Myosin-9 also known as myosin, heavy chain 9, non-muscle or non-muscle myosin heavy chain IIa (NMMHC-IIA) is a protein which in humans is encoded by the MYH9 gene.

Non-muscle myosin IIA (NM IIA) is expressed in most cells and tissues where it participates in a variety of processes requiring contractile force, such as cytokinesis, cell migration, polarization and adhesion, maintenance of cell shape, and signal transduction. Myosin IIs are motor proteins that are part of a superfamily composed of more than 30 classes. Class II myosins include muscle and non-muscle myosins that are organized as hexameric molecules consisting of two heavy chains (230 kDa), two regulatory light chains (20 KDa) controlling the myosin activity, and two essential light chains (17 kDa), which stabilize the heavy chain structure.

MYH9 is a large gene spanning more than 106 kilo base pairs on chromosome 22q12.3. It is composed of 41 exons with the first ATG of the open reading frame localized in exon 2 and the stop codon in exon 41. It encodes non-muscle myosin heavy chain IIA (NMHC IIA), a protein of 1,960 amino acids. Consistent with its wide expression in cells and tissues, the promoter region of MYH9 is typical of housekeeping genes having no TATA box but high GC content, with multiple GC boxes. MYH9 is a well-conserved gene through evolution. The mouse ortholog (Myh9) is localized in a syntenic region on chromosome 15 and has the same genomic organization as that of the human gene. It encodes a protein of the same length, with 97.1% amino acid identity with the human MYH9 protein.

There are significant differences in the relative abundance of the three NM II paralogs in various cells and tissues. However, NM IIA appears to be the predominant paralog in both tissues and cells in humans and mice. Mass spectroscopy analysis of the relative abundance of NMHC IIs in mouse tissues and human cell lines shows that NM IIA is predominant, although tissues like the heart vary from cell to cell; myocardial cells contain only NM IIB but NM IIA is more abundant in the non-myocyte cells. NM IIB is predominant in most parts of the brain and spinal cord but NM IIA is relatively more abundant in most other organs and cells lines. Both NM IIA and IIB are expressed early in development with NM IIC expression starting at E 11.5 in mice. Not only do most cells contain more than one paralog but there is evidence that the paralogs can co-assemble intracellularly into heterotypic filaments in a variety of settings in cultured cells.

Like all class II myosins, the two NMHC IIAs dimerize producing an asymmetric molecular structure recognizable by two heads and a tail domain: the N-terminal half of each heavy chain generates the head domain, which consists of the globular motor domain and the neck domain, and the C-terminal halves of the two heavy chains together form the tail domain. The motor domain, which is organized into four subdomains (SH3-like motif, the upper and the lower 50kDa subdomains, and the converter region) connected by flexible linkers, interacts with filamentous actin to generate force through magnesium-dependent hydrolysis of ATP. The neck acts as a lever arm that amplifies the movement produced by conformational changes of the motor domain, and is the binding site for the light chains through two IQ motifs. The tail domain is fundamental for both dimerization of the heavy chains and formation of NM IIA functional filaments. Two heavy chains dimerize through the tail domain forming a long alpha-helical coiled-coil rod composed of typical heptad repeats. Dimers self-associate though the coiled-coil rods to form myosin filaments. The tail domain ends at the C-terminus with a 34-residue non-helical tailpiece.

NM IIA plays a major role in early vertebrate development. Ablation of NM IIA in mice results in lethality by embryonic day (E) 6.5 due to abnormalities in the visceral endoderm which is disorganized due to a loss of E-cadherin mediated cell-cell adhesions. Lacking a normal polarized columnar layer of endoderm, the abnormal visceral endoderm of NM IIA knockout embryos fails to support the critical step of gastrulation. However, the development of a normal functioning visceral endoderm does not specifically depend on NM IIA since its function can be restored by genetically replacing the NMHC IIA with cDNA encoding NMHC II B (or NMHC IIC) that is under control of the NMHC IIA promoter. These "replacement" mice have a normal visceral endoderm and continue to proceed through gastrulation and undergo organogenesis. However, they die when they fail to develop a normal placenta. Absence of NM IIA results in a compact and underdeveloped labyrinthine layer in the placenta which lacks fetal blood vessel invasion. Moreover, mutant p.R702C NM IIA mice show similar defects in placental formation and mice specifically ablated for NM IIA in the mouse trophoblast-lineage cells demonstrate placental defects similar to mice in which NMHC IIA is genetically replaced by NMHC IIB.

There are three paralogs of non-muscle myosin II (NM II), NM IIA, IIB, and IIC, with each having the heavy chain encoded on a different chromosome. All three paralogs appear to bind the same or very similar light chains and share basic properties as to structure and activation, but all three play distinct roles during vertebrate development and adulthood (for general reviews on NM IIs, see ). All NM IIs have two important features: they are MgATPase enzymes that can hydrolyze ATP thereby converting chemical energy into mechanical movement. In addition, they can form bipolar filaments which can interact with and exert tension on actin filaments. These properties provide the basis for all NM II functions. The path to myosin filament formation, which is shared by NM II and smooth muscle myosin, starts with a folded inactive conformation of the NM II monomer which, upon phosphorylation of the 20 KDa light chain unfolds the molecule to produce a globular head region followed by an extended alpha-helical coiled-coil tail. The tail portion of the molecule can interact with other NM IIA hexamers to form bipolar filaments composed of 14-16 molecules.

Phosphorylation of the 20 KDa light chains on Serine 19 and Threonine 18 by a number of different kinases, but most prominently by Rho-dependent kinase and/or by the calcium-calmodulin-dependent myosin light chain kinase, not only linearizes the folded structure but removes the inhibition imposed on the MgATPase activity due to the folded conformation. In addition to phosphorylation of the 20 KDa light chains, the NMHC IIs can also be phosphorylated, but the sites phosphorylated differ among the paralogs. In most cases phosphorylation of NMHC IIA can act to either dissociate the myosin filaments or to prevent filament formation.