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Muscle contraction and different forms of motility of nonmuscle cells depend on cyclic interactions between actin filaments and myosin motors. Calcium ions are the main intracellular signal, which induces activation of actin-myosin interactions. Depending on the cell type and the class of myosin, the molecular mechanisms of regulation are different and take place on two levels - actin filament and myosin. In striated muscle, the actin thin filament is regulated by the troponin-tropomyosin complex. In smooth muscle and nonmuscle cells, actin filaments are predominantly regulated by caldesmon. The control of myosin activity in these cells also depends on the myosin light chain phosphorylation and the phosphorylation of the heavy chain. Direct binding of calcium ions to the myosin light chains (which could be calmodulin molecules) was observed in myosin from molluscan muscle and in some unconventional myosins. Intensive world-wide studies allow us to understand details of the mechanisms of actin-myosin interactions. In this article, we present the contemporary view on these mechanisms.

Chloroplast redistribution in the cell depends on direction, fluence-rate and spectral composition of the incident light. Two photoreceptors, phototropin 1 and 2, control the chloroplast responses in higher land plants. Actin and myosin form the motor system. Although numerous results point to calcium and phosphoinositides as the secondary messengers, the signal transduction pathways remain unidentified. Two possible roles of the acto-myosin system in the mechanism of chloroplast redistribution have been discussed. Reorganization of the actin cytoskeleton has been observed in water plants where the chloroplast movements depend on red light. This reorganization appears to be associated with cytoplasmic streaming. In higher land plants, e.g. in Arabidopsis thaliana, the chloroplast responses depend only on blue light. Neither specific light-induced reorganization of actin nor cytoplasmic streaming have been observed in the mesophyll of these species. A blue light-specific relocalization of myosins accompanies the chloroplast responses in Arabidopsis. Thus myosins might be potential targets of light signaling in higher land plants.

Myosins, actin-dependent molecular motors are expressed in almost all eukaryotic cells where are engaged in a panoply of cellular processes such as muscle contraction, cell migration and adhesion, intracellular trafficking, cytokinesis, endocytosis and secretion. In recent years a number of reports has been published revealing nuclear localization of myosins as well as actin and actin-binding proteins. Namely, nuclear form of myosin IC (NMI), myosin VI, myosin XVIB and myosin XVIIIB were found in nucleus. NMI and myosin VI seem to be involved in gene transcription and possibly intranuclear transport. In this paper, current knowledge on the role of myosin motors in nucleus has been presented.

Endocytosis is the way of uptaking and transporting substances from the immediate surroundings, as well as the way of removal and/or recycling of the plasma membrane components. For many years, it was thought that this process was not operating in plant cells in which high turgor pressure of the protoplast prevented invagination of the plasma membrane. Recent years, however, brought the evidence for intensive endocytosis in plants. So far, four types of endocytosis, among 5 known, have been identified in plants. Use of protein markers of different compartments of the endomembrane system, specific dyes, and the cytoskeleton inhibitors, made possible demonstration that, as opposed to animal and fungal systems, endocytosis in plant cells was dependent on actin and plant-specific myosins of VIII and XI classes. Although cell divisions in plants and animals differ considerably, they both are dependent on the proper endocytosis. It has been shown that during cytokinesis secretion and endocytosis act in concert, delivering polysaccharides directionally to the growing cell plate. What is more important, new cell walls are built from e.g. pectins transported by endocytosis from existing cell walls.

Actin interactions with myosin as well as dynamic polymerization and depolymerization of actin filaments underlie various forms of cellular motility. Both mechanisms are under control of regulatory proteins, among which tropomyosins take a central position. Tropomyosin isoforms can be tissue-specific or constitutive, expressed in different cell types. Tropomyosin expression pattern depends on the cell type and developmental stage. Several tropomyosin isoforms can simultanously co-exist in a single cell, and they are subjected to sorting to different cellular compartments. Due to specific localization, tropomyosins determine interactions of the filaments with actin binding proteins, and ascertain various filaments functions.