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 (DIOC₆(3)], the dynamic system of the endoplasmic reticulum (ER) in epidermal cells of onion bulb scales consists of long, tubular strands moving together with organelles in the deeper cytoplasm, and of a less mobile network composed of tubular and lamellar elements at the cell periphery. Treatment with the sulfhydryl-reagent N-ethylmaleimide (NEM) inhibited organelle and ER movement, and caused the fusion of ER-tubules into flat sheets. Fixed, long, tubular ER strands were formed by lowering the cytosolic pH of NEM-treated cells. Both these observations indicate the involvement of myosin in the dynamics of organelles and ER. Using a monoclonal antibody against murine skeletal muscle myosin (known to cross-react with plant myosin; Tang et al. 1989, J. Cell Sci. 92: 569–574), myosin was identified by immunofluorescence microscopy. Mapping the distribution of myosin, actin filaments, ER, and organelles in 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 bound motor protein exists in onion epidermal cells, which translocates organelles and the endoplasmic reticulum along actin filaments.