MYOSIN REORGANIZATION IN ACTIVATED RBL CELLS CORRELATES TEMPORALLY WITH STIMULATED SECRETION

Rat basophilic leukemia cells secrete histamine and serotonin in respo nse to cross-linking of the IgE receptor by multivalent antigen [Metzg er et al., 1986: Ann. Rev. Immunol. 4:419-470]. Receptor crosslinking also induces phosphorylation of the light and heavy chains of myosin I I with kinetics similar to that of secretion [Ludowyke et al., 1989: J . Biol. Chem. 264:12492-12501]. Here we show that myosin II localizati on changes after activation with similar kinetics. Furthermore, these changes are coincident with changes in cell shape and increase in moti le activity induced by activation. Within 2 min, activated cells begin to flatten, spread on their substratum, and extend lamellipodia which show active ruffling. Quantitation of the extent of eel spreading fro m video micrographs shows that 48% of the cells increase significantly in surface area by 5 min and 71% by 15 min. Myosin II is uniformly di stributed in unactivated cells but is deficient in newly formed lamell ipodia that start to appear at 2 min after activation. In contrast the se Iamellipodia show strong staining for actin. Further changes in myo sin organization are detected by 15 min after activation when myosin r eappears in the cell periphery, is concentrated in the perinuclear are a, and is also organized in punctate linear arrays that extend from th e nucleus to the cell periphery. The kinetics of the early cell shape changes and formation of the myosin-deficient lamellipodia correlate w ell with, and may relate to, the increase in the level of myosin II ph osphorylation reported by Ludowyke et al. [1989: J. Biol. Chem. 264: 1 2492-12501]. Changes in the distribution of cell surface-bound IgE als o occur upon antigen activation, and they correlate with the myosin di stribution in a manner that suggests that they may be driven by myosin II. (C) 1994 Wiley-Liss, Inc.