Phosphorylated 1-caldesmon is involved in disassembly of actin stress fibers and postmitotic spreading

The function of the ubiquitous actin-binding protein, caldesmon (I-CaD) in mammalian non-muscle cells remains elusive. During mitosis, I-CaD becomes markedly phosphorylated at Sei-497 and Ser527 (in the rat sequence), therefore, it has been Suggested that I-CaD is involved in cytokinesis by inhibiting the actomyosin interaction until it is phosphorylated, although direct in vivo evidence is still missing. In the present study, We used F-actin staining and specific antibodies against these two phosphorylation sites of I-CaD to simultaneously monitor actin assembly and I-CaD phosphorylation. Our observations demonstrated that the level of I-CaD phosphorylation undergoes dynamic changes during the cell cycle. The spatial and temporal distributions of phospho-CaD do not correlate with cytokinesis per se, but rather, with the level of actin bundles in a reciprocal manner. The highest I-CaD phosphorylation level coincides with the disassembly of actin cytoskeleton during mitotic cell rounding. Ser-to-Ala mutations at these two positions prevent stress fibers from disassembly upon migratory stimulation. In addition, phospho-CaD appears to colocalize with nascent focal adhesion complexes during postmitotic spreading. These findings suggest that I-CaD phosphorylation plays an important role not only in cytoskeleton remodeling during cell shape changes, but also in cell spreading and migration. (c) 2005 Elsevier Inc. All rights reserved.