[Abstract unavailable

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.

Depletion of a subset of 70K stress proteins in yeast mutants shows that they are involved in the post-translational import of precursor polypeptides into both mitochondria and the lumen of the endoplasmic reticulum. The identification of such a basic function may explain the remarkable evolutionary conservation of the gene family encoding these proteins.

The endoplasmic reticulum (ER) and the Golgi comprise the first two steps in protein secretion. Vesicular carriers mediate a continuous flux of proteins and lipids between these compartments, reflecting the transport of newly synthesized proteins out of the ER and the retrieval of escaped ER residents and vesicle machinery. Anterograde and retrograde transport is mediated by distinct sets of cytosolic coat proteins, the COPII and COPI coats, respectively, which act on the membrane to capture cargo proteins into nascent vesicles. We review the mechanisms that govern coat recruitment to the membrane, cargo capture into a transport vesicle, and accurate delivery to the target organelle.