- Email: [email protected]
Interorganellar Signal Transduction
Previews 319
Interorganellar Signal Transduction: The Arrest of Secretion Response Signals from the secretory pathway caused by “traffic jams” are transduced by the WSC1, WSC2/PKC1 signaling pathway and result in nuclear reorganization and ribosomal gene repression. The secretory pathway is a dynamic system of membrane-bound organelles and vesicles charged with the task of manufacturing proteins, lipids, and carbohydrates and transporting them to their sites of function. Nearly all secreted proteins and most membrane proteins are synthesized through this pathway. Not surprisingly, complex regulatory mechanisms are required to coordinate its vast array of activities. A major challenge for the cell is how to monitor functions and respond to events physically separated from the transcriptional and translational machinery. The recent discoveries of novel interorganellar signal transduction pathways have provided a glimpse of some of the elegant strategies used to achieve this (see Figure). Several of these pathways monitor functions of the endoplasmic reticulum (ER). The most intensely studied is an ER-to-nucleus signaling pathway called the unfolded protein response (UPR) (Patil and Walter, 2001). Induction of UPR target genes can be initiated by various insults to ER function including protein misfolding, transport blocks, and Ca2⫹ disregulation. In yeast, Ire1p, a kinase/nuclease that spans the ER/nuclear membrane, is a key player in activation. Ire1p splices and thereby activates a message encoding the UPR-specific transcriptional activator Hac1. Although Ire1p is conserved among eukaryotes, the UPR of mammals is more complex than yeast. A UPR transcription factor distinct from Hac1, called ATF6, lies dormant as an ER integral membrane protein. Upon ER stress, it is activated by proteolytic release through the action of the Site-1 and Site-2 proteases (Patil and Walter, 2001). This mechanism, termed regulated intramembrane proteolysis (RIP), was originally discovered as a part of a pathway regulating sterol biosynthesis. There, the SREBP (sterol regulatory element binding protein) is activated analogously when cholesterol levels are low (Brown et al., 2000). Regulation of the OLE pathway, which controls membrane fluidity, involves a slight variation on this theme. In this, as in RIP, ER membrane-bound transcription factors are activated by proteolytic release. However, the protease is the 26S proteasome, a complex better known for its role in general ubiquitin-dependent protein destruction (Hoppe et al., 2000). Further along the exocytic route, there is a distinct signaling pathway. This was discovered serendipitously through genetic studies investigating the regulation of ribosome biosynthesis. By screening a temperaturesensitive bank of yeast mutants, Warner and colleagues isolated strains that coordinately downregulated the transcription of ribosomal protein and ribosomal RNA genes (Mizuta and Warner, 1994). The effect was specific, as a variety of unrelated genes were transcribed normally under the same conditions. Surprisingly, the
complementing gene for one of the mutants was identified as SLY1, a gene required for ER-to-Golgi vesiclar transport. Further studies revealed that any disruption of protein trafficking from the ER to plasma membrane represses ribosomal genes. So how are these processes connected? A clue came from the observation that chlorpromazine, an agent that causes plasma membrane stress, also represses ribosomal genes (Nierras and Warner, 1999). This suggested that the signal might arise from disruption of membrane flow to the plasma membrane, which would lead to increased osmotic pressure from within the cell owing to continued synthesis of material. Indeed, inhibition of protein synthesis abrogates the observed repression (Mizuta and Warner, 1994). Disruption of the protein kinase C (PKC1) gene abolished the regulation, suggesting it is a key component of the signaling. This link was intriguing as PKC1 plays an established role in regulating cell wall biosynthesis. The idea that the pathway responds to changes to the tension of the plasma membrane was further strengthened by the participation of the WSC (cell wall integrity and stress response component) family of putative plasma membrane sensors, which are believed to act upstream of PKC1 (Li et al., 2000; Verna et al., 1997). Interestingly, the MAP kinase cascade that lies downstream of PKC1 does not appear to be required, suggesting that the response represents a new branch of PKC1 signaling. Disruption of the secretory pathway also has dramatic consequences on the organization of the nucleus. Tartakoff and coworkers recently found that secretory blocks strongly inhibit the import of several nuclear proteins (Nanduri et al., 1999). This is not an indirect effect of ER stress, as a sec1 mutant elicits a strong response without inducing the UPR pathway. They call the phenomenon the “arrest of secretion response,” or ASR. In a paper in the August issue of Molecular Cell, Tartakoff and colleagues make a connection between nuclear reorganization and the repression of ribosomal genes but also highlight some distinctions (Nanduri and Tartakoff, 2001). They found that PKC1 mutants alleviate the nuclear import defects in a sec1-1 mutant. As with the ribosomal gene repression response, the MAP kinase cascade does not appear to play an important role in the ASR. They also found that WSC2 and, to a lesser extent, WSC1 are required for the ASR in similar experiments. By contrast, WSC1 seems to be more important for ribosomal gene repression (Li et al., 2000). The two other members of the family, WSC3 and WSC4, are not required. Wsc proteins have a large serine/threonine rich extracellular domain, a single transmembrane domain and short cytoplasmic domain. Although the precise mechanism of WSC signaling is unclear, rather surprisingly, Wsc2p needs to be in intracellular compartments along the secretory pathway for its role in the ASR. Cells with a plasma membrane population of Wsc2p and none intracellularly were unable to elicit the response. By contrast, cells with an intracellular pool and little on the plasma membrane exhibit a robust response. Since the unfolded protein response has been shown to regulate the expression of genes throughout the secretory pathway (Travers et al., 2000), one might expect
Developmental Cell 320
Secretory Pathway to Nucleus Signal Transduction Pathways The known pathways including stimuli and targets are schematically depicted as described in the text. Depicted are the unfolded protein response, sterol responsive pathway, OLE pathway, and ASR in descending order.
significant overlap of gene targets between the UPR and ASR. However, whole genome expression analysis on sec1-1 cells showed a striking lack of overlap (Nanduri and Tartakoff, 2001). Although ribosomal genes are repressed as expected, many UPR target genes are unchanged, including those involved in vesicular traffic. Several membrane transporter genes are upregulated under either condition, but this is likely a consequence of feedback mechanisms due to their reduction at the plasma membrane. These data, coupled with evidence that UPR mutants do not block the ASR, make a convincing argument that the two pathways are distinct. The ASR illustrates yet another dramatic example of signaling from the secretory pathway to the nucleus. Unlike other ER-to-nuclear pathways, the ASR allows significant crosstalk with other pathways, notably those involving PKC1. The recent work by the Tartakoff group raises many new questions. The most intriguing is the unprecedented role of Wsc2p. How does it know when it is not at the plasma membrane? One possibility is that the local environment can passively regulate its activity. It is well known that both the membrane and luminal composition can differ depending on the stage of the secretory pathway. Alternatively, yet to be discovered cofactors that are site restricted may be required for signaling. As biochemical reconstitution may be necessary to adequately answer these questions, it is imperative that the precise activity of Wsc2p in signaling be defined. In addition, since the readout of the ASR pathway is rather dramatic (nuclear reorganization), we look
forward to the exploitation of the system to define all “missing links” including the protein kinase C-dependent “alternative pathway” and downstream targets affecting nuclear function. With GFP-labeled nucleoporins in hand, it seems straightforward to screen for mutants with either defective or constitutive ASRs. Davis T.W. Ng Department of Biochemistry and Molecular Biology Pennsylvania State University University Park, PA 16802 Selected Reading Brown, M.S., Ye, J., Rawson, R.B., and Goldstein, J.L. (2000). Cell 100, 391–398. Hoppe, T., Matuschewski, K., Rape, M., Schlenker, S., Ulrich, H.D., and Jentsch, S. (2000). Cell 102, 577–586. Li, Y., Moir, R.D., Sethy-Coraci, I.K., Warner, J.R., and Willis, I.M. (2000). Mol. Cell. Biol. 20, 3843–3851. Mizuta, K., and Warner, J.R. (1994). Mol. Cell. Biol. 14, 2493–2502. Nanduri, J., Mitra, S., Andrei, C., Liu, Y., Yu, Y., Hitomi, M., and Tartakoff, A.M. (1999). J. Biol. Chem. 274, 33785–33789. Nanduri, J., and Tartakoff, A.M. (2001). Mol. Cell 8, 281–289. Nierras, C.R., and Warner, J.R. (1999). J. Biol. Chem. 274, 13235– 13241. Patil, C., and Walter, P. (2001). Curr. Opin. Cell Biol. 13, 349–355. Travers, K.J., Patil, C.K., Wodicka, L., Lockhart, D.J., Weissman, J.S., and Walter, P. (2000). Cell 101, 249–258. Verna, J., Lodder, A., Lee, K., Vagts, A., and Ballester, R. (1997). Proc. Natl. Acad. Sci. USA 94, 13804–13809.
Interorganellar Signal Transduction: The Arrest of Secretion Response Signals from the secretory pathway caused by “traffic jams” are transduced by the WSC1, WSC2/PKC1 signaling pathway and result in nuclear reorganization and ribosomal gene repression. The secretory pathway is a dynamic system of membrane-bound organelles and vesicles charged with the task of manufacturing proteins, lipids, and carbohydrates and transporting them to their sites of function. Nearly all secreted proteins and most membrane proteins are synthesized through this pathway. Not surprisingly, complex regulatory mechanisms are required to coordinate its vast array of activities. A major challenge for the cell is how to monitor functions and respond to events physically separated from the transcriptional and translational machinery. The recent discoveries of novel interorganellar signal transduction pathways have provided a glimpse of some of the elegant strategies used to achieve this (see Figure). Several of these pathways monitor functions of the endoplasmic reticulum (ER). The most intensely studied is an ER-to-nucleus signaling pathway called the unfolded protein response (UPR) (Patil and Walter, 2001). Induction of UPR target genes can be initiated by various insults to ER function including protein misfolding, transport blocks, and Ca2⫹ disregulation. In yeast, Ire1p, a kinase/nuclease that spans the ER/nuclear membrane, is a key player in activation. Ire1p splices and thereby activates a message encoding the UPR-specific transcriptional activator Hac1. Although Ire1p is conserved among eukaryotes, the UPR of mammals is more complex than yeast. A UPR transcription factor distinct from Hac1, called ATF6, lies dormant as an ER integral membrane protein. Upon ER stress, it is activated by proteolytic release through the action of the Site-1 and Site-2 proteases (Patil and Walter, 2001). This mechanism, termed regulated intramembrane proteolysis (RIP), was originally discovered as a part of a pathway regulating sterol biosynthesis. There, the SREBP (sterol regulatory element binding protein) is activated analogously when cholesterol levels are low (Brown et al., 2000). Regulation of the OLE pathway, which controls membrane fluidity, involves a slight variation on this theme. In this, as in RIP, ER membrane-bound transcription factors are activated by proteolytic release. However, the protease is the 26S proteasome, a complex better known for its role in general ubiquitin-dependent protein destruction (Hoppe et al., 2000). Further along the exocytic route, there is a distinct signaling pathway. This was discovered serendipitously through genetic studies investigating the regulation of ribosome biosynthesis. By screening a temperaturesensitive bank of yeast mutants, Warner and colleagues isolated strains that coordinately downregulated the transcription of ribosomal protein and ribosomal RNA genes (Mizuta and Warner, 1994). The effect was specific, as a variety of unrelated genes were transcribed normally under the same conditions. Surprisingly, the
complementing gene for one of the mutants was identified as SLY1, a gene required for ER-to-Golgi vesiclar transport. Further studies revealed that any disruption of protein trafficking from the ER to plasma membrane represses ribosomal genes. So how are these processes connected? A clue came from the observation that chlorpromazine, an agent that causes plasma membrane stress, also represses ribosomal genes (Nierras and Warner, 1999). This suggested that the signal might arise from disruption of membrane flow to the plasma membrane, which would lead to increased osmotic pressure from within the cell owing to continued synthesis of material. Indeed, inhibition of protein synthesis abrogates the observed repression (Mizuta and Warner, 1994). Disruption of the protein kinase C (PKC1) gene abolished the regulation, suggesting it is a key component of the signaling. This link was intriguing as PKC1 plays an established role in regulating cell wall biosynthesis. The idea that the pathway responds to changes to the tension of the plasma membrane was further strengthened by the participation of the WSC (cell wall integrity and stress response component) family of putative plasma membrane sensors, which are believed to act upstream of PKC1 (Li et al., 2000; Verna et al., 1997). Interestingly, the MAP kinase cascade that lies downstream of PKC1 does not appear to be required, suggesting that the response represents a new branch of PKC1 signaling. Disruption of the secretory pathway also has dramatic consequences on the organization of the nucleus. Tartakoff and coworkers recently found that secretory blocks strongly inhibit the import of several nuclear proteins (Nanduri et al., 1999). This is not an indirect effect of ER stress, as a sec1 mutant elicits a strong response without inducing the UPR pathway. They call the phenomenon the “arrest of secretion response,” or ASR. In a paper in the August issue of Molecular Cell, Tartakoff and colleagues make a connection between nuclear reorganization and the repression of ribosomal genes but also highlight some distinctions (Nanduri and Tartakoff, 2001). They found that PKC1 mutants alleviate the nuclear import defects in a sec1-1 mutant. As with the ribosomal gene repression response, the MAP kinase cascade does not appear to play an important role in the ASR. They also found that WSC2 and, to a lesser extent, WSC1 are required for the ASR in similar experiments. By contrast, WSC1 seems to be more important for ribosomal gene repression (Li et al., 2000). The two other members of the family, WSC3 and WSC4, are not required. Wsc proteins have a large serine/threonine rich extracellular domain, a single transmembrane domain and short cytoplasmic domain. Although the precise mechanism of WSC signaling is unclear, rather surprisingly, Wsc2p needs to be in intracellular compartments along the secretory pathway for its role in the ASR. Cells with a plasma membrane population of Wsc2p and none intracellularly were unable to elicit the response. By contrast, cells with an intracellular pool and little on the plasma membrane exhibit a robust response. Since the unfolded protein response has been shown to regulate the expression of genes throughout the secretory pathway (Travers et al., 2000), one might expect
Developmental Cell 320
Secretory Pathway to Nucleus Signal Transduction Pathways The known pathways including stimuli and targets are schematically depicted as described in the text. Depicted are the unfolded protein response, sterol responsive pathway, OLE pathway, and ASR in descending order.
significant overlap of gene targets between the UPR and ASR. However, whole genome expression analysis on sec1-1 cells showed a striking lack of overlap (Nanduri and Tartakoff, 2001). Although ribosomal genes are repressed as expected, many UPR target genes are unchanged, including those involved in vesicular traffic. Several membrane transporter genes are upregulated under either condition, but this is likely a consequence of feedback mechanisms due to their reduction at the plasma membrane. These data, coupled with evidence that UPR mutants do not block the ASR, make a convincing argument that the two pathways are distinct. The ASR illustrates yet another dramatic example of signaling from the secretory pathway to the nucleus. Unlike other ER-to-nuclear pathways, the ASR allows significant crosstalk with other pathways, notably those involving PKC1. The recent work by the Tartakoff group raises many new questions. The most intriguing is the unprecedented role of Wsc2p. How does it know when it is not at the plasma membrane? One possibility is that the local environment can passively regulate its activity. It is well known that both the membrane and luminal composition can differ depending on the stage of the secretory pathway. Alternatively, yet to be discovered cofactors that are site restricted may be required for signaling. As biochemical reconstitution may be necessary to adequately answer these questions, it is imperative that the precise activity of Wsc2p in signaling be defined. In addition, since the readout of the ASR pathway is rather dramatic (nuclear reorganization), we look
forward to the exploitation of the system to define all “missing links” including the protein kinase C-dependent “alternative pathway” and downstream targets affecting nuclear function. With GFP-labeled nucleoporins in hand, it seems straightforward to screen for mutants with either defective or constitutive ASRs. Davis T.W. Ng Department of Biochemistry and Molecular Biology Pennsylvania State University University Park, PA 16802 Selected Reading Brown, M.S., Ye, J., Rawson, R.B., and Goldstein, J.L. (2000). Cell 100, 391–398. Hoppe, T., Matuschewski, K., Rape, M., Schlenker, S., Ulrich, H.D., and Jentsch, S. (2000). Cell 102, 577–586. Li, Y., Moir, R.D., Sethy-Coraci, I.K., Warner, J.R., and Willis, I.M. (2000). Mol. Cell. Biol. 20, 3843–3851. Mizuta, K., and Warner, J.R. (1994). Mol. Cell. Biol. 14, 2493–2502. Nanduri, J., Mitra, S., Andrei, C., Liu, Y., Yu, Y., Hitomi, M., and Tartakoff, A.M. (1999). J. Biol. Chem. 274, 33785–33789. Nanduri, J., and Tartakoff, A.M. (2001). Mol. Cell 8, 281–289. Nierras, C.R., and Warner, J.R. (1999). J. Biol. Chem. 274, 13235– 13241. Patil, C., and Walter, P. (2001). Curr. Opin. Cell Biol. 13, 349–355. Travers, K.J., Patil, C.K., Wodicka, L., Lockhart, D.J., Weissman, J.S., and Walter, P. (2000). Cell 101, 249–258. Verna, J., Lodder, A., Lee, K., Vagts, A., and Ballester, R. (1997). Proc. Natl. Acad. Sci. USA 94, 13804–13809.