Minipodia, the adhesive structures active in locomotion and endocytosis ofamoebae

Amoeba proteus cells, strains A and C-t, with well pronounced motor polarit y (the polytactic and orthotactic forms) develop dense coats of discrete mi nipodia which anchor them to the glass substratum. The scanning electron mi croscopy demonstrates that minipodia are surface microprotrusions about 0.5 mum thick and up to 8 mum long, covering mainly the middle-anterior area o f the ventral cell surface. They are few at the frontal zone and absent at the tail region. It means that the dense felt of discrete minipodia is loca ted in the same region which has been earlier described as the zone of most efficient adhesion of a directionally moving amoeba. The cells without sta ble motor polarity and those which adhere to the glass without moving, or j ust start locomotion, lack areas covered by discrete minipodia; instead, mi nipodia are grouped in rosette contacts, which have the form of papillae co mposed of supporting platforms with crones of minipodia projected from them . The cells detached from the substratum by simple experimental procedures: radiate heterotactic forms produced by mechanical shocks, anucleate fragme nts obtained by microdissection, and amoebae in course of cation-induced pi nocytosis, neither have separate minipodia nor rosette contacts. In contras t, during phagocytosis amoebae strongly adhere and produce dense sheaths of discrete minipodia extending along the whole surface up to the tail, excep t the naked front enclosing the phagosome. It was demonstrated by staining with the fluorescein-conjugated phalloidin that both types of adhesive stru ctures: the discrete minipodia as well as the rosette contacts are very ric h in the cytoskeletal F-actin.