The organization of endoplasmic reticulum export complexes

Estimation of Number and Deasity of Total ER-derived Buds. To estimate the number ER-derived buds, we used two distinct criteria: (a) a bud was considered an elevation on the surface of the ER with a width of 60-80 nm and covered with a characteristic coat on the external leaflet of the membrane: and (b) buds had a direct connection with the ER membrane and were extruded from the membrane by at least 50% of their circumference. The total number of ER buds per cell was determined as described (Lucocq et al., 1989) . To obtain this value, we multiplied the total number of ER buds detected in thin-secfions of ~300 cells by the mean volume of cells and divided that value by the total volume contributed by the ~300 cells. The latter value was found by multiplying the thickness of a section by the total surface area of the ~300 cell sections in which ER buds were counted.