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J. Cell Biol.,
Volume 140, Number 1, January 12, 1998 119-129
Department of Molecular Biology and Biochemistry, Rutgers University, Nelson Labs, Busch Campus, Piscataway, New
Jersey 08855
Phosphorylation of the regulatory light chain
of myosin II (RMLC) at Serine 19 by a specific enzyme,
MLC kinase, is believed to control the contractility of
actomyosin in smooth muscle and vertebrate nonmuscle cells. To examine how such phosphorylation is regulated in space and time within cells during coordinated cell movements, including cell locomotion and cell division, we generated a phosphorylation-specific antibody.
Motile fibroblasts with a polarized cell shape exhibit
a bimodal distribution of phosphorylated myosin along
the direction of cell movement. The level of myosin
phosphorylation is high in an anterior region near
membrane ruffles, as well as in a posterior region containing the nucleus, suggesting that the contractility of
both ends is involved in cell locomotion. Phosphorylated myosin is also concentrated in cortical microfilament bundles, indicating that cortical filaments are under tension. The enrichment of phosphorylated myosin
in the moving edge is shared with an epithelial cell
sheet; peripheral microfilament bundles at the leading
edge contain a higher level of phosphorylated myosin.
On the other hand, the phosphorylation level of circumferential microfilament bundles in cell-cell contacts is low. These observations suggest that peripheral
microfilaments at the edge are involved in force production to drive the cell margin forward while microfilaments in cell-cell contacts play a structural role. During cell division, both fibroblastic and epithelial cells
exhibit an increased level of myosin phosphorylation upon cytokinesis, which is consistent with our previous
biochemical study (Yamakita, Y., S. Yamashiro, and F. Matsumura. 1994. J. Cell Biol. 124:129-137). In the case
of the NRK epithelial cells, phosphorylated myosin first
appears in the midzones of the separating chromosomes during late anaphase, but apparently before the
formation of cleavage furrows, suggesting that phosphorylation of RMLC is an initial signal for cytokinesis.
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