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Regulation of the filament structure and assembly of Acanthamoeba myosin II by phosphorylation of serines in the heavy-chain nonhelical tailpiece

Significance Class II myosins are the only members of this superfamily of actin-associated molecular motors that form antiparallel bipolar filaments, which are essential for the biological functions of these myosins. Here we show that the assembly of Acanthamoeba myosin II monomers into minifilaments is modified by phosphorylation of one or more of four serine residues in the 27-residue nonhelical tailpiece at the end of each of the two heavy chains that form the coiled-coil helix characteristic of class II myosins. This regulatory mechanism might be applicable to other class II myosins that have a nonhelical tailpiece. Acanthamoeba myosin II (AMII) has two heavy chains ending in a 27-residue nonhelical tailpiece and two pairs of light chains. In a companion article, we show that five, and only five, serine residues can be phosphorylated both in vitro and in vivo: Ser639 in surface loop 2 of the motor domain and serines 1489, 1494, 1499, and 1504 in the nonhelical tailpiece of the heavy chains. In that paper, we show that phosphorylation of Ser639 down-regulates the actin-activated MgATPase activity of AMII and that phosphorylation of the serines in the nonhelical tailpiece has no effect on enzymatic activity. Here we show that bipolar tetrameric, hexameric, and octameric minifilaments of AMII with the nonhelical tailpiece serines either phosphorylated or mutated to glutamate have longer bare zones and more tightly clustered heads than minifilaments of unphosphorylated AMII, irrespective of the phosphorylation state of Ser639. Although antiparallel dimers of phosphorylated and unphosphorylated myosins are indistinguishable, phosphorylation inhibits dimerization and filament assembly. Therefore, the different structures of tetramers, hexamers, and octamers of phosphorylated and unphosphorylated AMII must be caused by differences in the longitudinal stagger of phosphorylated and unphosphorylated bipolar dimers and tetramers. Thus, although the actin-activated MgATPase activity of AMII is regulated by phosphorylation of Ser639 in loop 2 of the motor domain, the structure of AMII minifilaments is regulated by phosphorylation of one or more of four serines in the nonhelical tailpiece of the heavy chain.