Mating type switching in yeast controlled by asymmetric localization of ASH1 mRNA

Discovery of a mRNA mitochondrial localization element in Saccharomyces cerevisiae by nonhomologous random recombination and in vivo selection

In budding yeast, over 100 nuclear-encoded mRNAs are localized to the mitochondria. The determinants of mRNA localization to the mitochondria are not well understood, and protein factors involved in this process have not yet been identified. To reveal the sequence determinants for mitochondrial localization in a comprehensive and unbiased manner, we generated highly diversified libraries of 3′ UTR regions from a known mitochondrially localized mRNA by nonhomologous random recombination (NRR) and subjected the resulting sequences to an in vivo selection that links cell survival to mitochondrial mRNA localization. When applied to the yeast ATP2 mRNA, this approach rapidly identified a 50-nt consensus motif, designated Min2, as well as two Min2-homologous regions naturally present downstream of the ATP2 stop codon, which are sufficient when appended to the 3′ end of various reporter mRNAs to induce mitochondrial localization. Site-directed mutagenesis of Min2 revealed primary and secondary structure elements that contribute to localization activity. In addition, the Min2 motif may facilitate the identification of proteins involved in this mode of establishing cellular asymmetry.

Halting a cellular production line: responses to ribosomal pausing during translation

Cellular protein synthesis is a complex polymerization process carried out by multiple ribosomes translating individual mRNAs. The process must be responsive to rapidly changing conditions in the cell that could cause ribosomal pausing and queuing. In some circumstances, pausing of a bacterial ribosome can trigger translational abandonment via the process of trans-translation, mediated by tmRNA (transfer-messenger RNA) and endonucleases. Together, these factors release the ribosome from the mRNA and target the incomplete polypeptide for destruction. In eukaryotes, ribosomal pausing can initiate an analogous process carried out by the Dom34p and Hbs1p proteins, which trigger endonucleolytic attack of the mRNA, a process termed mRNA no-go decay. However, ribosomal pausing can also be employed for regulatory purposes, and controlled translational delays are used to help co-translational folding of the nascent polypeptide on the ribosome, as well as a tactic to delay translation of a protein while its encoding mRNA is being localized within the cell. However, other responses to pausing trigger ribosomal frameshift events. Recent discoveries are thus revealing a wide variety of mechanisms used to respond to translational pausing and thus regulate the flow of ribosomal traffic on the mRNA population.

Myo4p is a monomeric myosin with motility uniquely adapted to transport mRNA

The yeast Saccharomyces cerevisiae uses two class V myosins to transport cellular material into the bud: Myo2p moves secretory vesicles and organelles, whereas Myo4p transports mRNA. To understand how Myo2p and Myo4p are adapted to transport physically distinct cargos, we characterize Myo2p and Myo4p in yeast extracts, purify active Myo2p and Myo4p from yeast lysates, and analyze their motility. We find several striking differences between Myo2p and Myo4p. First, Myo2p forms a dimer, whereas Myo4p is a monomer. Second, Myo4p generates higher actin filament velocity at lower motor density. Third, single molecules of Myo2p are weakly processive, whereas individual Myo4p motors are nonprocessive. Finally, Myo4p self-assembles into multi-motor complexes capable of processive motility. We show that the unique motility of Myo4p is not due to its motor domain and that the motor domain of Myo2p can transport ASH1 mRNA in vivo. Our results suggest that the oligomeric state of Myo4p is important for its motility and ability to transport mRNA.

ZBP2 facilitates binding of ZBP1 to beta-actin mRNA during transcription

Cytoplasmic mRNA localization regulates gene expression by spatially restricting protein translation. Recent evidence has shown that nuclear proteins (such as hnRNPs) are required to form mRNPs capable of cytoplasmic localization. ZBP1 and ZBP2, two hnRNP K homology domain-containing proteins, were previously identified by their binding to the zipcode, the sequence element necessary and sufficient for beta-actin mRNA localization. ZBP1 colocalizes with nascent beta-actin mRNA in the nucleus but is predominantly a cytoplasmic protein. ZBP2, in contrast, is predominantly nuclear. We hypothesized that the two proteins cooperate to localize beta-actin mRNA and sought to address where and how this might occur. We demonstrate that ZBP2, a homologue of the splicing factor KSRP, binds initially to nascent beta-actin transcripts and facilitates the subsequent binding of the shuttling ZBP1. ZBP1 then associates with the RNA throughout the nuclear export and cytoplasmic localization process.

mRNA trafficking in fungi

Fungal growth depends on active transport of macromolecules along the actin and/or microtubule cytoskeleton. Thereby, molecular cargo such as proteins, lipids, and mRNAs is targeted to defined subcellular regions. Active transport and localisation of mRNAs mediate localised translation so that protein synthesis occurs where protein function is required. In Saccharomyces cerevisiae, actomyosin-dependent mRNA trafficking participates in polar growth, asymmetric cell division, targeting of membrane proteins and import of mitochondrial proteins. The best-understood example is transport of ASH1 mRNA to the distal pole of the incipient daughter cell. cis-acting RNA sequences are recognised by the RNA-binding protein She2p that is connected via the adaptor She3p to the molecular motor Myo4p. Local translation at the poles of daughter cells causes Ash1p to accumulate predominantly in nuclei of daughter cells, where this transcription factor inhibits mating-type switching. Recently, it was also shown that actomyosin-dependent ASH1 mRNA transport directs tip cell-specific gene expression in filaments of the human pathogen Candida albicans. Furthermore, in the plant pathogen Ustilago maydis microtubule-dependent shuttling of the RNA-binding protein Rrm4 is essential to determine the axis of polarity in infectious filaments. Thus, mRNA trafficking appears to be universally required for polar growth of fungi.

Local activation of yeast ASH1 mRNA translation through phosphorylation of Khd1p by the casein kinase Yck1p

In S. cerevisiae, the ASH1 mRNA is localized at the bud tip of late-anaphase cells, resulting in the exclusive sorting of Ash1p to the daughter cell nucleus. While the mechanism behind the localization of this transcript has been well studied, the regulation of its translation is still poorly understood. We now report that the RNA binding protein Khd1 interacts with the ASH1 mRNA localization element E1 and with the C-terminal domain of eIF4G1 to regulate the translation of this transcript. Khd1p reduces translation initiation on the ASH1 mRNA and diminishes Ash1p leakage into the mother cell nucleus. Furthermore, we show that the casein kinase Yck1p phosphorylates Khd1p at the plasma membrane, disrupting the Khd1p-RNA complex and releasing its translational repression on the ASH1 mRNA. This study reveals how, by linking mRNA sorting and translational activation, Khd1p and Yck1p regulate the spatiotemporal expression of a cell fate determinant.

Comparative analysis of structured RNAs in S. cerevisiae indicates a multitude of different functions

Background

Non-coding RNAs (ncRNAs) are an emerging focus for both computational analysis and experimental research, resulting in a growing number of novel, non-protein coding transcripts with often unknown functions. Whole genome screens in higher eukaryotes, for example, provided evidence for a surprisingly large number of ncRNAs. To supplement these searches, we performed a computational analysis of seven yeast species and searched for new ncRNAs and RNA motifs.

Results

A comparative analysis of the genomes of seven yeast species yielded roughly 2800 genomic loci that showed the hallmarks of evolutionary conserved RNA secondary structures. A total of 74% of these regions overlapped with annotated non-coding or coding genes in yeast. Coding sequences that carry predicted structured RNA elements belong to a limited number of groups with common functions, suggesting that these RNA elements are involved in post-transcriptional regulation and/or cellular localization. About 700 conserved RNA structures were found outside annotated coding sequences and known ncRNA genes. Many of these predicted elements overlapped with UTR regions of particular classes of protein coding genes. In addition, a number of RNA elements overlapped with previously characterized antisense transcripts. Transcription of about 120 predicted elements located in promoter regions and other, previously un-annotated, intergenic regions was supported by tiling array experiments, ESTs, or SAGE data.

Conclusion

Our computational predictions strongly suggest that yeasts harbor a substantial pool of several hundred novel ncRNAs. In addition, we describe a large number of RNA structures in coding sequences and also within antisense transcripts that were previously characterized using tiling arrays.

mRNAs encoding polarity and exocytosis factors are cotransported with the cortical endoplasmic reticulum to the incipient bud in Saccharomyces cerevisiae

Polarized growth in the budding yeast Saccharomyces cerevisiae depends upon the asymmetric localization and enrichment of polarity and secretion factors at the membrane prior to budding. We examined how these factors (i.e., Cdc42, Sec4, and Sro7) reach the bud site and found that their respective mRNAs localize to the tip of the incipient bud prior to nuclear division. Asymmetric mRNA localization depends upon factors that facilitate ASH1 mRNA localization (e.g., the 3' untranslated region, She proteins 1 to 5, Puf6, actin cytoskeleton, and a physical association with She2). mRNA placement precedes protein enrichment and subsequent bud emergence, implying that mRNA localization contributes to polarization. Correspondingly, mRNAs encoding proteins which are not asymmetrically distributed (i.e., Snc1, Mso1, Tub1, Pex3, and Oxa1) are not polarized. Finally, mutations which affect cortical endoplasmic reticulum (ER) entry and anchoring in the bud (myo4Delta, sec3Delta, and srp101) also affect asymmetric mRNA localization. Bud-localized mRNAs, including ASH1, were found to cofractionate with ER microsomes in a She2- and Sec3-dependent manner; thus, asymmetric mRNA transport and cortical ER inheritance are connected processes in yeast.

A genomic integration method to visualize localization of endogenous mRNAs in living yeast

mRNA localization may be an important determinant for protein localization. We describe a simple PCR-based genomic-tagging strategy (m-TAG) that uses homologous recombination to insert binding sites for the RNA-binding MS2 coat protein (MS2-CP) between the coding region and 3' untranslated region (UTR) of any yeast gene. Upon coexpression of MS2-CP fused with GFP, we demonstrate the localization of endogenous mRNAs (ASH1, SRO7, PEX3 and OXA1) in living yeast (Saccharomyces cerevisiae).

Why cells move messages: the biological functions of mRNA localization

RNA localization is a widespread mechanism that allows cells to spatially control protein function by determining their sites of synthesis. In embryos, localized mRNAs are involved in morphogen gradient formation or the asymmetric distribution of cell fate determinants. In somatic cell types, mRNA localization contributes to local assembly of protein complexes or facilitates protein targeting to organelles. Long-distance transport of specific mRNAs in plants allows coordination of developmental processes between different plant organs. In this review, we will discuss the biological significance of different patterns of mRNA localization.

Mitochondria-associated yeast mRNAs and the biogenesis of molecular complexes

The coherence of mitochondrial biogenesis relies on spatiotemporally coordinated associations of 800-1000 proteins mostly encoded in the nuclear genome. We report the development of new quantitative analyses to assess the role of local protein translation in the construction of molecular complexes. We used real-time PCR to determine the cellular location of 112 mRNAs involved in seven mitochondrial complexes. Five typical cases were examined by an improved FISH protocol. The proteins produced in the vicinity of mitochondria (MLR proteins) were, almost exclusively, of prokaryotic origin and are key elements of the core construction of the molecular complexes; the accessory proteins were translated on free cytoplasmic polysomes. These two classes of proteins correspond, at least as far as intermembrane space (IMS) proteins are concerned, to two different import pathways. Import of MLR proteins involves both TOM and TIM23 complexes whereas non-MLR proteins only interact with the TOM complex. Site-specific translation loci, both outside and inside mitochondria, may coordinate the construction of molecular complexes composed of both nuclearly and mitochondrially encoded subunits.

Visual screening for localized RNAs in yeast revealed novel RNAs at the bud-tip

Several RNAs, including rRNAs, snRNAs, snoRNAs, and some mRNAs, are known to be localized at specific sites in a cell. Although methods have been established to visualize RNAs in a living cell, no large-scale visual screening of localized RNAs has been performed. In this study, we constructed a genomic library in which random genomic fragments were inserted downstream of U1A-tag sequences under a GAL1 promoter. In a living yeast cell, transcribed U1A-tagged RNAs were visualized by U1A-GFP that binds the RNA sequence of the U1A-tag. In this screening, many RNAs showed nuclear signals. Since the nuclear signals of some RNAs were not seen when the U1A-tag was connected to the 3' ends of the RNAs, it is suggested that their nuclear signals correspond to nascent transcripts on GAL1 promoter plasmids. Using this screening method, we successfully identified two novel localized mRNAs, CSR2 and DAL81, which showed bud-tip localization.

Coordination of endoplasmic reticulum and mRNA localization to the yeast bud

Localization of messenger RNAs and local protein synthesis contribute to asymmetric protein distribution not only of cytoplasmic but also of membrane or secreted proteins. Since synthesis of the latter protein classes occurs at the rough endoplasmic reticulum (ER), mRNA localization and distribution of ER should be coordinated. However, this coordination is not yet understood. In yeast, mRNA localization to the growing bud depends on the myosin Myo4p, its adaptor She3p, and the specific RNA binding protein She2p. These proteins mediate the localization of 23 mRNAs including ASH1 mRNA and mRNAs encoding membrane proteins. In addition, Myo4p and She3p are required for segregation of cortical ER to the bud. Here we show, with ASH1 mRNA as a model mRNA, that localizing messenger ribonucleoprotein (mRNP) particles comigrate with tubular ER structures to the bud, which requires the RNA binding protein She2p. Coordinated movement of the ASH1 mRNP with ER tubules but not their association with each other depends on Myo4p and She3p. Subcellular fractionation experiments demonstrate a cosegregation of ER and She2p, which is independent of Myo4p, She3p, or polysomes. Our findings suggest a novel model for mRNA localization that involves association of She2p and mRNPs with ER tubules and myosin-dependent cotransport of tubules and localized mRNPs.

Loc1p is required for efficient assembly and nuclear export of the 60S ribosomal subunit

Loc1p is an exclusively nuclear dsRNA-binding protein that affects the asymmetric sorting of ASH1 mRNA to daughter cells in Saccharomyces cerevisiae. In addition to the role in cytoplasmic RNA localization, Loc1p is a constituent of pre-60S ribosomes. Cells devoid of Loc1p display a defect in the synthesis of 60S ribosomal subunits, resulting in "half-mer" polyribosomes. Previously, we reported that Loc1p is located throughout the entire nucleus; however, upon closer inspection we discovered that Loc1p is enriched in the nucleolus consistent with a role in 60S ribosome biogenesis. Given that Loc1p is an RNA-binding protein and presumably functions in the assembly of 60S ribosomal subunits, we investigated if Loc1p has a role in rRNA processing and nuclear export of 60S subunits. Analysis of pre-rRNA processing revealed that loc1Delta cells exhibit gross defects in 25S rRNA synthesis, specifically a delay in processing at sites A0, A1 and A2 in 35S pre-rRNA. Furthermore, loc1Delta cells exhibit nuclear export defects for 60S ribosomal subunits, again, consistent with a role for Loc1p in the assembly of 60S ribosomal subunits. It is attractive to hypothesize that the two phenotypes associated with loc1Delta cells, namely altered ASH1 mRNA localization and ribosome biogenesis, are not mutually exclusive, but that ribosome biogenesis directly impacts mRNA localization.

The temporal and spatial expression pattern of Myosin Va, Vb and VI in the mouse ovary

There are 16 classes of unconventional myosins. Class V myosins have been shown to be involved in transporting cargo to and from the cell periphery. Class VI myosins have also been shown to transport cargo from the cell periphery, although it seems that these proteins have many roles which include the mediation of cell migration and stereocillia stabilisation. With the requirement of myosin VI for Drosophila oogenesis, the localised expression of Myosin V in the developing egg chamber and recent mounting evidence which links myosin VI to the migration of human ovarian cancer cell lines, we wanted to investigate the expression pattern of these two myosin classes in the normal mouse ovary. Here we show that these myosins are expressed, localised and regulated within the oocyte and granulosa cells of the developing mouse follicle.

Miranda couples oskar mRNA/Staufen complexes to the bicoid mRNA localization pathway

The double-stranded RNA binding protein Staufen is required for the microtubule-dependent localization of bicoid and oskar mRNAs to opposite poles of the Drosophila oocyte and also mediates the actin-dependent localization of prospero mRNA during the asymmetric neuroblast divisions. The posterior localization of oskar mRNA requires Staufen RNA binding domain 2, whereas prospero mRNA localization mediated the binding of Miranda to RNA binding domain 5, suggesting that different Staufen domains couple mRNAs to distinct localization pathways. Here, we show that the expression of Miranda during mid-oogenesis targets Staufen/oskar mRNA complexes to the anterior of the oocyte, resulting in bicaudal embryos that develop an abdomen and pole cells instead of the head and thorax. Anterior Miranda localization requires microtubules, rather than actin, and depends on the function of Exuperantia and Swallow, indicating that Miranda links Staufen/oskar mRNA complexes to the bicoid mRNA localization pathway. Since Miranda is expressed in late oocytes and bicoid mRNA localization requires the Miranda-binding domain of Staufen, Miranda may play a redundant role in the final step of bicoid mRNA localization. Our results demonstrate that different Staufen-interacting proteins couple Staufen/mRNA complexes to distinct localization pathways and reveal that Miranda mediates both actin- and microtubule-dependent mRNA localization.

Role of cell cycle-regulated expression in the localized incorporation of cell wall proteins in yeast

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 G1 and S/G2 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/G2 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.

Drosophila myosin V is required for larval development and spermatid individualization

Class V myosins are multifunctional molecular motors implicated in vesicular traffic, RNA transport, and mechanochemical coupling of the actin and microtubule-based cytoskeletons. To assess the function of the single myosin V gene in Drosophila (MyoV), we have characterized both deletion and truncation alleles. Mutant animals exhibit no detectable defects during embryogenesis but are delayed in larval development; most die prior to 3rd instar. MyoV protein is widely distributed; however, there are no obvious cytological defects in mutant larval tissues where MyoV was normally highly expressed. Of the few adult MyoV mutant escapers, females were fertile but males were not. We examined the expression of MyoV during spermatogenesis. MyoV was associated with membranes, microtubule, and actin structures required for spermatid maturation; MyoV was strongly associated with the sperm nuclei during the maturation of the actin-rich investment cones that package spermatids in individual membranes. In MyoV mutant escaper males, the early stages of spermatogenesis were normal; however, in the later stages, the investment cones stained weakly for actin and their registration was disrupted; no mature sperm were produced. Our results suggest that MyoV contributes to the formation of the actin-based investment cones and acts to coordinate and/or anchor these structures and other components of the individualization complex.

Dynamics and inheritance of the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of a polygonal array of interconnected tubules and sheets that spreads throughout the eukaryotic cell and is contiguous with the nuclear envelope. This elaborate structure is created and maintained by a constant remodeling process that involves the formation of new tubules, their cytoskeletal transport and homotypic fusion. Since the ER is a large, single-copy organelle, it must be actively segregated into daughter cells during cell division. Recent analysis in budding yeast indicates that ER inheritance involves the polarized transport of cytoplasmic ER tubules into newly formed buds along actin cables by a type V myosin. The tubules then become anchored to a site at the bud tip and this requires the Sec3p subunit of the exocyst complex. The ER is then propagated along the cortex of the bud to yield a cortical ER structure similar to that of the mother cell. In animal cells, the ER moves predominantly along microtubules, whereas actin fibers serve a complementary role. It is not yet clear to what extent the other components controlling ER distribution in yeast might be conserved in animal cells.

Identification of a conserved RNA motif essential for She2p recognition and mRNA localization to the yeast bud

In Saccharomyces cerevisiae, over twenty mRNAs localize to the bud tip of daughter cells, playing roles in processes as different as mating type switching and plasma membrane targeting. The localization of these transcripts depends on interactions between a cis-acting localization element(s) or zipcodes and the RNA-binding protein She2p. While previous studies identified four different localization elements in the bud-localized ASH1 mRNA, the main determinants for She2p recognition are still unknown. To investigate the RNA-binding specificity of She2p, we isolated She2p-binding RNAs by in vivo selection from libraries of partially randomized ASH1 localization elements. The RNAs isolated contained a similar loop-stem-loop structure with a highly conserved CGA triplet in one loop and a single conserved cytosine in the other loop. Mutating these conserved nucleotides or the stem separating them resulted in the loss of She2p binding and in the delocalization of a reporter mRNA. Using this information, we identified the same RNA motif in two other known bud-localized transcripts, suggesting that this motif is conserved among bud-localized mRNAs. These results show that mRNAs with zipcodes lacking primary sequence similarity can rely on a few conserved nucleotides properly oriented in their three-dimensional structure in order to be recognized by the same localization machinery.

Developmental and regional expression and localization of mRNAs encoding proteins involved in RNA translocation

RNA localization is a regulated component of gene expression of fundamental importance in development and differentiation. Several RNA binding proteins involved in RNA localization during development in Drosophila have been identified, of which Y14, Mago, Pumilio, and IMP-1 are known to be expressed in adult mammalian intestine. The present study was undertaken to define the developmental and regional expression of these proteins, as well as Staufen-1, in mouse intestinal cells and in other tissues and cell lines using RT-PCR, and localization using in situ hybridization and immunohistochemistry. Staufen-1, Y14, Mago-m, and Pumilio-1 were expressed in intestinal epithelial cells of both villus and crypt and in Caco-2 and IEC-6 cells. In contrast, expression of IMP-1 was age- and region-specific, showing clear expression in distal fetal and newborn intestine, but very low or no expression in adult. The mRNAs were cytosolic, with more apical than basal expression in enterocytes. Staufen protein showed a similar localization pattern to that of its cognate mRNA. Overall, the data suggest an essential role for these proteins in intestinal cells. Age and regional expression of IMP-1 may indicate a role in regulation of site-specific translation of intestinal genes or in RNA localization.

Inp1p is a peroxisomal membrane protein required for peroxisome inheritance in Saccharomyces cerevisiae

Cells have evolved molecular mechanisms for the efficient transmission of organelles during cell division. Little is known about how peroxisomes are inherited. Inp1p is a peripheral membrane protein of peroxisomes of Saccharomyces cerevisiae that affects both the morphology of peroxisomes and their partitioning during cell division. In vivo 4-dimensional video microscopy showed an inability of mother cells to retain a subset of peroxisomes in dividing cells lacking the INP1 gene, whereas cells overexpressing INP1 exhibited immobilized peroxisomes that failed to be partitioned to the bud. Overproduced Inp1p localized to both peroxisomes and the cell cortex, supporting an interaction of Inp1p with specific structures lining the cell periphery. The levels of Inp1p vary with the cell cycle. Inp1p binds Pex25p, Pex30p, and Vps1p, which have been implicated in controlling peroxisome division. Our findings are consistent with Inp1p acting as a factor that retains peroxisomes in cells and controls peroxisome division. Inp1p is the first peroxisomal protein directly implicated in peroxisome inheritance.

RNA localization in yeast: moving towards a mechanism

RNA localization is a widely utilized strategy employed by cells to spatially restrict protein function. In Saccharomyces cerevisiae asymmetric sorting of mRNA to the bud has been reported for at least 24 mRNAs. The mechanism by which the mRNAs are trafficked to the bud, illustrated by ASH1 mRNA, involves recognition of cis-acting localization elements present in the mRNA by the RNA-binding protein, She2p. The She2p/mRNA complex subsequently associates with the myosin motor protein, Myo4p, through an adapter, She3p. This ribonucleoprotein complex is transported to the distal tip of the bud along polarized actin cables. While the mechanism by which ASH1 mRNA is anchored at the bud tip is unknown, current data point to a role for translation in this process, and the rate of translation of Ash1p during the transport phase is regulated by the cis-acting localization elements. Subcellular sorting of mRNA in yeast is not limited to the bud; certain mRNAs corresponding to nuclear-encoded mitochondrial proteins are specifically sorted to the proximity of mitochondria. Analogous to ASH1 mRNA localization, mitochondrial sorting requires cis-acting elements present in the mRNA, though trans-acting factors involved with this process remain to be identified. This review aims to discuss mechanistic details of mRNA localization in S. cerevisiae.

Moving messages: the intracellular localization of mRNAs

mRNA localization is a common mechanism for targeting proteins to regions of the cell where they are required. It has an essential role in localizing cytoplasmic determinants, controlling the direction of protein secretion and allowing the local control of protein synthesis in neurons. New methods for in vivo labelling have revealed that several mRNAs are transported by motor proteins, but how most mRNAs are coupled to these proteins remains obscure.

Daughter-specific repression of Saccharomyces cerevisiae HO: Ash1 is the commander

The GATA-1-like factor Ash1 is a repressor of the HO gene, which encodes an endonuclease that is responsible for mating-type switching in the yeast Saccharomyces cerevisiae. A multi-step programme, which involves a macromolecular protein complex, the secondary structure of ASH1 mRNA and the cell cytoskeleton, enables Ash1 to asymmetrically localize to the daughter cell nucleus in late anaphase and to repress HO transcription. The resulting Ash1 activity prevents the daughter cell from switching mating type. How does Ash1 inhibit transcription of HO exclusively in the daughter cell? In this review, a speculative model is proposed and discussed. Through its action as a daughter-specific repressor, Ash1 can be considered to be an ancestral regulator of cell fate in eukaryotes.

Perinuclear localization of slow troponin C m RNA in muscle cells is controlled by a cis-element located at its 3' untranslated region

The process of mRNA localization within a specific cytoplasmic region is an integral aspect of the regulation of gene expression. Furthermore, colocalization of mRNAs and their respective translation products may facilitate the proper assembly of multi-subunit complexes like the thick and thin filaments of muscle. This postulate was tested by investigating the cytoplasmic localization of three mRNAs-the alpha-actin, slow troponin C (sTnC), and slow troponin I (sTnI), which encode different poly-peptide partners of the thin filament. Using in situ hybridization we showed that all three thin filament mRNAs are localized in the perinuclear cytoplasm of cultured C2C12 muscle cells. Their localization differs from that of the nonmuscle beta-actin mRNA, which is localized in the peripheral region of both proliferating nondifferentiated myoblasts and the differentiated myocytes. Analysis of the localization signal of the sTnC mRNA showed that a 40-nucleotide-long region of the sTnC mRNA 3' UTR is sufficient to confer the perinuclear localization on a heterologous reporter beta-Gal mRNA. This localization signal showed tissue specificity and worked only in the differentiated myocytes, but not in the proliferating myoblasts or in HeLa cells. The predicted secondary structure of the localization signal suggests the presence of multiple stem and loop structures in this region of the 3' UTR. Mutations within the stem region of the localization signal, which abolish the base pairing in this region, significantly reduced its perinuclear mRNA localization activity. Using UV-induced photo-cross-linking of RNA and proteins we found that a myotube-specific 42-kDa polypeptide binds to the localization signal.

Neuronal activity regulates the developmental expression and subcellular localization of cortical BDNF mRNA isoforms in vivo

Activity-dependent changes in BDNF expression have been implicated in developmental plasticity. Although its expression is widespread in visual cortex, developmental regulation of its different transcripts by visual experience has not been investigated. Here, we investigated the cellular expression of different BDNF transcripts in rat visual cortex during postnatal development. We found that transcripts I and II are expressed only in adults but III and IV are expressed from early postnatal stage. Total BDNF mRNA is expressed throughout the age groups. Transcripts III and IV show a differential intracellular localization, while former was detected only in cell bodies, latter is present both in cell bodies and dendritic processes. Inhibition of visual activity decreases the levels of exons, with exon IV transcript almost disappearing from dendrites. In vitro experiments also confirmed the above results, indicating activity-dependent regulation of different BDNF promoters with specific temporal and cellular patterns of expression in developing visual cortex.

Recognition of the bcd mRNA localization signal in Drosophila embryos and ovaries

The process of mRNA localization, often used for regulation of gene expression in polarized cells, requires recognition of cis-acting signals by components of the localization machinery. Many known RNA signals are active in the contexts of both the Drosophila ovary and the blastoderm embryo, suggesting a conserved recognition mechanism. We used variants of the bicoid mRNA localization signal to explore recognition requirements in the embryo. We found that bicoid stem-loop IV/V, which is sufficient for ovarian localization, was necessary but not sufficient for full embryonic localization. RNAs containing bicoid stem-loops III/IV/V did localize within the embryo, demonstrating a requirement for dimerization and other activities supplied by stem-loop III. Protein complexes that bound specifically to III/IV/V and fushi tarazu localization signals copurified through multiple fractionation steps, suggesting that they are related. Binding to these two signals was competitive but not equivalent. Thus, the binding complexes are not identical but appear to have some components in common. We have proposed a model for a conserved mechanism of localization signal recognition in multiple contexts.

The Saccharomyces cerevisiae recombination enhancer biases recombination during interchromosomal mating-type switching but not in interchromosomal homologous recombination

Haploid Saccharomyces can change mating type through HO-endonuclease cleavage of an expressor locus, MAT, followed by gene conversion using one of two repository loci, HML or HMR, as donor. The mating type of a cell dictates which repository locus is used as donor, with a cells using HML and alpha cells using HMR. This preference is established in part by RE, a locus on the left arm of chromosome III that activates the surrounding region, including HML, for recombination in a cells, an activity suppressed by alpha 2 protein in alpha cells. We have examined the ability of RE to stimulate different forms of interchromosomal recombination. We found that RE exerted an effect on interchromosomal mating-type switching and on intrachromosomal homologous recombination but not on interchromosomal homologous recombination. Also, even in the absence of RE, MAT alpha still influenced donor preference in interchromosomal mating-type switching, supporting a role of alpha 2 in donor preference independent of RE. These results suggest a model in which RE affects competition between productive and nonproductive recombination outcomes. In interchromosome gene conversion, RE enhances both productive and nonproductive pathways, whereas in intrachromosomal gene conversion and mating-type switching, RE enhances only the productive pathway.

She2p Is a Novel RNA Binding Protein with a Basic Helical Hairpin Motif

Selective transport of mRNAs in ribonucleoprotein particles (mRNP) ensures asymmetric distribution of information within and among eukaryotic cells. Actin-dependent transport of ASH1 mRNA in yeast represents one of the best-characterized examples of mRNP translocation. Formation of the ASH1 mRNP requires recognition of zip code elements by the RNA binding protein She2p. We determined the X-ray structure of She2p at 1.95 A resolution. She2p is a member of a previously unknown class of nucleic acid binding proteins, composed of a single globular domain with a five alpha helix bundle that forms a symmetric homodimer. After demonstrating potent, dimer-dependent RNA binding in vitro, we mapped the RNA binding surface of She2p to a basic helical hairpin in vitro and in vivo and present a mechanism for mRNA-dependent initiation of ASH1 mRNP complex assembly.

Selective gene expression in multigene families from yeast to mammals

Exclusive gene expression, where only one member of a gene or gene cassette family is selected for expression, plays an important role in the establishment of cell identity in several biological systems. Here, we compare four such systems: mating-type switching in fission and budding yeast, where cells choose between expressing one of the two different mating-type cassettes, and immunoglobulin and odorant receptor gene expression in mammals, where the number of gene choices is substantially higher. The underlying mechanisms that establish this selective expression pattern in each system differ in almost every detail. In all four systems, once a successful gene activation event has taken place, a feedback mechanism affects the fate of the cell. In the mammalian systems, feedback is mediated by the expressed cell surface receptor to ensure monoallelic gene expression, whereas in the yeasts, the expressed gene cassette at the mating-type locus affects donor choice during the subsequent switching event.

ASH1 mRNA Anchoring Requires Reorganization of the Myo4p-She3p-She2p Transport Complex

One mechanism by which cells post-transcriptionally regulate gene expression is via intercellular and intracellular sorting of mRNA. In Saccharomyces cerevisiae, the localization of ASH1 mRNA to the distal tip of budding cells results in the asymmetric sorting of Ash1p to daughter cell nuclei. Efficient localization of ASH1 mRNA depends upon the activity of four cis-acting localization elements and also upon the activity of trans-factors She2p, She3p, and Myo4p. She2p, She3p, and Myo4p have been proposed to form an ASH1 mRNA localization particle. She2p directly and specifically binds each of the four ASH1 cis-acting localization elements, whereas She3p has been hypothesized to function as an adaptor by recruiting the She2p-mRNA complex to Myo4p, a type V myosin. The Myo4p-She3p-She2p heterotrimeric protein complex has been proposed to localize mRNA to daughter cells using polarized actin cables. Here we demonstrate that whereas the predicted Myo4p-She3p-She2p heterotrimeric complex forms in vivo, it represents a relatively minor species compared with the Myo4p-She3p complex. Furthermore, contrary to a prediction of the heterotrimeric complex model for ASH1 mRNA localization, ASH1 mRNA artificially tethered to She2p is not localized. Upon closer examination, we found that mRNA tightly associated with She2p is transported to daughter cells but is not properly anchored at the bud tip. These results are consistent with a model whereby anchoring of ASH1 mRNA requires molecular remodeling of the Myo4p-She3p-She2p heterotrimeric complex, a process that is apparently altered when mRNA is artificially tethered to She2p.

Arf1p provides an unexpected link between COPI vesicles and mRNA in Saccharomyces cerevisiae

The small GTPase Arf1p is involved in different cellular processes that require its accumulation at specific cellular locations. The recruitment of Arf1p to distinct points of action might be achieved by association of Arf1p with different proteins. To identify new interactors of Arf1p, we performed an affinity chromatography with GTP- or GDP-bound Arf1p proteins. A new interactor of Arf1p-GTP was identified as Pab1p, which binds to the polyA-tail of mRNAs. Pab1p was found to associate with purified COPI-coated vesicles generated from Golgi membranes in vitro. The stability of the Pab1p-Arf1p complex depends on the presence of mRNA. Both symmetrically distributed mRNAs as well as the asymmetrically localized ASH1 mRNA are found in association with Arf1p. Remarkably, Arf1p and Pab1p are both required to restrict ASH1 mRNA to the bud tip. Arf1p and coatomer play an unexpected role in localizing mRNA independent and downstream of the SHE machinery. Hereby acts the SHE machinery in long-range mRNA transport, whereas COPI vesicles could act as short-range and localization vehicles. The endoplasmic reticulum (ER)-Golgi shuttle might be involved in concentrating mRNA at the ER.

Differential cytoplasmic mRNA localisation adjusts pair-rule transcription factor activity to cytoarchitecture in dipteran evolution

Establishment of segmental pattern in the Drosophila syncytial blastoderm embryo depends on pair-rule transcriptional regulators. mRNA transcripts of pair-rule genes localise to the apical cytoplasm of the blastoderm via a selective dynein-based transport system and signals within their 3′-untranslated regions. However, the functional and evolutionary significance of this process remains unknown. We have analysed subcellular localisation of mRNAs from multiple dipteran species both in situ and by injection into Drosophila embryos. We find that although localisation of wingless transcripts is conserved in Diptera, localisation of even-skipped and hairy pair-rule transcripts is evolutionarily labile and correlates with taxon-specific changes in positioning of nuclei. We show in Drosophila that localised pair-rule transcripts target their proteins in close proximity to the nuclei and increase the reliability of the segmentation process by augmenting gene activity. Our data suggest that mRNA localisation signals in pair-rule transcripts affect nuclear protein uptake and thereby adjust gene activity to a variety of dipteran blastoderm cytoarchitectures.

[RNA localization in the cytoplasm]

RNA localization in subcytoplasmic areas is a process known for more than twenty years, and more than a hundred RNAs have now been shown to be spatially regulated. In most cases, RNA localization is involved in cell polarity, either by reading spatial clues and translating them into a spatial regulation of gene expression, or more directly by controlling cytoskeletal polarity. In this review, the various functions of RNA localization will be presented, and by analyzing two examples, Ash1 mRNA in yeast and retroviral genomic RNAs in mammals, the reader will be taken step by step into the detailed mechanisms of this fascinating process.

Quantitative measurement of mRNA at different loci within an individual living cell

Asymmetric localizations of cellular proteins and mRNAs are important for cell functions such as division, differentiation and development. The localization of specific mRNA generates cell polarity by controlling the translation sites of specific proteins and thereby restricting their locations to appropriate cellular regions. We have previously reported a novel method based on atomic force microscopy (AFM) for examining gene expression in a single living cell without killing or destroying it. An AFM tip was inserted into a living cell to extract mRNAs, which were analyzed after multiplication by RT-PCR and quantitative PCR. By applying this method, in this study we performed quantitative measurement of mRNA at different loci within individual living cells.

A new yeast PUF family protein, Puf6p, represses ASH1 mRNA translation and is required for its localization

In yeast Saccharomyces cerevisiae, Ash1p, a protein determinant for mating-type switching, is segregated within the daughter cell nucleus to establish asymmetry of HO expression. The accumulation of Ash1p results from ASH1 mRNA that is sorted as a ribonucleoprotein particle (mRNP or locasome) to the distal tip of the bud where translation occurs. To study the mechanism regulating ASH1 mRNA translation, we isolated the ASH1 locasome and characterized the associated proteins by MALDI-TOF. One of these proteins was Puf6p, a new member of the PUF family of highly conserved RNA-binding proteins such as Pumilio in Drosophila, responsible for translational repression, usually to effect asymmetric expression. Puf6p-bound PUF consensus sequences in the 3'UTR of ASH1 mRNA and repressed the translation of ASH1 mRNA both in vivo and in vitro. In the puf6 Delta strain, asymmetric localization of both Ash1p and ASH1 mRNA were significantly reduced. We propose that Puf6p is a protein that functions in the translational control of ASH1 mRNA, and this translational inhibition is necessary before localization can proceed.

Involvement of the late secretory pathway in actin regulation and mRNA transport in yeast

Both the delivery of secretory vesicles and asymmetric distribution of mRNA to the bud are dependent upon the actin cytoskeleton in yeast. Here we examined whether components of the exocytic apparatus play a role in mRNA transport. By screening secretion mutants in situ and in vivo, we found that all had an altered pattern of ASH1 mRNA localization. These included alleles of CDC42 and RHO3 (cdc42-6 and rho3-V51) thought to regulate specifically the fusion of secretory vesicles but were found to affect strongly the cytoskeleton as well. Most interestingly, mutations in late secretion-related genes not directly involved in actin regulation also showed substantial alterations in ASH1 mRNA distribution. These included mutations in genes encoding components of the exocyst (SEC10 and SEC15), SNARE regulatory proteins (SEC1, SEC4, and SRO7), SNAREs (SEC9 and SSO1/2), and proteins involved in Golgi export (PIK1 and YPT31/32). Importantly, prominent defects in the actin cytoskeleton were observed in all of these strains, thus implicating a known causal relationship between the deregulation of actin and the inhibition of mRNA transport. Our novel observations suggest that vesicular transport regulates the actin cytoskeleton in yeast (and not just vice versa) leading to subsequent defects in mRNA transport and localization.

Posttranscriptional regulation of HO expression by the Mkt1-Pbp1 complex

Cells of budding yeast give rise to mother and daughter cells, which differ in that only mother cells express the HO endonuclease gene and are thereby able to switch mating types. In this study, we identified the MKT1 gene as a positive regulator of HO expression. The MKT1 gene encodes a protein with two domains, XPG-N and XPG-I, which are conserved among a family of nucleases, including human XPG endonuclease. Loss of MKT1 had little effect on HO mRNA levels but resulted in decreased protein levels. This decrease was dependent on the 3' untranslated region of the HO transcript. We screened for proteins that associate with Mkt1 and isolated Pbp1, a protein that is known to associate with Pab1, a poly(A)-binding protein. Loss of PBP1 resembles an mkt1 Delta deletion, causing decreased expression of HO at the posttranscriptional level. Mkt1 and Pbp1 cosedimented with polysomes in sucrose gradients, with Mkt1 distribution in the polysomes dependent on Pbp1, but not vice versa. These observations suggest that a complex of Mkt1 and Pbp1 regulates the translation of HO mRNA.

mRNA localisation gets more complex

Recent advances in techniques for visualising mRNA movement in living cells have led to rapid progress in understanding the mechanism of mRNA localisation in the cytoplasm. There is an emerging consensus that in many cases the mRNA signals that determine intracellular destination are more complex and difficult to define than was first anticipated. Furthermore, the transacting factors that interpret the mRNA signals are numerous and their combinations change during the life of an mRNA, perhaps allowing the selection of many sub-destinations in the cell. Lastly, an emerging theme over the past few years is that many proteins that determine the destinations of mRNAs are recruited on nascent transcripts in the nucleus. They often function in many different processes in the biogenesis of mRNA and probably act in concert to provide specificity.

Regulation of local mRNA translation

Regulated local mRNA translation is one mechanism cells employ to concentrate proteins in particular locations. However, cells use many different strategies to accomplish this task; for example, some mRNAs are destroyed in regions where they are not wanted, other mRNAs are repressed in areas where their translation would be deleterious, and yet other mRNAs are transported, in a quiescent state, to the sites where their translation is activated. The importance of local translation cannot be overstated, for, depending on the species or cell type, it is required for cell division, establishment of mating type, development and memory formation.

A novel transport pathway for a yeast plasma membrane protein encoded by a localized mRNA

Generally, plasma membrane (PM) proteins are cotranslationally inserted into the endoplasmic reticulum (ER) and travel in vesicles via the Golgi apparatus to the PM. In the yeast Saccharomyces cerevisiae, the polytopic membrane protein Ist2p is encoded by an mRNA that is localized to the cortex of daughter cells. It has been suggested that IST2 mRNA localization leads to the accumulation of the protein at the PM of daughter cells. Since small- and medium-sized daughter cells only contain cortical, but not perinuclear ER, this implies the local translation of Ist2p specifically at the cortical ER. Here, we show that localization of constitutively expressed IST2 mRNA is required for delivery of Ist2p to the PM of daughter, but not mother cells and that it does not result in daughter-specific Ist2p accumulation. In contrast to a PM-located hexose transporter (Hxt1p) that follows the standard secretory pathway, the trafficking of Ist2p is independent of myosin-mediated vesicular transport. Furthermore, colocalization experiments in mutants of the secretory pathway demonstrate that trafficking of Ist2p does not require the classical secretory machinery. These data suggest the existence of a novel trafficking pathway connecting specialized domains of the ER with the PM.

Myosin Va and kinesin II motor proteins are concentrated in ribosomal domains (periaxoplasmic ribosomal plaques) of myelinated axons

Periaxoplasmic ribosomal plaques (PARPs) are discrete ribosome-containing domains distributed intermittently along the periphery of axoplasm in myelinated fibers. Thus, they are structural formations in which translational machinery is spatially organized to serve as centers of protein synthesis for local metabolic requirements and perhaps repair as well. Because of evidence that RNA is transported to putative PARP domains, involving both microtubule- and actin-based mechanisms, it was of interest to investigate whether cytoskeletal motor proteins exhibit a nonrandom localization within PARP domains. Axoplasm, from large Mauthner fibers and rat or rabbit spinal ventral nerve root fibers, removed from the myelin sheath in the form of an "axoplasmic whole-mount" was used for this analysis. PARP domains were identified either by specific immunofluorescence of rRNA, ribosomal P antigen, or by nonspecific RNA fluorescence using RNA binding dyes YOYO-1 or POPO-1. A polyclonal antibody (pAb) against the motor domain of myosin Va showed prominent nonrandom immunofluorescence labeling in PARP domains. Similarly, monoclonal antibodies (mAb) against kinesin KIF3A and a pan-specific antikinesin (mAb IBII) also showed a preponderant immunofluorescence in PARP domains. On the other hand, H2, a mAb antikinesin KIF5A, exhibited only random immunofluorescence labeling in axoplasm, as was also the case with pAb antidynein heavy chain immunofluorescence. Several possible explanations for these findings are considered, primary among which is targeted trafficking of translational machinery that results in local accumulation of motor proteins. Additional possibilities are trafficking functions intrinsic to the domain, and/or functions that govern dynamic organizational properties of PARPs.

Nuclear RNP complex assembly initiates cytoplasmic RNA localization

Cytoplasmic localization of mRNAs is a widespread mechanism for generating cell polarity and can provide the basis for patterning during embryonic development. A prominent example of this is localization of maternal mRNAs in Xenopus oocytes, a process requiring recognition of essential RNA sequences by protein components of the localization machinery. However, it is not yet clear how and when such protein factors associate with localized RNAs to carry out RNA transport. To trace the RNA-protein interactions that mediate RNA localization, we analyzed RNP complexes from the nucleus and cytoplasm. We find that an early step in the localization pathway is recognition of localized RNAs by specific RNA-binding proteins in the nucleus. After transport into the cytoplasm, the RNP complex is remodeled and additional transport factors are recruited. These results suggest that cytoplasmic RNA localization initiates in the nucleus and that binding of specific RNA-binding proteins in the nucleus may act to target RNAs to their appropriate destinations in the cytoplasm.

Xvelo1 uses a novel 75-nucleotide signal sequence that drives vegetal localization along the late pathway in Xenopus oocytes

Vegetally localized RNAs in Xenopus laevis oocytes are involved in the patterning of the early embryo as well as in cell fate specification. Here we report on the isolation and characterization of a novel, vegetally localized RNA in Xenopus oocytes termed Xvelo1. It encodes a protein of unknown biological function and it represents an antisense RNA for XPc1 over a length of more than 1.8 kb. Xvelo1 exhibits a localization pattern reminiscent of the late pathway RNAs Vg1 and VegT; it contains RNA localization elements (LE) which do not match with the consensus structural features as deduced from Vg1 and VegT LEs. Nevertheless, the protein binding pattern as observed for Xvelo1-LE in UV cross-linking experiments and coimmunoprecipitation assays is largely overlapping with the one obtained for Vg1-LE. These observations suggest that the structural features recognized by the protein machinery that drives localization of maternal mRNAs along the late pathway in Xenopus oocytes must be redefined.

Polarized distribution of intracellular components by class V myosins in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has three classes of myosins corresponding to three actin structures: class I myosin for endocytic actin structure, actin patches; class II myosin for contraction of the actomyosin contractile ring around the bud neck; and class V myosin for transport along a cable-like actin structure (actin cables), extending toward the growing cortex. Myo2p and Myo4p constitute respective class V myosins as the heavy chain and, like class V myosins in other organisms, function as actin-based motors for polarized distribution of organelles and intracellular molecules. Proper distribution of organelles is essential for autonomously replicating organelles that cannot be reproduced de novo, and is also quite important for other organelles to ensure their efficient segregation and proper positioning, even though they can be newly synthesized, such as those derived from endoplasmic reticulum. In the budding yeast, microtubule-based motors play limited roles in the distribution. Instead, the actin-based motor myosins, especially Myo2p, play a major role. Studies on Myo2p have revealed a wide variety of Myo2p cargo and Myo2p-interacting proteins and have established that Myo2p interacts with cargo and transfers it along actin cables. Moreover, recent findings suggest that Myo2p has another way to distribute cargo in that Myo2p conveys the attaching cargo along the actin track. Thus, the myosin have "dual paths" for distribution of a cargo. This dual path mechanism is proposed in the last section of this review.

Myo4p and She3p are required for cortical ER inheritance in Saccharomyces cerevisiae

Myo4p is a nonessential type V myosin required for the bud tip localization of ASH1 and IST2 mRNA. These mRNAs associate with Myo4p via the She2p and She3p proteins. She3p is an adaptor protein that links Myo4p to its cargo. She2p binds to ASH1 and IST2 mRNA, while She3p binds to both She2p and Myo4p. Here we show that Myo4p and She3p, but not She2p, are required for the inheritance of cortical ER in the budding yeast Saccharomyces cerevisiae. Consistent with this observation, we find that cortical ER inheritance is independent of mRNA transport. Cortical ER is a dynamic network that forms cytoplasmic tubular connections to the nuclear envelope. ER tubules failed to grow when actin polymerization was blocked with the drug latrunculin A (Lat-A). Additionally, a reduction in the number of cytoplasmic ER tubules was observed in Lat-A–treated and myo4Δ cells. Our results suggest that Myo4p and She3p facilitate the growth and orientation of ER tubules.

Nitrogen metabolism and virulence of Candida albicans require the GATA-type transcriptional activator encoded by GAT1

Nitrogen acquisition and metabolism is central to microbial growth. A conserved family of zinc-finger containing transcriptional regulators known as GATA-factors ensures efficient utilization of available nitrogen sources by fungi. GATA factors activate expression of nitrogen catabolic pathways when preferred nitrogen sources are absent or limiting, a phenomenon known as nitrogen catabolite repression. GAT1 of Candida albicans encodes a GATA-factor homologous to the AREA protein of Aspergillus nidulans and related transcription factors involved in nitrogen regulation. Two observations implicated GAT1 in nitrogen regulation. The growth of mutants lacking GAT1 was reduced when isoleucine, tyrosine or tryptophan were the sole source of nitrogen. Secondly, when cultured on a secondary nitrogen source, gat1Delta mutants were unable to activate expression of GAP1, UGA4 or DAL5, which were shown to be nitrogen regulated in C. albicans. This regulatory defect did not prevent filamentation of gat1Delta mutants in nitrogen repressing or non-repressing conditions, demonstrating that nitrogen catabolite repression does not influence dimorphism. The mutants were, however, highly attenuated in a murine model of disseminated candidiasis. Attenuation was not associated with any diminution of growth in serum or ability to utilize serum amino acids. The results indicate an important role for nitrogen regulation in the virulence of C. albicans.

Cellular Differentiation: The Violin Strikes up Another Tune

A switch in cellular identity in budding yeast requires the ubiquitin-dependent elimination of pre-existing master regulators encoded by the MAT locus. Failure to disassemble the prior state not only impairs the cell type transition but imparts a hybrid cellular fate. This theme will undoubtedly arise in many developmental and disease contexts.