Association between cytoskeletal microtubules and Ca2+-sequestering smooth ER in Semper cells of fly ommatidia

The Semper cells in the ommatidia of the blowfly contain an impressive cytoskeleton consisting of hundreds of microtubules. The microtubules are intimately interwoven by an extensive system of the smooth endoplasmic reticulum. The cells were permeabilized by saponin treatment, subsequently incubated in a loading medium containing 8.5 X 10(-7) M free Ca2+, MgATP and oxalate, and then processed for electron microscopy. As a result, microtubule associated SER elements and the nuclear envelope are heavily loaded with electron opaque Ca-oxalate deposits. Precipitate formation is ATP dependent. This experiment indicates that the microtubule associated SER can actively accumulate Ca2+ ions. The Ca pump exhibits a high affinity for Ca2+ ions. The SER may help to keep the cytosolic free Ca2+ in the submicromolar concentration range, in this way stabilizing the microtubules in the polymerized state.