Subplasmalemmal calcium-binding microregions in Acanthamoeba.

Acanthamoeba cells, fixed with glutaraldehyde supplemented with calcium ions, show Ca-dependent, electron-dense deposits (CaDD) at the cytoplasmic surface of the plasma membrane. The formation of CaDD is stimulated by pre-incubation and fixation of the cells in the presence of ATP. Chemical analysis of the trichloroacetic acid extracts of CaDD-containing cells demonstrates that adenosine monophosphate, pyrophosphate and inorganic phosphate are probably the compounds involved in the formation of CaDD. Treatment of CaDD-containing cells with exogenous phosphatase evokes the disappearance of electron-dense material and a fine fibrillar network can be observed inside the 'empty' CaDD. The ability to restore CaDD in the presence of calcium ions with either pyrophosphate or orthophosphate confirms the suggestion that calcium/phosphate salts may be deposited in special subplasmalemmal calcium-binding microregions.