Lsb1 is a negative regulator of las17 dependent actin polymerization involved in endocytosis

Regulation of the endocytosis and prion-chaperoning machineries by yeast E3 ubiquitin ligase Rsp5 as revealed by orthogonal ubiquitin transfer

Attachment of the ubiquitin (UB) peptide to proteins via the E1-E2-E3 enzymatic machinery regulates diverse biological pathways, yet identification of the substrates of E3 UB ligases remains a challenge. We overcame this challenge by constructing an "orthogonal UB transfer" (OUT) cascade with yeast E3 Rsp5 to enable the exclusive delivery of an engineered UB (xUB) to Rsp5 and its substrate proteins. The OUT screen uncovered new Rsp5 substrates in yeast, such as Pal1 and Pal2, which are partners of endocytic protein Ede1, and chaperones Hsp70-Ssb, Hsp82, and Hsp104 that counteract protein misfolding and control self-perpetuating amyloid aggregates (prions), resembling those involved in human amyloid diseases. We showed that prion formation and effect of Hsp104 on prion propagation are modulated by Rsp5. Overall, our work demonstrates the capacity of OUT to deconvolute the complex E3-substrate relationships in crucial biological processes such as endocytosis and protein assembly disorders through protein ubiquitination.

Mechanisms for Curing Yeast Prions

Prions are infectious proteins that self-propagate by changing from their normal folded conformation to a misfolded conformation. The misfolded conformation, which is typically rich in β-sheet, serves as a template to convert the prion protein into its misfolded conformation. In yeast, the misfolded prion proteins are assembled into amyloid fibers or seeds, which are constantly severed and transmitted to daughter cells. To cure prions in yeast, it is necessary to eliminate all the prion seeds. Multiple mechanisms of curing have been found including inhibiting severing of the prion seeds, gradual dissolution of the prion seeds, asymmetric segregation of the prion seeds between mother and daughter cells during cell division, and degradation of the prion seeds. These mechanisms, achieved by using different protein quality control machinery, are not mutually exclusive; depending on conditions, multiple mechanisms may work simultaneously to achieve curing. This review discusses the various methods that have been used to differentiate between these mechanisms of curing.

Prion-based memory of heat stress in yeast

Amyloids and amyloid-based prions are self-perpetuating protein aggregates which can spread by converting a normal protein of the same sequence into a prion form. They are associated with diseases in humans and mammals, and control heritable traits in yeast and other fungi. Some amyloids are implicated in biologically beneficial processes. As prion formation generates reproducible memory of a conformational change, prions can be considered as molecular memory devices.  We have demonstrated that in yeast, stress-inducible cytoskeleton-associated protein Lsb2 forms a metastable prion in response to high temperature. This prion promotes conversion of other proteins into prions and can persist in a fraction of cells for a significant number of cell generations after stress, thus maintaining the memory of stress in a population of surviving cells. Acquisition of an amino acid substitution required for Lsb2 to form a prion coincides with acquisition of increased thermotolerance in the evolution of Saccharomyces yeast. Thus the ability to form an Lsb2 prion in response to stress coincides with yeast adaptation to growth at higher temperatures. These findings intimately connect prion formation to the cellular response to environmental stresses.

Polarized Exocytosis Induces Compensatory Endocytosis by Sec4p-Regulated Cortical Actin Polymerization

Polarized growth is maintained by both polarized exocytosis, which transports membrane components to specific locations on the cell cortex, and endocytosis, which retrieves these components before they can diffuse away. Despite functional links between these two transport pathways, they are generally considered to be separate events. Using live cell imaging, in vivo and in vitro protein binding assays, and in vitro pyrene-actin polymerization assays, we show that the yeast Rab GTPase Sec4p couples polarized exocytosis with cortical actin polymerization, which induces endocytosis. After polarized exocytosis to the plasma membrane, Sec4p binds Las17/Bee1p (yeast Wiskott-Aldrich Syndrome protein [WASp]) in a complex with Sla1p and Sla2p during actin patch assembly. Mutations that inactivate Sec4p, or its guanine nucleotide exchange factor (GEF) Sec2p, inhibit actin patch formation, whereas the activating sec4-Q79L mutation accelerates patch assembly. In vitro assays of Arp2/3-dependent actin polymerization established that GTPγS-Sec4p overrides Sla1p inhibition of Las17p-dependent actin nucleation. These results support a model in which Sec4p relocates along the plasma membrane from polarized sites of exocytic vesicle fusion to nascent sites of endocytosis. Activated Sec4p then promotes actin polymerization and triggers compensatory endocytosis, which controls surface expansion and kinetically refines cell polarization.

Timely Endocytosis of Cytokinetic Enzymes Prevents Premature Spindle Breakage during Mitotic Exit

Cytokinesis requires the spatio-temporal coordination of membrane deposition and primary septum (PS) formation at the division site to drive acto-myosin ring (AMR) constriction. It has been demonstrated that AMR constriction invariably occurs only after the mitotic spindle disassembly. It has also been established that Chitin Synthase II (Chs2p) neck localization precedes mitotic spindle disassembly during mitotic exit. As AMR constriction depends upon PS formation, the question arises as to how chitin deposition is regulated so as to prevent premature AMR constriction and mitotic spindle breakage. In this study, we propose that cells regulate the coordination between spindle disassembly and AMR constriction via timely endocytosis of cytokinetic enzymes, Chs2p, Chs3p, and Fks1p. Inhibition of endocytosis leads to over accumulation of cytokinetic enzymes during mitotic exit, which accelerates the constriction of the AMR, and causes spindle breakage that eventually could contribute to monopolar spindle formation in the subsequent round of cell division. Intriguingly, the mitotic spindle breakage observed in endocytosis mutants can be rescued either by deleting or inhibiting the activities of, CHS2, CHS3 and FKS1, which are involved in septum formation. The findings from our study highlight the importance of timely endocytosis of cytokinetic enzymes at the division site in safeguarding mitotic spindle integrity during mitotic exit.

The cytokinesis machinery that is required for physical separation of mother-daughter cells during mitosis is highly conserved from yeast to humans. In budding yeast, cytokinesis is achieved via timely delivery of cytokinetic enzymes to the division site that eventually triggers the constriction of AMR. It has been previously demonstrated that cytokinesis invariably occurs after the disassembly of the mitotic spindle. Intriguingly, Chs2p that is responsible for laying down the primary septum has been shown to localize to the division site before mitotic spindle disassembly. In this study, we show that mitotic spindle integrity upon sister chromatid separation is dependent on the continuous endocytosis of cytokinetic enzymes. Failure in the internalization of cytokinetic proteins during mitotic exit causes premature AMR constriction that eventually contributes to the shearing of mitotic spindle. Consequently, cells fail to re-establish a bipolar spindle in the subsequent round of cell division cycle. Our findings provide insights into how the levels of secreted proteins at the division site impacts cytokinesis. We believe this regulation mechanism might be conserved in higher eukaryotic cells as a secreted protein, hemicentin, has been shown recently to be involved in regulating cytokinesis in both Caenorhabditis elegans and mouse embryos.

Actin, Membrane Trafficking and the Control of Prion Induction, Propagation and Transmission in Yeast

The model eukaryotic yeast Saccharomyces cerevisiae has proven a useful model system in which prion biogenesis and elimination are studied. Several yeast prions exist in budding yeast and a number of studies now suggest that these alternate protein conformations may play important roles in the cell. During the last few years cellular factors affecting prion induction, propagation and elimination have been identified. Amongst these, proteins involved in the regulation of the actin cytoskeleton and dynamic membrane processes such as endocytosis have been found to play a critical role not only in facilitating de novo prion formation but also in prion propagation. Here we briefly review prion formation and maintenance with special attention given to the cellular processes that require the functionality of the actin cytoskeleton.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.

Stress-dependent proteolytic processing of the actin assembly protein Lsb1 modulates a yeast prion

Yeast prions are self-propagating amyloid-like aggregates of Q/N-rich protein that confer heritable traits and provide a model of mammalian amyloidoses. [PSI(+)] is a prion isoform of the translation termination factor Sup35. Propagation of [PSI(+)] during cell division under normal conditions and during the recovery from damaging environmental stress depends on cellular chaperones and is influenced by ubiquitin proteolysis and the actin cytoskeleton. The paralogous yeast proteins Lsb1 and Lsb2 bind the actin assembly protein Las17 (a yeast homolog of human Wiskott-Aldrich syndrome protein) and participate in the endocytic pathway. Lsb2 was shown to modulate maintenance of [PSI(+)] during and after heat shock. Here, we demonstrate that Lsb1 also regulates maintenance of the Sup35 prion during and after heat shock. These data point to the involvement of Lsb proteins in the partitioning of protein aggregates in stressed cells. Lsb1 abundance and cycling between actin patches, endoplasmic reticulum, and cytosol is regulated by the Guided Entry of Tail-anchored proteins pathway and Rsp5-dependent ubiquitination. Heat shock-induced proteolytic processing of Lsb1 is crucial for prion maintenance during stress. Our findings identify Lsb1 as another component of a tightly regulated pathway controlling protein aggregation in changing environments.

Eng2 is a component of a dynamic protein complex required for endocytic uptake in fission yeast

Eng2 is a glucanase required for spore release, although it is also expressed during vegetative growth, suggesting that it might play other cellular functions. Its homology to the Saccharomyces cerevisiae Acf2 protein, previously shown to promote actin polymerization at endocytic sites in vitro, prompted us to investigate its role in endocytosis. Interestingly, depletion of Eng2 caused profound defects in endocytic uptake, which were not due to the absence of its glucanase activity. Analysis of the dynamics of endocytic proteins by fluorescence microscopy in the eng2Δ strain unveiled a previously undescribed phenotype, in which assembly of the Arp2/3 complex appeared uncoupled from the internalization of the endocytic coat and resulted in a fission defect. Strikingly also, we found that Eng2-GFP dynamics did not match the pattern of other endocytic proteins. Eng2-GFP localized to bright cytosolic spots that moved around the cellular poles and occasionally contacted assembling endocytic patches just before recruitment of Wsp1, the Schizosaccharomyces pombe WASP. Interestingly, Csh3-YFP, a WASP-interacting protein, interacted with Eng2 by co-immunoprecipitation and was recruited to Eng2 in bright cytosolic spots. Altogether, our work defines a novel endocytic functional module, which probably couples the endocytic coat to the actin module.