Genome Analysis and Human Health by Leena Rawal & Sher Ali

Author:Leena Rawal & Sher Ali
Language: eng
Format: epub
Publisher: Springer Singapore, Singapore

5.6 Subcellular Localization of Non-muscle Myosin II Motor Proteins

Subcellular localization of a protein provides clues about its cellular functions in a given tissue. Non-muscle myosin II motor proteins associate mainly with actin filaments to generate intracellular contractile forces that are required for mediating cellular functions. Therefore, the subcellular localization of these molecular machines is linked to actin cytoskeleton dynamics that depend upon the state of a given cell. For instance, in a nonmotile quiescent cell, non-muscle myosin II motor proteins localize mostly in the cytoplasm, and some remain associated with actin filaments. However, these motor proteins are also found in the nuclei of proliferating myoblasts (Rodgers 2005). Interestingly, these cytosolic myosin II motor proteins undergo transient localization to cleavage furrow during cytokinesis by unknown mechanisms. It is largely believed that the myosin II-generated contractile forces mediate the separation of two daughter cells during cytokinesis; however, their specific roles and underlying mechanisms are not clearly understood. Recent studies have shown that non-muscle myosin IIB is required for the completion of meiotic cytokinesis in male but not in female mice (Yang et al. 2012). Myosin IIB has been shown to prevent endomitosis or polyploidization during differentiation of megakaryocytes (Lordier et al. 2012). Myosin IIB also regulates enucleation process in erythrocytes that is akin to cytokinesis in other cells (Ubukawa et al. 2012).

Cell migration has been viewed as an index of cellular life because of its importance in maintaining growth, development, normal physiology, and immunity of a given organism. Cells during this complex process display frequent changes in their shapes and make attachment with matrix and neighboring cells. The cell cytoskeleton plays critical roles in driving this essential cellular process. The actomyosin system, an essential component of the cytoskeleton, is extensively studied in migrating cells. The actomyosin system generates contractile forces to mediate cell dynamics during migration. However, nature has limited this extraordinary ability of migration to animal and other eukaryotic cells but not to plant cells. Unlike animal and other eukaryotic cells, plant cells cannot change their shapes, interact with matrix, and extend lamellipodia probably due to their rigid cell wall and absence of class II myosins. Both mechanical and functional roles of myosin IIA and myosin IIB are extensively investigated in migrating cells for the past two decades. Many laboratories reported that myosin IIA and myosin IIB play specific roles in mediating cell dynamics during migration. Cells initiate migration by extending their membrane in the form of lamellipodia perhaps as a part of strategy to probe environment for favorable conditions and proper directions. During migration, cells display periodic extension and retraction of their lamellipodia by unknown mechanism. Interestingly, the cytosolic myosin IIA and myosin IIB translocate to the lamellipodia during cell migration. Both myosin IIA and myosin IIB display distinct localization and drive lamellipodia extension in opposite direction. On one hand, myosin IIB promotes lamellipodia and growth cone extension, and on the other, myosin IIA drives retraction of the lamellipodia during cell migration (Betapudi 2010; Brown and Bridgman 2003; Rochlin et al.

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