PROTRUSION OF CELL-SURFACE COUPLED WITH SINGLE EXOCYTOTIC EVENTS OF SECRETION OF THE SLIME IN PHYSARUM PLASMODIA

Exocytosis has been proposed to participate in the formation of pseudo pods, Using video-enhanced microscopy, we directly visualized exocytos is of single vesicles in living Physarum plasmodia migrating on a subs trate, Vesicles containing slime, the plasmodial extracellular matrix, of similar to 3.5 mu m in diameter, shrank at the cell periphery at t he average rate of similar to 1 mu m/second, and became invisible, Imm ediately after exocytotic events, the neighboring cell surface extende d to form a protrusion, The rate of extension was similar to 1 mu m/se cond. The protrusion showed lamella-like morphology, and contained act in microfilaments, Electron microscopy suggested that the organization of microfilaments in such protrusions may be a random meshwork rather than straight bundles, These morphologies suggest that protruded regi ons are pseudopods. importantly, only the slime-containing vesicle pre ferentially invaded the hyaline layer that consists of dense actin mic rofilaments while the other vesicular organelles remained in the granu loplasm. Quantitative analysis demonstrated a linear relationship in t erms of their surface area, between individual protrusions and single slime-containing vesicles, It is, therefore, likely that most of the p lasma membrane of the protrusion was supplied by fusion of the slime-c ontaining vesicle during exocytosis.