Secretion of slime, the extracellular matrix of the plasmodium, as visualized with a fluorescent probe and its correlation with locomotion on the substratum.

Slime, the extracellular matrix of Physarum plasmodium, is secreted by the exocytosis of a vesicles that contain a slime precursor. Using an antibody raised against biochemically purified slime, we detected the intracellular localization of the slime vesicle. Slime vesicles are abundant in the advancing front of the plasmodium, as confirmed by electron microscopic observation in two different cross-sectional angles. Screening various reagents, we found that rhodamine-phosphatidylethanolamine (Rh-PE) binds specifically to slime in both its intravesicular and extracellular forms, as confirmed by immunoelectron microscopy using an antibody against fluorochrome rhodamine. The plasmodia vitally stained with Rh-PE exhibited dynamic fluorescent patterns during the course of locomotion. The fluorescence was conspicuous at the periphery of the leading pseudopods and oscillated according to the shuttle streaming that accompanied the relaxation and contraction of the periphery; it was intense in the relaxation phase when pseudopods extended, and became weak in the contraction phase when pseudopods contracted. The results collectively mean that the slime vesicles carried by the cytoplasmic streaming accumulated prior to secretion at the advancing margin of the plasmodium.