The organization of endoplasmic reticulum export complexes

While there is an apparent random distribution of export complexes in the cytoplasm, we found a very high degree of organization in the distribution of E R buds in the cell. We observed not only a high local density on the same stretch of E R membrane (Fig. 11 , tier I, dotted boxes), but found distantly connected E R bud-bearing zones encircling the same VTC (Fig. 11 , tier II, dashed cylindrical region), suggestive of a regional specialization of the COPII export machinery. Consistent with this interpretation, buds and vesicles accumulated in semi-intact cells in the pres-ence of GTP~/S or the Sar1[H79G] mutant strongly labeled with antibodies specific for mammalian homologues of the yeast components Secl3p and Sec23p.