II and R-actin filaments, the assembly and constriction of cytokinetic actomyosin ring are perturbed in the R-Sp-actin cells. Thus, despite the overall structural similarity of arginylated and non-arginylated actin filaments, actin arginylation affects actin filament assortment into distinct subcellular structures and its interaction with myosin II.