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A role for clathrin in the sorting of vacuolar proteins in the Golgi complex of yeast
he dliiTes Nopp140 shuttles on tracks between nucleolus and cytoplasm U. T. Meier and G. Blobel Cell 70, 127-138 In 1985, Gunter Blobel proposed, in the 'gene gating' hypothesis, that a connection exists between transcriptionally active genes and individual nuclear pores, such that the transcripts from a specified gene are exported to the cytoplasm through a single pore. In less extreme forms, the idea that a detailed functional organization of the chromatin within the nucleus exists has long been popular among cell biologists. A few
years ago, the development of sensitive in situ hybridization techniques allowed the visuahzation of the paths taken by particular transcripts to exit from the nucleus. Strikingly, some RNAs followed curvilinear tracks from the gene to the nuclear periphery, consistent with the gene gating hypothesis. Now, Meier and Blobel have demonstrated by immunoelectron microscopy that Nopp140, a nucleo-
lar phosphoprotein, shuttles between the nucleolus and the cytoplasm on a limited number of tracks. This strengthens the case that tracks may be a fundamental aspect of nuclear organization, and that the identity of the traffic flowing through distinct nuclear pores may be quite specific. Nopp140 is particularly interesting because it binds to nuclear localization sequences that target proteins for import to the nucleus. Meier and Blobel suggest that it may play a key role in delivering ribosomal proteins from the cytoplasm to the nucleolus (the site of ribosomal RNA synthesis and preribosome assembly) or perhaps in returning assembled ribosomal subunits to the cytoplasm.
Characterization of TPM1 disrupted yeast cells Indicates an involvement of tropomyosin in directed vesicular transport H. Liu and A. Bretscher J. Cell Biol. 118, 285-299 The role of actm in directing vesicular transport to the bud in Saccharomyces cerevisiae has recently been highlighted. In this paper, the authors describe the effects of disruption of the gene for tropomyosin, a protein associated with actin microfilaments, in budding yeast. Cells lacking functional tropomyosin grew slowly and accumulated intracellular vesicles reminiscent of secretory vesicles. In addition, the deposition of chitin, a cell wall component, was delocalized in the mutants compared with wild-
type cells, in which chitin is specifically deposited in a ring around the emerging bud. The accumulation of vesiclesin cells lacking tropomyosin required that the early stages of the secretory pathway were intact, as demonstrated by the use of strains with mutations affecting early or late events in secretion. The cz-factorinduced secretion of a-agglutinin, which is normally directed to the growing bud, was partially impaired in the tropomyosin-deficient cells. Overall, no block in transport to the
A role for clathrln In the sorting of vacuolar proteins in the Golgi complex of yeast M. Seeger and G. S. Payne EMBO J. 11,2811-2818 The role of clathrin in endocytosis in higher eukaryotes is well established. A similar role in membrane traffic from the Golgi complex to lysosomes, or to regulated secretory granules, is also generally accepted. In yeast, however, the role of clathrin in intracelluiar membrane traff0c has been less clear. In this paper, the authors provide evidence
that implicates clathnn in the transport of lysosomal enzymes through the Golgi complex in Saccharomyces cerevlslae. Seeger and Payne examined ceils with a temperature-sensitive defect in the clathrin heavy chain. They had previously shown that cells in which the clathrin heavy chain gene had been deleted showed no defect in
TRENDS IN CELLBIOLOGYVOL. 2 OCTOBER1992
cell surface or to vacuoles was observed, supporting the idea that tropomyosin and actin are involved in directing vesicles to the bud, rather than mediating efficient vesicle fusion events. These findings lend support to the increasing weight of evidence that yeast uses the actln-based cytoskeleton to direct vesicles to particular delivery sites. This contrasts with higher eukaryotes, in which microtubules seem more likely to direct vesicular transport.
vacuolar protein sorting. However, when the temperature-sensitive cells were examined five minutes after incubation at the restrictive temperature a severe defect in the sorting of soluble vacuolar proteins was observed. After longer periods at the restrictive temperature the sorting defect was repaired. At both times, sorting of vacuolar membrane proteins was unaffected. Therefore, yeast cells with defective clathrin can induce a clathnn-independent pathway that allows efficient sorting of soluble vacuolar proteins. The nature of this pathway, and its relationship to the pathway utilized by vacuolar membrane proteins, will probably be the subject of many studies in the next few years. 291
years ago, the development of sensitive in situ hybridization techniques allowed the visuahzation of the paths taken by particular transcripts to exit from the nucleus. Strikingly, some RNAs followed curvilinear tracks from the gene to the nuclear periphery, consistent with the gene gating hypothesis. Now, Meier and Blobel have demonstrated by immunoelectron microscopy that Nopp140, a nucleo-
lar phosphoprotein, shuttles between the nucleolus and the cytoplasm on a limited number of tracks. This strengthens the case that tracks may be a fundamental aspect of nuclear organization, and that the identity of the traffic flowing through distinct nuclear pores may be quite specific. Nopp140 is particularly interesting because it binds to nuclear localization sequences that target proteins for import to the nucleus. Meier and Blobel suggest that it may play a key role in delivering ribosomal proteins from the cytoplasm to the nucleolus (the site of ribosomal RNA synthesis and preribosome assembly) or perhaps in returning assembled ribosomal subunits to the cytoplasm.
Characterization of TPM1 disrupted yeast cells Indicates an involvement of tropomyosin in directed vesicular transport H. Liu and A. Bretscher J. Cell Biol. 118, 285-299 The role of actm in directing vesicular transport to the bud in Saccharomyces cerevisiae has recently been highlighted. In this paper, the authors describe the effects of disruption of the gene for tropomyosin, a protein associated with actin microfilaments, in budding yeast. Cells lacking functional tropomyosin grew slowly and accumulated intracellular vesicles reminiscent of secretory vesicles. In addition, the deposition of chitin, a cell wall component, was delocalized in the mutants compared with wild-
type cells, in which chitin is specifically deposited in a ring around the emerging bud. The accumulation of vesiclesin cells lacking tropomyosin required that the early stages of the secretory pathway were intact, as demonstrated by the use of strains with mutations affecting early or late events in secretion. The cz-factorinduced secretion of a-agglutinin, which is normally directed to the growing bud, was partially impaired in the tropomyosin-deficient cells. Overall, no block in transport to the
A role for clathrln In the sorting of vacuolar proteins in the Golgi complex of yeast M. Seeger and G. S. Payne EMBO J. 11,2811-2818 The role of clathrin in endocytosis in higher eukaryotes is well established. A similar role in membrane traffic from the Golgi complex to lysosomes, or to regulated secretory granules, is also generally accepted. In yeast, however, the role of clathrin in intracelluiar membrane traff0c has been less clear. In this paper, the authors provide evidence
that implicates clathnn in the transport of lysosomal enzymes through the Golgi complex in Saccharomyces cerevlslae. Seeger and Payne examined ceils with a temperature-sensitive defect in the clathrin heavy chain. They had previously shown that cells in which the clathrin heavy chain gene had been deleted showed no defect in
TRENDS IN CELLBIOLOGYVOL. 2 OCTOBER1992
cell surface or to vacuoles was observed, supporting the idea that tropomyosin and actin are involved in directing vesicles to the bud, rather than mediating efficient vesicle fusion events. These findings lend support to the increasing weight of evidence that yeast uses the actln-based cytoskeleton to direct vesicles to particular delivery sites. This contrasts with higher eukaryotes, in which microtubules seem more likely to direct vesicular transport.
vacuolar protein sorting. However, when the temperature-sensitive cells were examined five minutes after incubation at the restrictive temperature a severe defect in the sorting of soluble vacuolar proteins was observed. After longer periods at the restrictive temperature the sorting defect was repaired. At both times, sorting of vacuolar membrane proteins was unaffected. Therefore, yeast cells with defective clathrin can induce a clathnn-independent pathway that allows efficient sorting of soluble vacuolar proteins. The nature of this pathway, and its relationship to the pathway utilized by vacuolar membrane proteins, will probably be the subject of many studies in the next few years. 291