One example of a process in which the motor proteins are involved is in the formation and function of contractile bundles.
thick filaments and they are arranged within a scaffold of actin thin filaments (along with numerous other proteins) into higher order fibrous structures known as sarcomeres. Each sarcomere contains numerous repeating units of interlinked thick and thin filaments, and the opposite orientation of the myosin heads causes adjacent actin filaments to slide past each other during muscle contraction. Each sarcomere is ~2 µm long in resting muscle, but this length is shortened by as much as 70% after muscle contraction. Muscle contraction is regulated by calcium levels  and by the troponin regulatory system (see figure "Tropomyosin stabilizes thin filaments"). Although actin subunits continue to turn-over at both ends of the thin filament, this exchange is relatively slow, making the actin filaments in sarcomeres relatively more stable when compared to the actin filaments found in other cell types.
Video: Sliding filament theory of muscle contraction. [Video was uploaded to YouTube by avice01 and created by Sara Egner using electron micrography from P M Motta, P M Andrews, K R Porter and J Vial.]
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; see also "adherens junction"), 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 . 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 ; these initial studies showed the rate of ATP hydrolysis by myosin (and hence myosin movement) varies directly with the actin concentration . Further studies using isolated stress fibers from fibroblasts confirmed that stress fibers are contractile and shorten by as much as 25% . Myosin II bundle formation and contractile activity in nonmuscle cells is regulated by phosphorylation .
[2, 5]. (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 (for simplicity, the myosin filament is shown as a single bipolar molecule). (C) A high resolution view of the bipolar myosin filament heads interspersed between the regions high in α-actinin content. Relative to α-actinin, the more flexible actin crosslinking protein, filamin, is dispersed throughout the stress fiber.
- Luchi RJ. & Kritcher EM. Drug effects on cardiac myosin adenosine triphosphatase activity. J. Pharmacol. Exp. Ther. 1967; 158(3):540-5. [PMID: 4294509]
- Langanger G., Moeremans M., Daneels G., Sobieszek A., De Brabander M. & De Mey J. The molecular organization of myosin in stress fibers of cultured cells. J. Cell Biol. 1986; 102(1):200-9. [PMID: 3510218]
- Woolley DE. An actin-like protein from amoebae of dictyostelium discoideum. Arch. Biochem. Biophys. 1972; 150(2):519-30. [PMID: 4261413]
- Spudich JA. Biochemical and structural studies of actomyosin-like proteins from non-muscle cells. II. Purification, properties, and membrane association of actin from amoebae of Dictyostelium discoideum. J. Biol. Chem. 1974; 249(18):6013-20. [PMID: 4278010]
- Kreis TE. & Birchmeier W. Stress fiber sarcomeres of fibroblasts are contractile. Cell 1980; 22(2 Pt 2):555-61. [PMID: 6893813]
- Katoh K., Kano Y., Masuda M., Onishi H. & Fujiwara K. Isolation and contraction of the stress fiber. Mol. Biol. Cell 1998; 9(7):1919-38. [PMID: 9658180]