Contractile Bundles in Nonmuscle Cells[Edit]
In non-muscle cells, myosin II associates with actin filaments to form contractile structures known as stress fibers along the lower surfaces where the cell is anchored to its substrate. In epithelial cells, contractile bundles are also prominent in the adhesion belt (aka adherens belt), which helps to maintain the stability and integrity of epithelial cell sheets. The contractile bundles in nonmuscle cells are similar to skeletal muscle fibers, but they are smaller (~0.4 µm in fibroblasts), less organized, and they contain different accessory proteins [1].
Figure 1. Stress fiber structure: A. Isolated stress fibers have a banded appearance, with bundles of actin filaments interspersed with semiperiodic electron-dense regions. B. The electron-dense regions are rich in actin crosslinking proteins, namely α-actinin. Bipolar myosin II filaments lie between the loosely packed actin filaments in the regions that lack α-actinin . C. A high resolution view of the bipolar myosin filament heads interspersed between the regions rich in α-actinin. Relative to α-actinin, the more flexible actin crosslinking protein, filamin, is dispersed throughout the entire stress fiber.
Figure 1. Stress fiber structure: A. Isolated stress fibers have a banded appearance, with bundles of actin filaments interspersed with semiperiodic electron-dense regions. B. The electron-dense regions are rich in actin crosslinking proteins, namely α-actinin. Bipolar myosin II filaments lie between the loosely packed actin filaments in the regions that lack α-actinin . C. A high resolution view of the bipolar myosin filament heads interspersed between the regions rich in α-actinin. Relative to α-actinin, the more flexible actin crosslinking protein, filamin, is dispersed throughout the entire stress fiber.Historically speaking, the mechanism of actomyosin contraction for nonmuscle actin was examined using amoebae proteins Dictyostelium, Acanthamoeba) because the actin is very similar to muscle actin [2]; these initial studies showed the rate of ATP hydrolysis by myosin (and hence myosin movement) varies directly with the actin concentration [3]. Further studies using isolated stress fibers from fibroblasts confirmed that stress fibers are contractile and shorten by as much as 25% [4]. Myosin II bundle formation and contractile activity in nonmuscle cells is regulated by phosphorylation [5].