Subcellular Localization of Calcium Repositories in Plasmodia of the Acellular Slime Mold Physarum Polycephalum

Ul t ras t ruc tura l fibrils consisting of densely packed microfilaments, s imdar in size to the th in filaments of striated muscle, have been observed in the p lasmodium of the acellular slime mold Ph)'sarum pol)'cephalum (Rhea, 1966; Wohl fa r th Bot termann, 1964) A prote in wi th sedimenta t ion characteristics and ATPase activity typical of muscle myosin is also extractable f rom shme mold plasmodia (Ha tano and Tazawa, 1968, A d e l man and Taylor , 1969 b). Addi t ional electron microscope observations made by Anderson (1964) and %Vohlfarth-Bottermann (1964) described memb rane -bounded vesicles in the vicinity of the mtcrofi lamentous bundles of Physa~um. This study represents a n effort to de termine whe ther these vesicles migh t serve as repositories for ca lc ium or other ions tha t migh t be involved in the regulat ion of mechanochemica l activity in the microfi lamentous bundles.