High-resolution secretory timeline from vesicle formation at the Golgi to fusion at the plasma membrane in S. cerevisiae

Most of the components in the yeast secretory pathway have been studied, yet a high-resolution temporal timeline of their participation is lacking. Here, we define the order of acquisition, lifetime, and release of critical components involved in late secretion from the Golgi to the plasma membrane. Of particular interest is the timing of the many reported effectors of the secretory vesicle Rab protein Sec4, including the myosin-V Myo2, the exocyst complex, the lgl homolog Sro7, and the small yeast-specific protein Mso1. At the trans-Golgi network (TGN) Sec4's GEF, Sec2, is recruited to Ypt31-positive compartments, quickly followed by Sec4 and Myo2 and vesicle formation. While transported to the bud tip, the entire exocyst complex, including Sec3, is assembled on to the vesicle. Before fusion, vesicles tether for 5 s, during which the vesicle retains the exocyst complex and stimulates lateral recruitment of Rho3 on the plasma membrane. Sec2 and Myo2 are rapidly lost, followed by recruitment of cytosolic Sro7, and finally the SM protein Sec1, which appears for just 2 s prior to fusion. Perturbation experiments reveal an ordered and robust series of events during tethering that provide insights into the function of Sec4 and effector exchange.

The Sur7/PalI family transmembrane protein Tos7 (Yol019w) plays a role in secretion in budding yeast

Tos7 (Yol019w) is a Sur7/PalI family transmembrane protein in the budding yeast Saccharomyces cerevisiae. Since the deletion of TOS7 did not affect growth or cell morphology, the cellular roles of Tos7 have not been established previously. Here, we show that high-copy TOS7 expression suppressed the growth defect of the secretion-defective RGA1-C term-overexpressing mutant and sec15-1 mutant. Moreover, Tos7 physically interacted with Boi2 and the Rho GTPase Rho3, two key regulators of exocyst assembly, suggesting that Tos7 plays a role in secretion. We also show that the deletion of TOS7 rendered the cells more sensitive to the cell wall-disrupting agents Congo red and calcofluor white while high-copy TOS7 expression had an opposite effect, suggesting that Tos7 affects cell wall organization. Finally, we show that Tos7 localized to punctate patches on the plasma membrane that were largely co-localized with the plasma membrane microdomains named MCC (membrane compartment of Can1). Together, these results suggest that Tos7 contributes to cell surface-related functions. Tos7 is likely an auxiliary component of MCC/eisosome that specifically interacts with the secretory pathway.

Exocyst structural changes associated with activation of tethering downstream of Rho/Cdc42 GTPases

The exocyst complex plays a critical role in determining both temporal and spatial dynamics of exocytic vesicle tethering and fusion with the plasma membrane. However, the mechanism by which the exocyst functions and how it is regulated remain poorly understood. Here we describe a novel biochemical assay for the examination of exocyst function in vesicle tethering. Importantly, the assay is stimulated by gain-of-function mutations in the Exo70 component of the exocyst, selected for their ability to bypass Rho/Cdc42 activation in vivo. Single-particle electron microscopy and 3D reconstructions of negatively stained exocyst complexes reveal a structural change in the mutant exocyst that exposes a binding site for the v-SNARE. We demonstrate a v-SNARE requirement in our tethering assay and increased v-SNARE binding to exocyst gain-of-function complexes. Together, these data suggest an allosteric mechanism for activation involving a conformational change in one subunit of the complex, which is relayed through the complex to regulate its biochemical activity in vitro, as well as overall function in vivo.

A time-resolved interaction analysis of Bem1 reconstructs the flow of Cdc42 during polar growth

Cdc42 organizes cellular polarity and directs the formation of cellular structures in many organisms. By locating Cdc24, the source of active Cdc42, to the growing front of the yeast cell, the scaffold protein Bem1, is instrumental in shaping the cellular gradient of Cdc42. This gradient instructs bud formation, bud growth, or cytokinesis through the actions of a diverse set of effector proteins. To address how Bem1 participates in these transformations, we systematically tracked its protein interactions during one cell cycle to define the ensemble of Bem1 interaction states for each cell cycle stage. Mutants of Bem1 that interact with only a discrete subset of the interaction partners allowed to assign specific functions to different interaction states and identified the determinants for their cellular distributions. The analysis characterizes Bem1 as a cell cycle-specific shuttle that distributes active Cdc42 from its source to its effectors. It further suggests that Bem1 might convert the PAKs Cla4 and Ste20 into their active conformations.

Septin architecture and function in budding yeast

The septins constitute a conserved family of guanosine phosphate-binding and filament-forming proteins widespread across eukaryotic species. Septins appear to have two principal functions. One is to form a cortical diffusion barrier, like the septin collar at the bud neck of Saccharomyces cerevisiae, which prevents movement of membrane-associated proteins between the mother and daughter cells. The second is to serve as a polymeric scaffold for recruiting the proteins required for critical cellular processes to particular subcellular areas. In the last decade, structural information about the different levels of septin organization has appeared, but crucial structural determinants and factors responsible for septin assembly remain largely unknown. This review highlights recent findings on the architecture and function of septins and their remodeling with an emphasis on mitotically dividing budding yeasts.

Cdc24 interacts with septins to create a positive feedback loop during bud site assembly in yeast

Yeast cells select the position of their new bud at the beginning of each cell cycle. The recruitment of septins to this prospective bud site is one of the critical events in a complex assembly pathway that culminates in the outgrowth of a new daughter cell. During recruitment, septin rods follow the high concentration of Cdc42GTP that is generated by the focused localization of the Cdc42 guanine-nucleotide-exchange factor Cdc24. We show that, shortly before budding, Cdc24 not only activates Cdc42 but also transiently interacts with Cdc11, the septin subunit that caps both ends of the septin rods. Mutations in Cdc24 that reduce affinity to Cdc11 impair septin recruitment and decrease the stability of the polarity patch. The interaction between septins and Cdc24 thus reinforces bud assembly at sites where septin structures are formed. Once the septins polymerize to form the septin ring, Cdc24 is found at the cortex of the bud and directs further outgrowth from this position.

The cell polarity proteins Boi1 and Boi2 direct an actin nucleation complex to sites of exocytosis in Saccharomyces cerevisiae

Owing to the local enrichment of factors that influence its dynamics and organization, the actin cytoskeleton displays different shapes and functions within the same cell. In yeast cells, post-Golgi vesicles ride on long actin cables to the bud tip. The proteins Boi1 and Boi2 (Boi1/2) participate in tethering and docking these vesicles to the plasma membrane. Here, we show in Saccharomyces cerevisiae that Boi1/2 also recruit nucleation and elongation factors to form actin filaments at sites of exocytosis. Disrupting the connection between Boi1/2 and the nucleation factor Bud6 impairs filament formation, reduces the directed movement of the vesicles to the tip and shortens the vesicles' tethering time at the cortex. Transplanting Boi1 from the bud tip to the peroxisomal membrane partially redirects the actin cytoskeleton and the vesicular flow towards the peroxisome, and creates an alternative, rudimentary vesicle-docking zone. We conclude that Boi1/2, through interactions with Bud6 and Bni1, induce the formation of a cortical actin structure that receives and aligns incoming vesicles before fusion with the membrane.

YFR016c/Aip5 is part of an actin nucleation complex in yeast

The polarisome comprises a network of proteins that organizes polar growth in yeast and filamentous fungi. The yeast formin Bni1 and the actin nucleation-promoting factor Bud6 are subunits of the polarisome that together catalyze the formation of actin cables below the tip of yeast cells. We identified YFR016c (Aip5) as an interaction partner of Bud6 and the polarisome scaffold Spa2. Yeast cells lacking Aip5 display a reduced number of actin cables. Aip5 binds with its N-terminal region to Spa2 and with its C-terminal region to Bud6. Both interactions collaborate to localize Aip5 at bud tip and neck, and are required to stimulate the formation of actin cables. Our experiments characterize Aip5 as a novel subunit of a complex that regulates the number of actin filaments at sites of polar growth.

A cell separation checkpoint that enforces the proper order of late cytokinetic events

Eukaryotic cell division requires dependency relationships in which late processes commence only after early ones are appropriately completed. We have discovered a system that blocks late events of cytokinesis until early ones are successfully accomplished. In budding yeast, cytokinetic actomyosin ring contraction and membrane ingression are coupled with deposition of an extracellular septum that is selectively degraded in its primary septum immediately after its completion by secreted enzymes. We find this secretion event is linked to septum completion and forestalled when the process is slowed. Delay of septum degradation requires Fir1, an intrinsically disordered protein localized to the cytokinesis site that is degraded upon septum completion but stabilized when septation is aberrant. Fir1 protects cytokinesis in part by inhibiting a separation-specific exocytosis function of the NDR/LATS kinase Cbk1, a key component of "hippo" signaling that induces mother-daughter separation. We term this system enforcement of cytokinesis order, a checkpoint ensuring proper temporal sequence of mechanistically incompatible processes of cytokinesis.

Dissecting the nucleotide binding properties of the septins from S. cerevisiae

Septins are a conserved family of guanosine triphosphate (GTP)-binding proteins that assemble into an ordered array of filaments at the mother bud neck in Saccharomyces cerevisiae cells. They are present in all higher eukaryotes except plants. Septins belong structurally to the P-Loop nucleoside triphosphatase (NTPases) like Rab and Ras. However, unlike other small guanosine triphosphatase (GTPases) septins are supposed to act as scaffolds rather than signalling mediators. This is why they are considered as the fourth class of cytoskeletal proteins. It is assumed that septins fulfil their functions independently of the bound nucleotide. The role of guanosine diphosphosphate (GDP) and GTP binding and subsequent hydrolysis was controversial debated in the last couple of years. Lack of crystal structures of yeast septin subunits or rods and difficulties to isolate single monomeric septin subunits often hindered the correlation of results obtained from in vivo studies with biochemical data. Recently, nucleotide binding and hydrolysis was connected to the formation of septin rods from its subunits. However, the evidence was only indirectly obtained through the use of septin mutants in the context of intact cells. We provide here mechanistic insight into the nucleotide binding of the yeast septins by in vitro assays using purified septin rods and building blocks, thereby adding further insights to the already available models on septin filament formation.

Distinct roles of the polarity factors Boi1 and Boi2 in the control of exocytosis and abscission in budding yeast

The budding yeast cortical proteins Boi1 and Boi2 were previously implicated in polarized growth and in inhibition of cytokinesis as part of the NoCut abscission checkpoint. This report establishes an essential role of Boi1 and Boi2 in vesicle exocytosis during bud growth, whereas Boi2 (but not Boi1) is required for abscission inhibition.

Boi1 and Boi2 (Boi1/2) are budding yeast plasma membrane proteins that function in polarized growth, and in cytokinesis inhibition in response to chromosome bridges via the NoCut abscission checkpoint. How Boi1/2 act in these two distinct processes is not understood. We demonstrate that Boi1/2 are required for a late step in the fusion of secretory vesicles with the plasma membrane of the growing bud. Cells lacking Boi1/2 accumulate secretory vesicles and are defective in bud growth. In contrast, Boi2 is specifically required for abscission inhibition in cells with chromatin bridges. The SH3 domain of Boi2, which is dispensable for bud growth and targets Boi2 to the site of abscission, is necessary and sufficient for abscission inhibition. Gain of function of the exocyst, a conserved protein complex involved in tethering of exocytic vesicles to the plasma membrane, rescued secretion and bud growth defects in boi mutant cells, and abrogated NoCut checkpoint function. Thus Boi2 functions redundantly with Boi1 to promote the fusion of secretory vesicles with the plasma membrane at sites of polarized growth, and acts as an abscission inhibitor during cytokinesis in response to chromatin bridges.