Membrane trafficking in the yeast Saccharomyces cerevisiae model

Abstract

The yeast Saccharomyces cerevisiae is one of the best characterized eukaryotic models. The secretory pathway was the first trafficking pathway clearly understood mainly thanks to the work done in the laboratory of Randy Schekman in the 1980s. They have isolated yeast sec mutants unable to secrete an extracellular enzyme and these SEC genes were identified as encoding key effectors of the secretory machinery. For this work, the 2013 Nobel Prize in Physiology and Medicine has been awarded to Randy Schekman; the prize is shared with James Rothman and Thomas Südhof. Here, we present the different trafficking pathways of yeast S. cerevisiae. At the Golgi apparatus newly synthesized proteins are sorted between those transported to the plasma membrane (PM), or the external medium, via the exocytosis or secretory pathway (SEC), and those targeted to the vacuole either through endosomes (vacuolar protein sorting or VPS pathway) or directly (alkaline phosphatase or ALP pathway). Plasma membrane proteins can be internalized by endocytosis (END) and transported to endosomes where they are sorted between those targeted for vacuolar degradation and those redirected to the Golgi (recycling or RCY pathway). Studies in yeast S. cerevisiae allowed the identification of most of the known effectors, protein complexes, and trafficking pathways in eukaryotic cells, and most of them are conserved among eukaryotes.

Figure 1

Phylogenetic tree of eukaryotes representing the distribution of cellular biology models.

Figure 2

Representation of the different yeast intracellular trafficking pathways. Organellar soluble and membrane proteins are synthesized at the endoplasmic reticulum (ER) and transported to the unstacked Golgi (grey arrow). At the Golgi, these proteins are sorted into anterograde transport vesicles for ER resident proteins (grey arrow), into secretory (SEC) vesicles for plasma membrane (PM) and extracellular proteins (blue arrow), and into vacuolar protein sorting (VPS) vesicles for vacuolar proteins passing through endosomes (red arrow). The endocytic pathway (END) is used for internalization of PM proteins and extracellular medium components (green arrow). At the early endosomes (EE), proteins are sorted between those targeted for degradation into the vacuole, after maturation of the EE into the late endosome or multivesicular body (MVB), and those that are following the recycling pathway (RCY) to avoid degradation by being targeted to the Golgi (orange arrow).

Figure 3

The Golgi to vacuole trafficking pathways. Two pathways transport proteins from the unstacked Golgi to the vacuole, either directly via the ALP pathway, or via the endosomes for the VPS CPY pathway. In the ALP pathway, the alkaline phosphatase ALP is packaged into vesicles through adaptor AP-3 interaction which in turn recruits clathrin. Vesicles are then fusing with the vacuole. The CPY pathway transports different vacuolar proteins, either membrane-bound as the CPS (carboxypeptidase S) or soluble as the CPY (carboxytpeptidase Y) pro-protease. The soluble CPY is recruited into the Golgi lumen through the binding of its receptor Vps10. Both CPY and CPS are loaded onto vesicles via the AP-1 adaptor and clathrin coat. After budding, vesicles are transported to the endosomes where they fuse liberating CPS to the endosomal membrane and CPY into the lumen. Vps10 is sorted away from the endosomal membrane to be recycled back to the Golgi for a new round of CPY transport by being loaded onto vesicles initiated by the Retromer complex. On the other hand CPS is loaded onto vesicles, which will bud into the endosomal lumen to give rise to the multivesicular body (MVB). This process requires first the ubiquitination of CPS as a sorting signal for the ESCRT (endosomal sorting complex required for transport) machinery (ESCRT-0 to -III), which will cluster CPS into invaginated endosomal membrane and pinch off vesicles. The MVB will then fuse with the vacuole and liberate its content into the vacuolar lumen to be processed.

Figure 4

The endocytic internalization process. Endocytosis sites are first initiated by the recruitment of clathrin via the AP-2 adaptor complex (early coat) to cluster cargo proteins to the endocytic invagination sites at the plasma membrane (PM). Initiation is terminated by the recruitment of ANTH- (AP180 N-terminal homology) and ENTH- (epsin N-terminal homology) domain containing proteins Sla2 and epsins Ent1 and Ent2 to form the late coat. Invagination starts by the recruitment of the WASP/Myosin module to initiate actin branched polymerization by the actin module (Arp2/3 complex and actin). Myosin is squeezing the PM while the actin module expands the invagination. Once the invagination in long enough, the fission module starts pinching off vesicles that are released into the cytoplasm.