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

Visualization of single RNA transcripts in situ

Fluorescence in situ hybridization (FISH) and digital imaging microscopy were modified to allow detection of single RNA molecules. Oligodeoxynucleotide probes were synthesized with five fluorochromes per molecule, and the light emitted by a single probe was calibrated. Points of light in exhaustively deconvolved images of hybridized cells gave fluorescent intensities and distances between probes consistent with single messenger RNA molecules. Analysis of beta-actin transcription sites after serum induction revealed synchronous and cyclical transcription from single genes. The rates of transcription initiation and termination and messenger RNA processing could be determined by positioning probes along the transcription unit. This approach extends the power of FISH to yield quantitative molecular information on a single cell.

Asymmetric cell division

With the recent identification of intrinsic cell-fate determinants for asymmetric cell division in several systems, biologists have begun to gain insight into the cellular mechanisms by which these determinants are preferentially segregated into one of the two daughter cells during mitosis so that the daughter cells acquire different fates.

Integrin binding and mechanical tension induce movement of mRNA and ribosomes to focal adhesions

The extracellular matrix (ECM) activates signalling pathways that control cell behaviour by binding to cell-surface integrin receptors and inducing the formation of focal adhesion complexes (FACs). In addition to clustered integrins, FACs contain proteins that mechanically couple the integrins to the cytoskeleton and to immobilized signal-transducing molecules. Cell adhesion to the ECM also induces a rapid increase in the translation of preexisting messenger RNAs. Gene expression can be controlled locally by targeting mRNAs to specialized cytoskeletal domains. Here we investigate whether cell binding to the ECM promotes formation of a cytoskeletal microcompartment specialized for translational control at the site of integrin binding. High-resolution in situ hybridization revealed that mRNA and ribosomes rapidly and specifically localized to FACs that form when cells bind to ECM-coated microbeads. Relocation of these protein synthesis components to the FAC depended on the ability of integrins to mechanically couple the ECM to the contractile cytoskeleton and on associated tension-moulding of the actin lattice. Our results suggest a new type of gene regulation by integrins and by mechanical stress which may involve translation of mRNAs into proteins near the sites of signal reception.

A homologue of the yeast SHE4 gene is essential for the transition between the syncytial and cellular stages during sexual reproduction of the fungus Podospora anserina

The Podospora anserina cro1 gene was identified as a gene required for sexual sporulation. Crosses homozygous for the cro1-1 mutation yield fruiting bodies which produce few asci due to the formation of giant plurinucleate cells instead of dikaryotic cells after fertilization. This defect does not impair karyogamy, but meioses of the resultant polyploid nuclei are most often abortive. Cytological studies suggest that the primary defect of the mutant is its inability to form septa between the daughter nuclei after each mitosis, a step specific for normal dikaryotic cell divisions. The cro1-1 mutant would thus be unable to leave the syncytial vegetative state while abiding by the meiotic programme. cro1-1 also shows defects in ascospore germination and growth rate. GFP-tagging of the CRO1 protein reveals that it is a cytosolic protein mainly expressed at the beginning of the dikaryotic stage and at the time of ascospore maturation. The CRO1 protein exhibits significant similarity to the SHE4 protein, which is required for asymmetric mating-type switching in budding yeast cells. Thus, a gene involved in asymmetric cell divisions in a unicellular organism plays a key role at the transition between the syncytial (vegetative) state and the cellular (sexual) state in a filamentous fungus.

FH proteins as cytoskeletal organizers

Regulation of cell shape is a poorly understood yet central issue in cell biology. Recent experiments indicate that FH proteins link cellular signalling pathways to changes in cell shape. Members of the FH protein family play essential roles in cytokinesis and in driving alterations in cell polarity. This review discusses the structure and function of these proteins and examines the evidence that they interact specifically with Rho GTPases and profilin to organize the actin-based cytoskeleton.

Staufen-dependent localization of prospero mRNA contributes to neuroblast daughter-cell fate

The generation of cellular diversity is essential in embryogenesis, especially in the central nervous system. During neurogenesis, cell interactions or asymmetric protein localization during mitosis can generate daughter cells with different fates. Here we describe the asymmetric localization of a messenger RNA and an RNA-binding protein that creates molecular and developmental differences between Drosophila neural precursors (neuroblasts) and their daughter cells, ganglion mother cells (GMCs). The prospero (pros) mRNA and the RNA-binding protein Staufen (Stau) are asymmetrically localized in mitotic neuroblasts and are specifically partitioned into the GMC, as is Pros protein. Stau is required for localization of pros RNA but not of Pros protein. Loss of localization of Stau or of pros RNA alters GMC development, but only in embryos with reduced levels of Pros protein, suggesting that pros RNA and Pros protein act redundantly to specify GMC fate. We also find that GMCs do not transcribe the pros gene, showing that inheritance of pros RNA and/or Pros protein from the neuroblast is essential for GMC specification.

Myosins: matching functions with motors

It is an exciting time to be studying myosins and their roles in the function of cells and organisms. Past efforts aimed at finding new members of this family have now given way to a focus on identifying individual functions for each motor protein. These actin-based motors are now known to be intimately involved in the following processes: neurosensory function; vesicle trafficking; determinant partitioning; and cortical function. The following article reviews the inroads made into the functions of myosins in these processes over the past several years.

Unconventional myosins in cell movement, membrane traffic, and signal transduction

In the past few years genetic, biochemical, and cytolocalization data have implicated members of the myosin superfamily of actin-based molecular motors in a variety of cellular functions including membrane trafficking, cell movements, and signal transduction. The importance of myosins is illustrated by the identification of myosin genes as targets for disease-causing mutations. The task at hand is to decipher how the multitude of myosins function at both the molecular and cellular level-a task facilitated by our understanding of myosin structure and function in muscle.

Application of mating type gene technology to problems in fungal biology

In ascomycetes, the single mating type locus (MAT) controls sexual development. This locus is structurally unusual because the two alternate forms ("alleles") are completely dissimilar sequences, encoding different transcription factors, yet they occupy the same chromosomal position. Recently developed procedures allow efficient cloning of MAT genes from a wide array of filamentous ascomycetes, thereby providing MAT-based technology for application to several ongoing issues in fungal biology. This article first outlines the basic nature of MAT genes, then addresses the following topics: efficient cloning of MAT genes; the unusual molecular characteristics of these genes; phylogenetics using MAT; the issues of why some fungi are self-sterile, others self-fertile, and yet others asexual; the long-standing mystery of possible mating type switching in filamentous fungi; and finally the evolutionary origins of pathogenic capability.

Localization of mRNAs to the oocyte is common in Drosophila ovaries

Patterns of mRNA accumulation are sometimes dictated post-transcriptionally. Striking examples of mRNA localized to subcellular sites within cells have been described in animal oocytes, a variety of somatic cells in both animals and plants, and most recently in yeast (St Johnston, 1995; Bouget et al., 1996; Long et al., 1997; Takizawa et al., 1997). Some of the best characterized of the localized mRNAs come from the Drosophila ovary, where localization of three mRNAs--bicoid, oskar and gurken--within the oocyte defines the anteroposterior and dorsoventral body axes (reviewed in St Johnston, 1995). About 20 other mRNAs display a related pattern of localization, in which they become concentrated in the oocyte after transcription in the adjacent and interconnected nurse cells. Despite considerable work on the roles and mechanisms of mRNA localization, there have been no clear indications of the prevalence of this phenomenon, even in the much studied Drosophila system. Here we address this issue. We have examined the distributions of a random collection of individual Drosophila ovarian mRNAs, and find that a significant fraction, almost a tenth, are localized to the oocyte.

Actin-binding verprolin is a polarity development protein required for the morphogenesis and function of the yeast actin cytoskeleton

Yeast verprolin, encoded by VRP1, is implicated in cell growth, cytoskeletal organization, endocytosis and mitochondrial protein distribution and function. We show that verprolin is also required for bipolar bud-site selection. Previously we reported that additional actin suppresses the temperature-dependent growth defect caused by a mutation in VRP1. Here we show that additional actin suppresses all known defects caused by vrp1-1 and conclude that the defects relate to an abnormal cytoskeleton. Using the two-hybrid system, we show that verprolin binds actin. An actin-binding domain maps to the LKKAET hexapeptide located in the first 70 amino acids. A similar hexapeptide in other acting-binding proteins was previously shown to be necessary for actin-binding activity. The entire 70- amino acid motif is conserved in novel higher eukaryotic proteins that we predict to be actin-binding, and also in the actin-binding proteins, WASP and N-WASP. Verprolin-GFP in live cells has a cell cycle-dependent distribution similar to the actin cortical cytoskeleton. In fixed cells hemagglutinin-tagged Vrp1p often co-localizes with actin in cortical patches. However, disassembly of the actin cytoskeleton using Latrunculin-A does not alter verprolin's location, indicating that verprolin establishes and maintains its location independent of the actin cytoskeleton. Verprolin is a new member of the actin-binding protein family that serves as a polarity development protein, perhaps by anchoring actin. We speculate that the effects of verprolin upon the actin cytoskeleton might influence mitochondrial protein sorting/function via mRNA distribution.

Mechanisms of asymmetric cell division during animal development

Recent years have seen the discovery of machineries for asymmetric cell division in a number of different organisms. The Inscuteable protein is a central component of such a machinery in Drosophila. Within dividing Drosophila neural precursor cells, Inscuteable directs both the orientation of the mitotic spindle and the asymmetric segregation of the proteins Numb, Prospero and Miranda into one of the two daughter cells. Numb can act by repressing signalling via the transmembrane receptor Notch, whereas Miranda localizes the transcription factor Prospero which initiates daughter cell specific gene expression. The identification of Numb homologs in other species has suggested that this machinery might be conserved from Drosophila to vertebrates.

The N terminus of the Drosophila Numb protein directs membrane association and actin-dependent asymmetric localization

Drosophila Numb is a membrane associated protein of 557 amino acids (aa) that localizes asymmetrically into a cortical crescent in mitotic neural precursor cells and segregates into one of the daughter cells, where it is required for correct cell fate specification. We demonstrate here that asymmetric localization but not membrane localization of Numb in Drosophila embryos is inhibited by latrunculin A, an inhibitor of actin assembly. We also show that deletion of either the first 41 aa or aa 41-118 of Numb eliminates both localization to the cell membrane and asymmetric localization during mitosis, whereas C-terminal deletions or deletions of central portions of Numb do not affect its subcellular localization. Fusion of the first 76 or the first 119 aa of Numb to beta-galactosidase results in a fusion protein that localizes to the cell membrane, but fails to localize asymmetrically during mitosis. In contrast, a fusion protein containing the first 227 aa of Numb and beta-galactosidase localizes asymmetrically during mitosis and segregates into the same daughter cell as the endogenous Numb protein, demonstrating that the first 227 aa of the Numb protein are sufficient for asymmetric localization.

Extrinsic cues, intrinsic cues and microfilaments regulate asymmetric protein localization in Drosophila neuroblasts

BACKGROUND

The Drosophila central nervous system develops from stem cell like precursors called neuroblasts, which divide unequally to bud off a series of smaller daughter cells called ganglion mother cells. Neuroblasts show cell-cycle-specific asymmetric localization of both RNA and proteins: at late interphase, prospero RNA and Inscuteable, Prospero and Staufen proteins are all apically localized; at mitosis, Inscuteable remains apical whereas prospero RNA, Prospero protein and Staufen protein form basal cortical crescents. Here we use in vitro culture of neuroblasts to investigate the role of intrinsic and extrinsic cues and the cytoskeleton in asymmetric localization of Inscuteable, Prospero and Staufen proteins.

RESULTS

Neuroblast cytokinesis is normal in vitro, producing a larger neuroblast and a smaller ganglion mother cell. Apical localization of Inscuteable, Prospero and Staufen in interphase neuroblasts is reduced or eliminated in vitro, but all three proteins are localized normally during mitosis (apical Inscuteable, basal Prospero and Staufen). Microfilament inhibitors result in delocalization of all three proteins. Inscuteable becomes uniform at the cortex, whereas Prospero and Staufen become cytoplasmic; inhibitor washout leads to recovery of microfilaments and asymmetric localization of all three proteins. Microtubule disruption has no effect on protein localization, but disruption of both microtubules and microfilaments results in cytoplasmic localization of Inscuteable.

CONCLUSIONS

Both extrinsic and intrinsic cues regulate protein localization in neuroblasts. Microfilaments, but not microtubules, are essential for asymmetric protein anchoring (and possibly localization) in mitotic neuroblasts. Our results highlight the similarity between Drosophila, Caenorhabditis elegans, vertebrates, plants and yeast: in all organisms, asymmetric protein or RNA localization and/or anchoring requires microfilaments.