MYOSIN IN ONION (ALLIUM-CEPA) BULB SCALE EPIDERMAL-CELLS - INVOLVEMENT IN DYNAMICS OF ORGANELLES AND ENDOPLASMIC-RETICULUM

Studied with the fluorochrome 3,3-dihexyloxacarbocyanine iodide [(DIOC 6(3)], the dynamic system of the endoplasmic reticulum (ER) in epiderm al cells of onion bulb scales consists of long, tubular strands moving together with organelles in the deeper cytoplasm, and of a less mobil e network composed of tubular and lamellar elements at the cell periph ery. Treatment with the sulfhydryl-reagent N-ethylmaleimide (NEM) inhi bited organelle and ER movement, and caused the fusion of ER-tubules i nto flat sheets. Fixed, long, tubular ER strands were formed by loweri ng the cytosolic pH of NEM-treated cells. Both these observations indi cate the involvement of myosin in the dynamics of organelles and ER. U sing a monoclonal antibody against murine skeletal muscle myosin (know n to cross-react with plant myosin; Tang et al. 1989, J. Cell Sci. 92: 569-574), myosin was identified by immunofluorescence microscopy. Map ping the distribution of myosin, actin filaments, ER, and organelles i n different phases of recovery after centrifugation of epidermal cells , co-localization of myosin with ER and organelles but not with actin filaments was observed, supporting the hypothesis that a membrane boun d motor protein exists in onion epidermal cells, which translocates or ganelles and the endoplasmic reticulum along actin filaments.