Randy Schekman

Identification of 23 complementation groups required for post-translational events in the yeast secretory pathway

Cells of a Saccharomyces cerevisiae mutant that is temperature-sensitive for secretion and cell surface growth become dense during incubation at the non-permissive temperature (37 degrees C). This property allows the selection of additional secretory mutants by sedimentation of mutagenized cells on a Ludox density gradient. Colonies derived from dense cells are screened for conditional growth and secretion of invertase and acid phosphatase. The sec mutant strains that accumulate an abnormally large intracellular pool of invertase at 37 degrees C (188 mutant clones) fall into 23 complementation groups, and the distribution of mutant alleles suggests that more complementation groups could be found. Bud emergence and incorporation of a plasma membrane sulfate permease activity stop quickly after a shift to 37 degrees C. Many of the mutants are thermoreversible; upon return to the permissive temperature (25 degrees C) the accumulated invertase is secreted. Electron microscopy of sec mutant cells reveals, with one exception, the temperature-dependent accumulation of membrane-enclosed secretory organelles. We suggest that these structures represent intermediates in a pathway in which secretion and plasma membrane assembly are colinear.

COPII: a membrane coat formed by Sec proteins that drive vesicle budding from the endoplasmic reticulum.

In vitro synthesis of endoplasmic reticulum-derived transport vesicles has been reconstituted with washed membranes and three soluble proteins (Sar1p, Sec13p complex, and Sec23p complex). Vesicle formation requires GTP but can be driven by nonhydrolyzable analogs such as GMP-PNP. However, GMP-PNP vesicles fail to target and fuse with the Golgi complex whereas GTP vesicles are functional. All the cytosolic proteins required for vesicle formation are retained on GMP-PNP vesicles, while Sar1p dissociates from GTP vesicles. Thin section electron microscopy of purified preparations reveals a uniform population of 60-65 nm vesicles with a 10 nm thick electron dense coat. The subunits of this novel coat complex are molecularly distinct from the constituents of the nonclathrin coatomer involved in intra-Golgi transport. Because the overall cycle of budding driven by these two types of coats appears mechanistically similar, we propose that the coat structures be called COPI and COPII.

Coat Proteins and Vesicle Budding

The trafficking of proteins within eukaryotic cells is achieved by the capture of cargo and targeting molecules into vesicles that bud from a donor membrane and deliver their contents to a receiving compartment. This process is bidirectional and may involve multiple organelles within a cell. Distinct coat proteins mediate each budding event, serving both to shape the transport vesicle and to select by direct or indirect interaction the desired set of cargo molecules. Secretion, which has been viewed as a default pathway, may require sorting and packaging signals on transported molecules to ensure their rapid delivery to the cell surface.

Order of events in the yeast secretory pathway

Abstract The sequence of posttranslational events in the export of yeast glycoproteins has been determined with the aid of mutants that affect the secretory apparatus. Temperature-sensitive secretory mutants ( sec ) of S. cerevisiae, when incubated at a nonpermissive growth temperature (37°C), accumulate intracellular precursor forms of exported glycoproteins, such as invertase, and expand or amplify one or more of three different secretory organelles. Characterization of haploid double- sec -mutant strains, with regard to the structure of the accumulated invertase and the morphology of the exaggerated organelles, allows assessment of the order in which the gene products are required, the sequence of invertase maturation steps and a pathway of secretory organelles. The transitions from one organelle to the next require energy and sec gene products. One of the mutants ( sec7 ) accumulates a different organelle depending on the concentration of glucose in the medium. In normal growth medium (2% glucose), a thermally irreversible structure, the Berkeley body, predominates; in low glucose (0.1%), Golgi structures accumulate thermoreversibly. The results are consistent with the following model. Secretory proteins enter the ER, where the initial steps of glycosylation occur. Nine or more sec gene products and energy are required to transfer material to a Golgi-like structure, where further glycosylation occurs. Two or more functions and energy are required to package nearly fully glycosylated proteins into vesicles that are then transported into the bud, where they fuse with the plasma membrane in a process that requires at least ten additional gene products and energy.

Distinct sets of SEC genes govern transport vesicle formation and fusion early in the secretory pathway

A vesicular intermediate in protein transport from the endoplasmic reticulum is detected in a subset of temperature-sensitive mutants blocked early in the yeast secretory pathway. By electron microscopy three of the mutants, sec18, sec17, and sec22, accumulate 50 nm vesicles at the nonpermissive temperature. Vesicle accumulation is blocked by the mutations sec12, sec13, sec16, and sec23 as shown by analysis of double-mutant strains. Thus the early SEC genes can be divided into vesicle forming and vesicle fusion functions. Synthetic lethal interactions between sec mutations define two groups of SEC genes, corresponding to the groups involved in vesicle formation or fusion. Mutations in two of the genes involved in vesicle fusion, SEC17 and SEC18, are lethal in combination, and five of six possible pairwise combinations of mutations in genes required for vesicle formation, SEC12, SEC13, SEC16, and SEC23, are lethal. These interactions suggest cooperation between different SEC gene products in vesicle budding and vesicle fusion processes.

COPII-coated vesicle formation reconstituted with purified coat proteins and chemically defined liposomes.

COPII vesicle formation requires only three coat assembly subunits: Sar1p, Sec13/31p, and Sec23/24p. PI 4-phosphate or PI 4,5-bisphosphate is required for the binding of these proteins to liposomes. The GTP-bound form of Sar1p recruits Sec23/24p to the liposomes as well as to the ER membranes, and this Sar1p-Sec23/24p complex is required for the binding of Sec13/31p. Ultrastructural analysis shows that the binding of COPII coat proteins to liposomes results in coated patches, coated buds, and coated vesicles of 50-90 nm in diameter. Budding proceeds without rupture of the donor liposome or vesicle product. These observations suggest that the assembly of the COPII coat on the ER occurs by a sequential binding of coat proteins to specific lipids and that this assembly promotes the budding of COPII-coated vesicles.

Early stages in the yeast secretory pathway are required for transport of carboxypeptidase Y to the vacuole

Temperature-sensitive secretory mutants (sec) of S. cerevisiae have been used to evaluate the organelles and cellular functions involved in transport of the vacuolar glycoprotein, carboxypeptidase Y (CPY). Others have shown that CPY (61 kd) is synthesized as an inactive proenzyme (69 kd) that is matured by cleavage of an 8 kd amino-terminal propeptide. sec mutants that are blocked in either of two early stages in the secretory process and accumulate endoplasmic reticulum or Golgi bodies also accumulate precursor forms of CPY when cells are incubated at the nonpermissive temperature (37 degrees C). These forms are converted to a proper size when cells are returned to a permissive temperature (25 degrees C). Vacuoles isolated from sec mutant cells do not contain the proCPY produced at 37 degrees C. These results suggest that vacuolar and secretory glycoproteins require the same cellular functions for transport from the endoplasmic reticulum and from the Golgi body. The Golgi body represents a branch point in the pathway: from this organelle, vacuolar proenzymes are transported to the vacuole for proteolytic processing and secretory proteins are packaged into vesicles.

Multiple Cargo Binding Sites on the COPII Subunit Sec24p Ensure Capture of Diverse Membrane Proteins into Transport Vesicles

We have characterized the mechanisms of cargo selection into ER-derived vesicles by the COPII subunit Sec24p. We identified a site on Sec24p that recognizes the v-SNARE Bet1p and show that packaging of a number of cargo molecules is disrupted when mutations are introduced at this site. Surprisingly, cargo proteins affected by these mutations did not share a single common sorting signal, nor were proteins sharing a putative class of signal affected to the same degree. We show that the same site is conserved as a cargo-interaction domain on the Sec24p homolog Lst1p, which only packages a subset of the cargoes recognized by Sec24p. Finally, we identified an additional mutation that defines another cargo binding domain on Sec24p, which specifically interacts with the SNARE Sec22p. Together, our data support a model whereby Sec24p proteins contain multiple independent cargo binding domains that allow for recognition of a diverse set of sorting signals.

Sec61p mediates export of a misfolded secretory protein from the endoplasmic reticulum to the cytosol for degradation

Degradation of misfolded secretory proteins has long been assumed to occur in the lumen of the endoplasmic reticulum (ER). Recent evidence, however, suggests that such proteins are instead degraded by proteasomes in the cytosol, although it remains unclear how the proteins are transported out of the ER. Here we provide the first genetic evidence that Sec61p, the pore‐forming subunit of the protein translocation channel in the ER membrane, is directly involved in the export of misfolded secretory proteins. We describe two novel mutants in yeast Sec61p that are cold‐sensitive for import into the ER in both intact yeast cells and a cell‐free system. Microsomes derived from these mutants are defective in exporting misfolded secretory proteins. These proteins become trapped in the ER and are associated with Sec61p. We conclude that misfolded secretory proteins are exported for degradation from the ER to the cytosol via channels formed by Sec61p.

COPII–cargo interactions direct protein sorting into ER-derived transport vesicles

Vesicles coated with coat protein complex II (COPII) selectively transport molecules (cargo) and vesicle fusion proteins from the endoplasmic reticulum (ER) to the Golgi complex. We have investigated the role of coat proteins in cargo selection and recruitment. We isolated integral membrane and soluble cargo proteins destined for transport from the ER in complexes formed in the presence of Sar1 and Sec23/24, a subset of the COPII components, and GTP or GMP-PNP. Vesicle fusion proteins of the vSNARE family and Emp24, a member of a putative cargo carrier family, were also found in COPII complexes. The inclusion of amino-acid permease molecules into the complex depended on the presence of Shr3, a protein required for the permease to leave the ER,. Resident ER proteins Sec61, BiP (Kar2) and Shr3 were not included in the complexes, indicating that the COPII components bound specifically to vesicle cargo. COPII–cargo complexes and putative cargo adaptor–cargo complexes were also isolated from COPII vesicles. Our results indicate that cargo packaging signals and soluble cargo adaptors are recognized by a recruitment complex comprising Sar1–GTP and Sec23/24.