Non-muscle myosin II (NMII) is a motor protein present in nearly all animal cells that converts chemical energy into mechanical force. Unlike its relative in muscle cells, NMII’s primary function is to interact with the actin cytoskeleton to generate force and tension. This activity supports a wide array of cellular processes.
Structure and Assembly of NMII
Non-muscle myosin II is a hexameric protein, constructed from six protein chains. This complex consists of two identical heavy chains (approx. 200 kDa), two essential light chains (17 kDa), and two regulatory light chains (20 kDa). The N-terminal portion of each heavy chain forms a globular head, which contains the motor domain for binding to actin and hydrolyzing ATP. Following the head is the neck region, which acts as a lever arm and is where the two types of light chains attach.
The remainder of the heavy chain forms a long, rod-like tail. The tails of the two heavy chains intertwine to form a coiled-coil structure. This dimerization creates a two-headed molecule with motor domains at one end and a unified tail at the other.
Individual NMII hexamers self-assemble into larger structures called bipolar filaments. This assembly occurs through tail-to-tail interactions, resulting in a filament with motor heads pointing in opposite directions at each end. These filaments are around 300 nanometers long and contain 15 to 30 NMII molecules. This structure allows them to pull on oppositely oriented actin filaments to generate contractile force.
The Contractile Mechanism
Non-muscle myosin II generates force through a cyclical process known as the cross-bridge cycle, which is powered by adenosine triphosphate (ATP). The cycle begins with the NMII motor head strongly attached to an actin filament in a temporary state of rigor.
When a molecule of ATP binds to the motor head, a conformational change causes it to detach from the actin filament. The NMII head then hydrolyzes the ATP into adenosine diphosphate (ADP) and an inorganic phosphate (Pi). This released energy is used to “cock” the myosin head, moving it into a high-energy, pre-power stroke position.
Containing ADP and Pi, the repositioned head rebinds to the actin filament at a new location. The release of the inorganic phosphate triggers the force-producing “power stroke,” causing the head to pivot and pull the actin filament along with it by about 5 to 10 nanometers. This movement is the basis of the contraction generated by NMII.
Following the power stroke, the ADP molecule is released from the myosin head. This returns the head to its original state, tightly bound to the actin filament and ready for a new molecule of ATP to restart the cycle. The coordinated action of many myosin heads generates the contractile forces within the cell.
Cellular Roles of NMII
NMII’s force generation is used for many cellular tasks, including cytokinesis, the final stage of cell division. During this process, NMII and actin filaments form a contractile ring at the cell’s equator. The constriction of this ring, driven by NMII’s motor activity, pinches the parent cell into two daughter cells.
Cell migration also depends on NMII activity. As a cell moves, it forms adhesions to the underlying surface. NMII generates tension within actin bundles called stress fibers, which connect to these adhesions and help pull the rear of the cell forward in a process known as tail retraction.
NMII is also involved in maintaining cell shape and the integrity of cell-cell and cell-extracellular matrix adhesions. The tension generated by the actomyosin network helps establish the overall form of the cell. At cell junctions, such as adherens junctions, NMII-driven contraction strengthens the connection between neighboring cells, which contributes to the structural integrity of tissues.
These contractile forces are used during embryonic development in a process called tissue morphogenesis. Coordinated contractions in sheets of cells can cause the tissue to fold and bend, forming structures like tubes and vesicles. One such mechanism, apical constriction, involves contraction at the apical surface of epithelial cells, driving the formation of various organs.
Regulation of NMII Activity
The cell controls where and when NMII generates force, primarily through the phosphorylation of its regulatory light chains (RLC). This event is carried out by enzymes called kinases, such as Myosin Light Chain Kinase (MLCK) and Rho-associated kinase (ROCK). The addition of a phosphate group to a specific site (Serine 19) on the RLC unfolds the NMII molecule from its inactive state, allowing it to assemble into bipolar filaments and interact with actin. Conversely, enzymes called phosphatases remove the phosphate group, deactivating NMII and causing the filaments to disassemble.
Specialization of NMII function is provided by three different isoforms: NMIIA, NMIIB, and NMIIC, each encoded by a distinct gene. While structurally similar, these isoforms have different enzymatic properties. They are often found in different locations within the cell or are active during different processes.
NMIIA is a faster motor associated with dynamic structures, such as those at the leading edge of a migrating cell. In contrast, NMIIB is a slower motor that can bear more load and is found in more stable, tension-holding structures like stress fibers. NMIIC is less abundant and its roles are still being fully characterized.
Relevance to Human Disease
Disruptions in NMII function are linked to a range of human diseases. Genetic mutations in the MYH9 gene, which codes for the NMIIA isoform’s heavy chain, cause a group of rare conditions known as MYH9-related disorders. These disorders are characterized by symptoms that can include abnormally large platelets (macrothrombocytopenia), hearing loss, kidney problems, and cataracts.
The role of NMII in cancer is complex. By maintaining tissue integrity and strengthening cell-cell adhesions, NMIIA can act as a tumor suppressor, preventing cells from breaking away from a primary tumor. Loss of NMIIA function in some cancers is linked to a poorer prognosis.
However, cancer cells can exploit NMII’s functions. The same mechanisms that drive normal cell migration can be hijacked by cancer cells to increase their motility and invade surrounding tissues. Elevated levels of NMIIA are observed in several cancers and are often associated with increased metastasis, the process of cancer spreading to other parts of the body.