Many genes required for cell polarity development in budding yeast have been identified and arranged into a functional hierarchy. Core elements of the hierarchy are widely conserved, underlying cell polarity development in diverse eukaryotes. To enumerate more fully the protein–protein interactions that mediate cell polarity development, and to uncover novel mechanisms that coordinate the numerous events involved, we carried out a large-scale two-hybrid experiment. 68 Gal4 DNA binding domain fusions of yeast proteins associated with the actin cytoskeleton, septins, the secretory apparatus, and Rho-type GTPases were used to screen an array of yeast transformants that express ∼90% of the predicted Saccharomyces cerevisiae open reading frames as Gal4 activation domain fusions. 191 protein–protein interactions were detected, of which 128 had not been described previously. 44 interactions implicated 20 previously uncharacterized proteins in cell polarity development. Further insights into possible roles of 13 of these proteins were revealed by their multiple two-hybrid interactions and by subcellular localization. Included in the interaction network were associations of Cdc42 and Rho1 pathways with proteins involved in exocytosis, septin organization, actin assembly, microtubule organization, autophagy, cytokinesis, and cell wall synthesis. Other interactions suggested direct connections between Rho1- and Cdc42-regulated pathways; the secretory apparatus and regulators of polarity establishment; actin assembly and the morphogenesis checkpoint; and the exocytic and endocytic machinery. In total, a network of interactions that provide an integrated response of signaling proteins, the cytoskeleton, and organelles to the spatial cues that direct polarity development was revealed.

The bipolar budding pattern of a/alpha Saccharomyces cerevisiae cells appears to depend on persistent spatial markers in the cell cortex at the two poles of the cell. Previous analysis of mutants with specific defects in bipolar budding identified BUD8 and BUD9 as potentially encoding components of the markers at the poles distal and proximal to the birth scar, respectively. Further genetic analysis reported here supports this hypothesis. Mutants deleted for BUD8 or BUD9 grow normally but bud exclusively from the proximal and distal poles, respectively, and the double-mutant phenotype suggests that the bipolar budding pathway has been totally disabled. Moreover, overexpression of these genes can cause either an increased bias for budding at the distal (BUD8) or proximal (BUD9) pole or a randomization of bud position, depending on the level of expression. The structures and localizations of Bud8p and Bud9p are also consistent with their postulated roles as cortical markers. Both proteins appear to be integral membrane proteins of the plasma membrane, and they have very similar overall structures, with long N-terminal domains that are both N- and O-glycosylated followed by a pair of putative transmembrane domains surrounding a short hydrophilic domain that is presumably cytoplasmic. The putative transmembrane and cytoplasmic domains of the two proteins are very similar in sequence. When Bud8p and Bud9p were localized by immunofluorescence and tagging with GFP, each protein was found predominantly in the expected location, with Bud8p at presumptive bud sites, bud tips, and the distal poles of daughter cells and Bud9p at the necks of large-budded cells and the proximal poles of daughter cells. Bud8p localized approximately normally in several mutants in which daughter cells are competent to form their first buds at the distal pole, but it was not detected in a bni1 mutant, in which such distal-pole budding is lost. Surprisingly, Bud8p localization to the presumptive bud site and bud tip also depends on actin but is independent of the septins.

In yeast, the accelerated rate of decay of nonsense mutant mRNAs, called nonsense-mediated mRNA decay, requires three proteins, Upf1p, Upf2p, and Upf3p. Single, double, and triple disruptions of the UPF genes had nearly identical effects on nonsense mRNA accumulation, suggesting that the encoded proteins function in a common pathway. We examined the distribution of epitope-tagged versions of Upf proteins by sucrose density gradient fractionation of soluble lysates and found that all three proteins co-distributed with 80 S ribosomal particles and polyribosomes. Treatment of lysates with RNase A caused a coincident collapse of polyribosomes and each Upf protein into fractions containing 80 S ribosomal particles, as expected for proteins that are associated with polyribosomes. Mutations in the cysteine-rich (zinc finger) and RNA helicase domains of Upf1p caused loss of function, but the mutant proteins remained polyribosome-associated. Density gradient profiles for Upf1p were unchanged in the absence of Upf3p, and although similar, were modestly shifted to fractions lighter than those containing polyribosomes in the absence of Upf2p. Upf2p shifted toward heavier polyribosome fractions in the absence of Upf1p and into fractions containing 80 S particles and lighter fractions in the absence of Upf3p. Our results suggest that the association of Upf2p with polyribosomes typically found in a wild-type strain depends on the presence and opposing effects of Upf1p and Upf3p.The notion of a global pathway for eukaryotic mRNA decay suggested by early work in animal cells has recently been greatly advanced by studies in the yeast Saccharomyces cerevisiae (1-4). Using an in vivo transcriptional pulse, the temporal fate of newly synthesized mRNA was established by monitoring poly(A) tail length, loss of the m 7 Gppp cap, disappearance of the mRNA, and the appearance of degradation intermediates. mRNAs with shorter half-lives were generally subject to faster rates of deadenylation and decapping. Once the poly(A) tails were reduced to a short oligo(A) length (10 -12 nucleotides), the mRNAs were decapped and digested from the 5Ј end. Decapping requires Dcp1p (5). Processive degradation from the 5Ј end requires the product of the XRN1 gene, which is known to encode a 5Ј 3 3Ј exoribonuclease (6). Deadenylation-dependent decapping followed by 5Ј 3 3Ј exonucleolytic decay is likely to be the global default pathway for the degradation of most eukaryotic mRNAs.Yeast mRNAs containing a premature stop codon decay more rapidly than their wild-type counterparts (7). This accelerated decay, called nonsense-mediated mRNA decay (NMD), 1 requires cis-acting elements in the mRNA in addition to a premature stop codon (8). Premature translational termination triggers decapping at the 5Ј end of nonsense mRNAs with kinetics that are independent of deadenylation (9). Following decapping, decay proceeds through the Xrn1p-mediated nucleolysis that is common to intrinsic decay. These results support the view that when translation is prematurely terminat...

In Saccharomyces cerevisiae, Bud8p and Bud9p are homologous plasma membrane glycoproteins that appear to mark the distal and proximal cell poles, respectively, as potential sites for budding in the bipolar pattern. Here we provide evidence that Bud8p is delivered to the presumptive bud site (and thence to the distal pole of the bud) just before bud emergence, and that Bud9p is delivered to the bud side of the mother-bud neck (and thence to the proximal pole of the daughter cell) after activation of the mitotic exit network, just before cytokinesis. Like the delivery of Bud8p, that of Bud9p is actin dependent; unlike the delivery of Bud8p, that of Bud9p is also septin dependent. Interestingly, although the transcription of BUD8 and BUD9 appears to be cell cycle regulated, the abundance of BUD8 mRNA peaks in G2/M and that of BUD9 mRNA peaks in late G1, suggesting that the translation and/or delivery to the cell surface of each protein is delayed and presumably also cell cycle regulated. The importance of time of transcription in localization is supported by promoter-swap experiments: expression of Bud8p from the BUD9 promoter leads to its localization predominantly to the sites typical for Bud9p, and vice versa. Moreover, expression of Bud8p from the BUD9 promoter fails to rescue the budding-pattern defect of a bud8 mutant but fully rescues that of a bud9 mutant. However, although expression of Bud9p from the BUD8 promoter fails to rescue a bud9 mutant, it also rescues only partially the budding-pattern defect of a bud8 mutant, suggesting that some feature(s) of the Bud8p protein is also important for Bud8p function. Experiments with chimeric proteins suggest that the critical element(s) is somewhere in the extracytoplasmic domain of Bud8p.

The yeast cell wall is an essential organelle that protects the cell from mechanical damage and antimicrobial peptides, participates in cell recognition and adhesion, and is important for the generation and maintenance of normal cell shape. We studied the localization of three covalently bound cell wall proteins in Saccharomyces cerevisiae. Tip1p was found only in mother cells, whereas Cwp2p was incorporated in small-to-medium-sized buds. When the promoter regions of TIP1 and CWP2 (responsible for transcription in early G 1 and S/G 2 phases, respectively) were exchanged, the localization patterns of Tip1p and Cwp2p were reversed, indicating that the localization of cell wall proteins can be completely determined by the timing of transcription during the cell cycle. The third protein, Cwp1p, was incorporated into the birth scar, where it remained for several generations. However, we could not detect any role of Cwp1p in strengthening the birth scar wall or any functional interaction with the proteins that mark the birth scar pole as a potential future budding site. Promoter-exchange experiments showed that expression in S/G 2 phase is necessary but not sufficient for the normal localization of Cwp1p. Studies of mutants in which septum formation is perturbed indicate that the normal asymmetric localization of Cwp1p also depends on the normal timing of septum formation, composition of the septum, or both. INTRODUCTIONThe establishment and maintenance of asymmetry are of vital importance for the growth, differentiation, and morphogenesis of cells and organisms. For example, the yeast Saccharomyces cerevisiae grows asymmetrically, producing a bud that becomes the daughter cell (reviewed by Lew and Reed, 1995; Pruyne and Bretscher 2000a,b). In the G 1 phase of the cell cycle, a bud site is selected in a mating type-dependent manner. After the cell cycle-commitment point START, proteins required for bud initiation are recruited to this site, and a bud is formed in a process that depends largely on the delivery of vesicles containing new cell surface material by the polarized actin cytoskeleton. Growth occurs exclusively in the bud, first preferentially at the tip and later isotropically, until the time of cytokinesis. The actin cytoskeleton and cell surface growth are then redirected toward the mother-bud neck, where a chitinous primary septum is formed, followed by the deposition of secondary septa on both sides of the primary septum. Cells then separate by partial digestion of the primary septum, and a new cell cycle is initiated.The cell wall of yeast is essential for growth and is involved in the establishment and maintenance of asymmetry, as illustrated by the loss of both buds and polarized localization of cytoskeletal proteins when the cell wall is removed (Reck-Peterson et al., 1999). The cell wall is comprised of glucans, mannoproteins, and a small amount of chitin (reviewed by Orlean, 1997;Smits et al., 2001;Klis et al., 2006). The mannoproteins can be divided into four classes: 1) noncovalently linked prote...

In Saccharomyces cerevisiae, Upf3p is required for nonsense-mediated mRNA decay (NMD). Although localized primarily in the cytoplasm, Upf3p contains three sequence elements that resemble nuclear localization signals (NLSs) and two sequence elements that resemble nuclear export signals (NESs). We found that a cytoplasmic reporter protein localized to the nucleus when fused to any one of the three NLS-like sequences of Upf3p. A nuclear reporter protein localized to the cytoplasm when fused to one of the NES-like sequences (NES-A). We present evidence that NES-A functions to signal the export of Upf3p from the nucleus. Combined alanine substitutions in the NES-A element caused a re-distribution of Upf3p to a subnuclear location identified as the nucleolus and conferred an Nmd- phenotype. Single mutations in NES-A failed to affect the distribution of Upf3p and were Nmd+. When an NES element from HIV-1 Rev was inserted near the C terminus of a mutant Upf3p containing multiple mutations in NES-A, the cytoplasmic distribution typical of wild-type Upf3p was restored but the cells remained phenotypically Nmd-. These results suggest that NES-A is a functional nuclear export signal. Combined mutations in NES-A may cause multiple defects in protein function leading to an Nmd- phenotype even when export is restored.

The accelerating expansion of online bioinformatics tools has profoundly impacted molecular biology, with such tools becoming integral to the modern life sciences. As a result, molecular biology laboratory education must train students to leverage bioinformatics in meaningful ways to be prepared for a spectrum of careers. Institutions of higher learning can benefit from a flexible and dynamic instructional paradigm that blends up-to-date bioinformatics training with best practices in molecular biology laboratory pedagogy. At North Carolina State University, the campus-wide interdisciplinary Biotechnology (BIT) Program has developed cutting-edge, flexible, inquiry-based Molecular Biology Laboratory Education Modules (MBLEMs). MBLEMs incorporate relevant online bioinformatics tools using evidenced-based pedagogical practices and in alignment with national learning frameworks. Students in MBLEMs engage in the most recent experimental developments in modern biology (e.g., CRISPR, metagenomics) through the strategic use of bioinformatics, in combination with wet-lab experiments, to address research questions. MBLEMs are flexible educational units that provide a menu of inquiry-based laboratory exercises that can be used as complete courses or as parts of existing courses. As such, MBLEMs are designed to serve as resources for institutions ranging from community colleges to research-intensive universities, involving a diverse range of learners. Herein, we describe this new paradigm for biology laboratory education that embraces bioinformatics as a critical component of inquiry-based learning for undergraduate and graduate students representing the life sciences, the physical sciences, and engineering.

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