Golgi stacks are often located near sites of “transitional ER” (tER), where COPII transport vesicles are produced. This juxtaposition may indicate that Golgi cisternae form at tER sites. To explore this idea, we examined two budding yeasts: Pichia pastoris, which has coherent Golgi stacks, and Saccharomyces cerevisiae, which has a dispersed Golgi. tER structures in the two yeasts were visualized using fusions between green fluorescent protein and COPII coat proteins. We also determined the localization of Sec12p, an ER membrane protein that initiates the COPII vesicle assembly pathway. In P. pastoris, Golgi stacks are adjacent to discrete tER sites that contain COPII coat proteins as well as Sec12p. This arrangement of the tER-Golgi system is independent of microtubules. In S. cerevisiae, COPII vesicles appear to be present throughout the cytoplasm and Sec12p is distributed throughout the ER, indicating that COPII vesicles bud from the entire ER network. We propose that P. pastoris has discrete tER sites and therefore generates coherent Golgi stacks, whereas S. cerevisiae has a delocalized tER and therefore generates a dispersed Golgi. These findings open the way for a molecular genetic analysis of tER sites.

Our data suggest that Sec16 helps to organize patches of COPII-coat proteins into clusters that represent tER sites. The Golgi disruption that occurs in the sec16 mutant provides evidence that Golgi structure in budding yeasts depends on tER organization.

The budding yeast Pichia pastoris contains ordered Golgi stacks next to discrete transitional endoplasmic reticulum (tER) sites, making this organism ideal for structure-function studies of the secretory pathway. Here, we have used P. pastoris to test various models for Golgi trafficking.The experimental approach was to analyze P. pastoris tER-Golgi units by using cryofixed and freeze-substituted cells for electron microscope tomography, immunoelectron microscopy, and serial thin section analysis of entire cells. We find that tER sites and the adjacent Golgi stacks are enclosed in a ribosome-excluding "matrix." Each stack contains three to four cisternae, which can be classified as cis, medial, trans, or trans-Golgi network (TGN). No membrane continuities between compartments were detected. This work provides three major new insights. First, two types of transport vesicles accumulate at the tER-Golgi interface. Morphological analysis indicates that the center of the tER-Golgi interface contains COPII vesicles, whereas the periphery contains COPI vesicles. Second, fenestrae are absent from cis cisternae, but are present in medial through TGN cisternae. The number and distribution of the fenestrae suggest that they form at the edges of the medial cisternae and then migrate inward. Third, intact TGN cisternae apparently peel off from the Golgi stacks and persist for some time in the cytosol, and these "free-floating" TGN cisternae produce clathrin-coated vesicles. These observations are most readily explained by assuming that Golgi cisternae form at the cis face of the stack, progressively mature, and ultimately dissociate from the trans face of the stack. INTRODUCTIONThe Golgi apparatus consists of flattened membrane cisternae that are usually organized into stacks (Berger and Roth, 1997). Newly synthesized biosynthetic cargo molecules exit the endoplasmic reticulum (ER) in COPII-coated vesicles and enter the cis cisterna of the Golgi (Farquhar and Hauri, 1997;Barlowe, 2002). These cargo molecules then occur in medial and trans-cisternae. In the trans-Golgi network (TGN), the cargo molecules are sorted into different types of carriers for delivery to the plasma membrane and other destinations (Mellman and Simons, 1992). Although this basic scheme is well established, the pathway of intra-Golgi transport is still being investigated. Anterograde Golgi transport has been proposed to occur via vesicular intermediates, membrane continuities, cisternal maturation, or a combination of these mechanisms (Beznoussenko and Mironov, 2002). The extent to which a given mechanism operates may vary with the cell type and the stage of the cell cycle (Pelham and Rothman, 2000;Marsh and Howell, 2002). Considerable evidence now favors cisternal maturation as a major route for intra-Golgi transport (Pelham, 2001;Storrie and Nilsson, 2002), but the generality and several key predictions of this model remain to be verified.We are using electron microscopy to test predictions of the different Golgi-trafficking models. Because some of the...

EVI1, located at chromosome band 3q26, encodes a 1051 amino DNA-binding zinc fingers in the central part of the molecule.acid zinc finger protein inappropriately expressed in the leu-EVI1 also contains an acidic domain at the carboxyl terkemic cells of 2-5% of acute myeloid leukemia (AML) and myeminus. 4 The first group of zinc fingers recognizes and binds As expressed in some tissues, and it is activated inappropriately a reporter gene, we used chloramphenicol acetyl transferase by chromosomal rearrangements involving band 3q26 in (CAT) linked to a human normal genomic promoter and to a about 2-5% of human myeloid leukemia with an abnormal synthetic promoter both of which have been previously karyotype.2,3 Expression of EVI1 in hematopoietic cells is condescribed. 11 Our results show that MDS1/EVI1 can activate troversial, but it has been reported for CD34+ cells. 4 The most the promoters almost as strongly as GATA-1. Activation of the frequent rearrangements affecting EVI1 expression are promoters depends strictly on the segment of the PR domain t(3;3)(q21;q26) and inv(3)(q21q26), in which activation of encoded by the second and third untranslated exons of EVI1 EVI1 is due to juxtaposition of the gene to the enhancer region and on the first group of seven zinc fingers. Furthermore, actiof the ribophorin gene. 5 The gene can also be activated folvation by MDS1/EVI1 is repressed by expression of EVI1. To lowing the t(3;21)(q26;q22) in which EVI1 is fused downexamine the relative amounts of the two genes in the bone stream of the runt homology domain of the AML1 gene. 6,7 In marrow, we have analyzed bone marrow samples of a normal addition, the gene can also be activated in cells with an apparindividual and of seven patients with the t(3;3) or the inv(3). ently normal karyotype by unknown mechanisms.8 EVI1 is a Our results indicate that in some of the patients the rearrangenuclear protein containing a group of seven DNA-binding ments affect the expression of EVI1 but not MDS1/EVI1. These zinc fingers at the amino terminus and a second group of three results combined are especially important in view of the fact that recurring chromosomal translocations in leukemia with abnormalities at chromosome band 3q26 often disrupt the Correspondence: G Nucifora, Oncology Institute, Loyola University region between MDS1 and EVI1 and result in truncation

COPII vesicles assemble at ER subdomains called transitional ER (tER) sites, but the mechanism that generates tER sites is unknown. To study tER biogenesis, we analyzed the transmembrane protein Sec12, which initiates COPII vesicle formation. Sec12 is concentrated at discrete tER sites in the budding yeast Pichia pastoris. We find that P. pastoris Sec12 exchanges rapidly between tER sites and the general ER. The tER localization of Sec12 is saturable and is mediated by interaction of the Sec12 cytosolic domain with a partner component. This interaction apparently requires oligomerization of the Sec12 lumenal domain. Redistribution of P. pastoris Sec12 to the general ER does not perturb the localization of downstream tER components, suggesting that Sec12 and other COPII proteins associate with a tER scaffold. These results provide evidence that tER sites form by a network of dynamic associations at the cytosolic face of the ER.