The organization of endoplasmic reticulum export complexes

The apparent nonrandom distribution of ER buds led us to quantitatively estimate the probability of a given bud having proximity to a second bud in the cell. For this purpose, we constructed a series of concentric shells of equal volume that radiate outward from the center of randomly chosen buds. The radius of the first internal shell was arbitrarily assigned a value of 0.1 txm to encompass the entire tip of a budding structure. Each increment in the diameter of successive shells extending outward from the first shell progressively decreased in dimension to encompass the same volume in three-dimensional space. Given the average number of buds in a cell (~250), the volume of cell cytoplasm (560 i~m3), and the volume of a shell (4.2 × 10 .3 i~m3), if buds assumed a strictly random distribution, then the probability of encountering another bud in a given concentric shell would remain equal with a value of 0.18% ( Fig. 6, diamonds) . However, if buds were confined to regional foci, the probability of encountering a second bud would be high in the first series of concentric shells, and then fall off very rapidly with increasing distance in threedimensional space.