- Email: [email protected]
Timing of fungal invasion using host's repining hormone as a signal
HFJ~.DLINES
Life w i t h o u t a kinase COKER, K. l., STAROS,J. V. and GUYER,C. A. (1994) A kinase-negative epidermal growth factor receptor that retains the capacity to stimulate DNA synthesis Proc. Natl Acad. Sci. USA 91, 6967-6971 The epidermal growth factor receptor (EGFR)is a transmembrane tyrosine kinase that, upon activation, autophosphorylates at specific residues, thus providing binding sites for signalling proteins with Src homology 2 (SH2) domains. Recruitment of these molecules at the plasma membrane turns on the Rasdependent mitogen-activated protein kinase (MAPK) cascade, causing cell proliferation. To investigate the importance of the kinase activity for the signalling properties of the EGFR,Coker et ol. mutated a highly conserved residue within the catalytic domain that is thought to participate in the phosphotransfer reaction. The partially purified receptor was unable to autophosphorylate or to phosphorylate exogenous substrates in vitro. However, in intact CHO cells (which lack endogenous EGFRs)transfected with the mutant gene, EGF induced a low level of tyrosine phosphorylation of the receptor. As suggested by the authors, this could be catalysed by an associated kinase. In these cells, EGF activated MAPK and DNA replication as efficiently as in cells expressing the wild-type receptor. In contrast, a kinase.defectivereceptor mutated at the ATP-binding site was capable of stimulating MAPK but not DNA synthesis. Therefore, different point mutations in the catalytic domain of the EGFRmay have distinct biological consequences in terms of signal transduction, depending on whether the receptor retains the ability to bind ATP. It has been shown that EGF can also activate a Ras.independent signalling pathway leading to the tyrosine phosphorylation and nuclear translocation of the transcription factor p91. One critical issue that should be addressed is whether these mutants are still able to activate this second pathway.
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388
Fungal attack of ripe fruit FLAISHMAN, M. A. and KOLATrUKUDY, P. E. (1994) Timing of fungal invasion usi~,g host's ripening hormone as a signal Proc. Not~Acad. Sci. USA 91, 6579-6583 Extensive crop losses occur after Iharvest. In the case of fruit, the spores of many fungal pathogens that attach to its surface during the growing sea',;on wait for the L:ruitto ripen before penetrating it and causing damage. How does the pathogen time its attack on the host? The results in this paper provide convincing evidence that for the Colletotrichum fungi that infect climacteric I'ruit (those that increase respiration and ethylene production at ripening) ethylene is the trigger. When spores of these fungi were placed in a chamber together (but not in contact) with climacteric ripening fruits, such as banana, tomato and avocado, they germinated and produced appressoria (infection structures). This observation suggested that a gaseous metabolite released during fruit ripening is the signal sensed by the pathogen. Further studies demonstrated that the metabollte is ethylene. Fruit that do not release ethylene during ripening, such as orange or a transgenic tomato, did not induce appressoria formation. Moreover, incubation with physiological concentrations ..c ethylene, ethephon (an ethylene-forming compound) and propylene (an ethylene analogue), but not methane, caused spores to germinate and form appressoria. This is not a general effect, since fungal pathogens of non-climacteric fruits such as pea and cucumber did not respond significantly to ethephon. Norbornadlene (which is thought to interact with the plant ethylene receptor) and silver ions inhibit both ethylene action in plants and fungal appressorla formation; and higher concentrations of ethylene can overcome the inhibition caused by norbornadiene in both cases. Thus, the fungal ethylene receptor seems to share characteristics with the plant receptor, Indeed, the pathogen might have acquired ethylene-response genes from its host. It will be interesting to compare the signal transduction pathways in the two organisms.
TRENDSIN CELLBIOLOGYVOL. 4 NOVEMBER1994
REVIEWS
The Drosophila compound eye is a highly regular array of approximately 800 unit eyes, called ommatidia. Each ommatidium is composed of eight photoreceptor (R) cells, which are sensory neurons that transduce visual information to the brain, and a variety of non-neuronal accessory cells arranged in a very precise pattern. The cellular simplicity of the compound eye and its accessibility to genetic analysis have made it a favourable system for studying the developmental signals that regulate pattern formation within an epithelium.
Development of the
Early pattern formation in the developing Drosophila eye
Dnuophi.locompound eye
Differentiation of the compound eye begins in a monolayer columnar epithelium called the eye iraaginal disc (for a recent general review, see Ref. 1). The imaginal disc is formed from a small group of about 20 cells that are set aside early during embryogenesis. These cells proliferate during the larval stages, or instars, with pattern formation commencing during the third and final instar (Fig. 1). The onset of differentiation is marked by a dorsoventral contraction in the apical surface of the eye disc epithelium called the morphogenetic furrow (MF), which forms at the posterior edge of the disc. Before MF formation, cells are undifferentiated and unpatterned, and cell division occurs asynchronously. The formation of the MF is associated with a synchronization of the cell division cycle just ahead of the MF In a domain defined by expression of the cell cycle gene string (stg)z. Within the MF, cells are synchronized in the G1 phase of the cell cycle. Cells in this G 1 domain begin to aggregate into regularly spaced clusters. Immature clusters, called preclusters, emerge from the posterior edge of the MF (Fig. 1). They contain postmitotlc precursors for the first five R cells that differentiate, R2-S and R8. Between the clusters, the remaining cells enter $ phase synchronously, and their division completes the pool of precursors needed to assemble a complete adult eye. These cells will be added to the preclusters In a highly ordered spatlotemporal fashion to give rise to the mature ommatldlum. Hence, the MF represents a transition from asynchronously dividing and unpatterned cells to a synchronous wave of cell division and pattern formation. Within the clusters, differentiation of R zells proceeds In a strict developmental sequence. R8 is the first cell to express many neuronal and cell-typespecific markers; it is followed by differentiation of R2 and R5, then R3 and R4. The remaining R cells R1, R6 and R7 - as well as the non.neuronal accessory cells are recruited from the pool of uncommitted precursors derived from the synchronous wave of division behind the MF. Because there are no obligatory lineage relationships between cells within an ~mmatidium, R cell fate is thought to be mediated by Inductive interactions between developing cells within a cluster s. Inductive signalling has been most clearly demonstrated for the last R cell to join the cluster, the R7 cell4. I n this review, we focus on recent studies of earlier events occurring in the developing eye disc and the genes regulating them. TRENDS IN CELL BIOLOGYVOL. 4 NOVEMBER 1994
The formation of complex cellular arrays from unpattemed epithelia is a widespread developmental phenomenon. Insights into the mechanisms regulating this transformation have come from studying the development of the Drosophila compound eye. Pattern formation in the eye primordium is a highly ordered process in which the onset of differentiation is coordinated with synchronization of cell cycle progression. Recent studies have identified a number of genes that are required for early patterning events, and provide a link between the regulation of proliferation and pattern formation.
The MF: Initiation and propagation of pattern formation Recent studies have suggested that formation of the MF and the expansion and differentiation of the eye field posterior to it can be divided lnto two processes: initiation and propagation. Although very few genes are expressed before the appearance of the MF that might play a role In its initiation, some possible candidates have recently been identified. Both eyes absent s (eya) and sine oculis6 (so) are expressed very early In eye development in the unpatterned epithelium before formation of the MF (Fig. 2). The eya gene encodes a novel protein product that is localized to the nucleus. The so gene encodes a homeodomaincontaining protein and hence is likely to be a transcription factor, eya expression occurs slightly earlier in development than so expression, and is first detected late in the second larval instar at the posterior edge of the eye disc, extending laterally around the disc margins. In older discs, eya is expressed ahead of and within the MF, extending to the posterior edge of the disc. Expression of so is first detected in the posterior and lateral edges of the early thirdinstar eye disc before the formation of the MF. Severe alleles of eya and so result in massive apoptosis before the onset of pattern formation, resulting in flies that are completely eyeless. Less severe alleles produce a band of apoptosis just ahead of the MF, resulting in adult e~¢es that are much smaller than normal. These observations suggest that the eya and so gene products play a role in both the
The authorsare at the Dept of Biological Chemistry, University of California, Los Angeles, CA 90024, USA; LawrenceZipursky is also at The Howard Hughes Medical ;nstitute.
0 1994 ElsevierScienceLtd 0962.8924/94/$07.00
389
Life w i t h o u t a kinase COKER, K. l., STAROS,J. V. and GUYER,C. A. (1994) A kinase-negative epidermal growth factor receptor that retains the capacity to stimulate DNA synthesis Proc. Natl Acad. Sci. USA 91, 6967-6971 The epidermal growth factor receptor (EGFR)is a transmembrane tyrosine kinase that, upon activation, autophosphorylates at specific residues, thus providing binding sites for signalling proteins with Src homology 2 (SH2) domains. Recruitment of these molecules at the plasma membrane turns on the Rasdependent mitogen-activated protein kinase (MAPK) cascade, causing cell proliferation. To investigate the importance of the kinase activity for the signalling properties of the EGFR,Coker et ol. mutated a highly conserved residue within the catalytic domain that is thought to participate in the phosphotransfer reaction. The partially purified receptor was unable to autophosphorylate or to phosphorylate exogenous substrates in vitro. However, in intact CHO cells (which lack endogenous EGFRs)transfected with the mutant gene, EGF induced a low level of tyrosine phosphorylation of the receptor. As suggested by the authors, this could be catalysed by an associated kinase. In these cells, EGF activated MAPK and DNA replication as efficiently as in cells expressing the wild-type receptor. In contrast, a kinase.defectivereceptor mutated at the ATP-binding site was capable of stimulating MAPK but not DNA synthesis. Therefore, different point mutations in the catalytic domain of the EGFRmay have distinct biological consequences in terms of signal transduction, depending on whether the receptor retains the ability to bind ATP. It has been shown that EGF can also activate a Ras.independent signalling pathway leading to the tyrosine phosphorylation and nuclear translocation of the transcription factor p91. One critical issue that should be addressed is whether these mutants are still able to activate this second pathway.
It's easy to subscribe to Trends In Cell Biology For y o . r o w n personal copy of TCB simply: Phone:
+44 865 843300 (UK) +1 914 524 9200 (USA) +61 (02) 958 4429 (Australia)
Fax:
+44 865 843940 (UK) +1 914 333 2444 (USA) +61 (02) 967 2126 (Australia)
or Emaih
[email protected]
and give your name, address, the month you would like your subscription to start, and your credit card details.
Current prices are listed in the information boxes on page vi.
388
Fungal attack of ripe fruit FLAISHMAN, M. A. and KOLATrUKUDY, P. E. (1994) Timing of fungal invasion usi~,g host's ripening hormone as a signal Proc. Not~Acad. Sci. USA 91, 6579-6583 Extensive crop losses occur after Iharvest. In the case of fruit, the spores of many fungal pathogens that attach to its surface during the growing sea',;on wait for the L:ruitto ripen before penetrating it and causing damage. How does the pathogen time its attack on the host? The results in this paper provide convincing evidence that for the Colletotrichum fungi that infect climacteric I'ruit (those that increase respiration and ethylene production at ripening) ethylene is the trigger. When spores of these fungi were placed in a chamber together (but not in contact) with climacteric ripening fruits, such as banana, tomato and avocado, they germinated and produced appressoria (infection structures). This observation suggested that a gaseous metabolite released during fruit ripening is the signal sensed by the pathogen. Further studies demonstrated that the metabollte is ethylene. Fruit that do not release ethylene during ripening, such as orange or a transgenic tomato, did not induce appressoria formation. Moreover, incubation with physiological concentrations ..c ethylene, ethephon (an ethylene-forming compound) and propylene (an ethylene analogue), but not methane, caused spores to germinate and form appressoria. This is not a general effect, since fungal pathogens of non-climacteric fruits such as pea and cucumber did not respond significantly to ethephon. Norbornadlene (which is thought to interact with the plant ethylene receptor) and silver ions inhibit both ethylene action in plants and fungal appressorla formation; and higher concentrations of ethylene can overcome the inhibition caused by norbornadiene in both cases. Thus, the fungal ethylene receptor seems to share characteristics with the plant receptor, Indeed, the pathogen might have acquired ethylene-response genes from its host. It will be interesting to compare the signal transduction pathways in the two organisms.
TRENDSIN CELLBIOLOGYVOL. 4 NOVEMBER1994
REVIEWS
The Drosophila compound eye is a highly regular array of approximately 800 unit eyes, called ommatidia. Each ommatidium is composed of eight photoreceptor (R) cells, which are sensory neurons that transduce visual information to the brain, and a variety of non-neuronal accessory cells arranged in a very precise pattern. The cellular simplicity of the compound eye and its accessibility to genetic analysis have made it a favourable system for studying the developmental signals that regulate pattern formation within an epithelium.
Development of the
Early pattern formation in the developing Drosophila eye
Dnuophi.locompound eye
Differentiation of the compound eye begins in a monolayer columnar epithelium called the eye iraaginal disc (for a recent general review, see Ref. 1). The imaginal disc is formed from a small group of about 20 cells that are set aside early during embryogenesis. These cells proliferate during the larval stages, or instars, with pattern formation commencing during the third and final instar (Fig. 1). The onset of differentiation is marked by a dorsoventral contraction in the apical surface of the eye disc epithelium called the morphogenetic furrow (MF), which forms at the posterior edge of the disc. Before MF formation, cells are undifferentiated and unpatterned, and cell division occurs asynchronously. The formation of the MF is associated with a synchronization of the cell division cycle just ahead of the MF In a domain defined by expression of the cell cycle gene string (stg)z. Within the MF, cells are synchronized in the G1 phase of the cell cycle. Cells in this G 1 domain begin to aggregate into regularly spaced clusters. Immature clusters, called preclusters, emerge from the posterior edge of the MF (Fig. 1). They contain postmitotlc precursors for the first five R cells that differentiate, R2-S and R8. Between the clusters, the remaining cells enter $ phase synchronously, and their division completes the pool of precursors needed to assemble a complete adult eye. These cells will be added to the preclusters In a highly ordered spatlotemporal fashion to give rise to the mature ommatldlum. Hence, the MF represents a transition from asynchronously dividing and unpatterned cells to a synchronous wave of cell division and pattern formation. Within the clusters, differentiation of R zells proceeds In a strict developmental sequence. R8 is the first cell to express many neuronal and cell-typespecific markers; it is followed by differentiation of R2 and R5, then R3 and R4. The remaining R cells R1, R6 and R7 - as well as the non.neuronal accessory cells are recruited from the pool of uncommitted precursors derived from the synchronous wave of division behind the MF. Because there are no obligatory lineage relationships between cells within an ~mmatidium, R cell fate is thought to be mediated by Inductive interactions between developing cells within a cluster s. Inductive signalling has been most clearly demonstrated for the last R cell to join the cluster, the R7 cell4. I n this review, we focus on recent studies of earlier events occurring in the developing eye disc and the genes regulating them. TRENDS IN CELL BIOLOGYVOL. 4 NOVEMBER 1994
The formation of complex cellular arrays from unpattemed epithelia is a widespread developmental phenomenon. Insights into the mechanisms regulating this transformation have come from studying the development of the Drosophila compound eye. Pattern formation in the eye primordium is a highly ordered process in which the onset of differentiation is coordinated with synchronization of cell cycle progression. Recent studies have identified a number of genes that are required for early patterning events, and provide a link between the regulation of proliferation and pattern formation.
The MF: Initiation and propagation of pattern formation Recent studies have suggested that formation of the MF and the expansion and differentiation of the eye field posterior to it can be divided lnto two processes: initiation and propagation. Although very few genes are expressed before the appearance of the MF that might play a role In its initiation, some possible candidates have recently been identified. Both eyes absent s (eya) and sine oculis6 (so) are expressed very early In eye development in the unpatterned epithelium before formation of the MF (Fig. 2). The eya gene encodes a novel protein product that is localized to the nucleus. The so gene encodes a homeodomaincontaining protein and hence is likely to be a transcription factor, eya expression occurs slightly earlier in development than so expression, and is first detected late in the second larval instar at the posterior edge of the eye disc, extending laterally around the disc margins. In older discs, eya is expressed ahead of and within the MF, extending to the posterior edge of the disc. Expression of so is first detected in the posterior and lateral edges of the early thirdinstar eye disc before the formation of the MF. Severe alleles of eya and so result in massive apoptosis before the onset of pattern formation, resulting in flies that are completely eyeless. Less severe alleles produce a band of apoptosis just ahead of the MF, resulting in adult e~¢es that are much smaller than normal. These observations suggest that the eya and so gene products play a role in both the
The authorsare at the Dept of Biological Chemistry, University of California, Los Angeles, CA 90024, USA; LawrenceZipursky is also at The Howard Hughes Medical ;nstitute.
0 1994 ElsevierScienceLtd 0962.8924/94/$07.00
389