Role of the nonhelical tailpiece of myosin-II in regulating filament architecture and function.

Mammalian nonmuscle myosin-II isoforms (NM-IIA, NM-IIB, and NM-IIC) each contain a nonhelical tailpiece (NHT) at their C terminus. Stop-codon mutations in the NHT of NM-IIA are linked to diseases such as macrothrombocytopenia. However, the role of the NHT in NM-II filament assembly and function remains poorly understood. Here, we show that NM-II isoforms lacking the NHT, including disease-associated NM-IIA truncations, form enlarged bipolar filaments with reduced bare zones. NHT length emerges as a key determinant of filament size. Moreover, NM-IIA NHT truncations generate stress fibers composed of enlarged bipolar filaments that exhibit reduced FRAP recovery and an increased tendency to aggregate, resulting in impaired cell migration. We further provide in vivo evidence that NM-IIA assembles into bipolar filaments in the absence of RLC phosphorylation, a property enhanced by NHT deletion. Together, these findings establish the NHT as a critical regulator of NM-II filament architecture, dynamics, and function and provide mechanistic insight into NM-IIA NHT-associated diseases.