The organization of endoplasmic reticulum export complexes

Morphometric analysis of peripheral export complexes revealed that these sites typically contained two to six buds emanating from the ER, although this could approach a value of 20 for some exceptional, larger clusters ( Fig. 5 A, closed circles) . These sites had an average number of buds per site of 4.4 ___ 0.3 (Table II) , a value that was slightly smaller than the average number of buds per site found in export complexes adjacent to Golgi (6.1 +_ 0.4 buds per site; Table II) . By including only ER-budding profiles facing VTCs, we were able to estimate a "local" bud density in these export complexes. On average, this value was 17.2 +__ 2.5 buds per i~m 2 of ER surface, which, on a relative scale, is 125 times higher than the average bud density found on the total ER surface (0.14 buds per ixm 2) and approximately five times higher than the bud density found on the surface of the ER within the Golgi exclusion zone.