MYH9 mutations differentially stabilize non-muscle myosin II filaments and induce distinct cellular aggregation phenotypes.

Mutations in the MYH9 gene, which encodes the heavy chain of the actin-based molecular motor non-muscle myosin II-A (NM2-A), cause a spectrum of rare blood disorders collectively termed MYH9-related diseases (MYH9-RD). Previous data indicate that mutations in the motor domain result in more severe phenotypes than those in the dimerization/filamentation domain. Here, we show that N93K mutation, previously described as motor-impairing, has only a minor effect on myosin motor function in vitro, but it significantly enhances NM2-A filament stability and interaction with the chaperone UNC45a. When expressed in stress fiber-competent cells, e.g., COS7 or U2OS, and stress fiber-incompetent cells, e.g., MEG-01, NM2-A N93K formed amorphous aggregates that colocalized with wild-type NM2-A, NM2-B and UNC45a, but not F-actin, tropomyosin-4 or phosphoSer19, active, RLC. Another motor mutant, NM2-A R705H, caused milder defects in filament stability, adhesion maturation and aggregation in both types of cells. Conversely, the tail domain mutant NM2-A E1841K enhanced filament stability in COS7 cells without forming aggregates, while it promoted the formation of small, elongated aggregates in MEG-01 cells that did not co-localize with wild type NM2-A. These data indicate that the molecular defect caused by NM2A-N93K affects the distribution and function of the wild type allele, whereas NM2-A E1841K aggregates do not affect the wild type allele, which could constitute the molecular basis of the differences in severity in mutant MYH9 allele carriers.