Membrane pinch-off and reinsertion observed in living cells of Drosophila.

The garland cell of Drosophila is a nephrocyte which takes up waste products from the haemolymph. Endocytosis is thought to occur by the pinch-off of coated vesicles from deep invaginations of the plasma membrane called labyrinthine channels. Electron microscopic studies show that the length of these channels is variable, depending on the relative rates of membrane pinch-off and reinsertion (recycling). Thus, in wild-type garland cells, if the temperature is raised from 19 degrees C to 30 degrees C, the channels shorten, because at high temperature the pinch-off rate exceeds the reinsertion rate. On the other hand, in garland cells of the temperature-sensitive, single-gene mutant shibirets1 (shi), in which endocytosis is reversibly blocked at the pinch-off stage at 30 degrees C, the labyrinthine channels elongate considerably, as membrane insertion proceeds while pinch-off is blocked. The rates of membrane pinch-off and insertion were quantitated in living garland cells by observing the changes in the capacitance of the whole cell membrane which occur as a result of changes in the total area of the plasma membrane. In wild-type cells, the capacitance gradually decreased as the temperature was raised to 30 degrees C, reflecting the shortening of the channels. In shi cells, the capacitance decreased between 19 degrees C and 26 degrees C but then began to increase at higher temperatures as the blockage of endocytosis caused by the shi gene took effect, causing the channels to elongate. The observations suggest that in shi cells the surface area of the cell more than doubles in 12 min by channel elongation. Estimates of the amount of membrane which is pinched off and reinserted were made.