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Centriole construction needs new type of tubulin
HEADLINES
Signal transduction meets nuclear export ENGEL, K. et al. (1998) Leptomycin B-sensitive nuclear export of MAPKAP kinase 2 is regulated by phosphorylation EMBO J. 17, 3363–3371 Mechanistic aspects of nuclear transport have progressed rapidly over the past few years, with a plethora of receptors and signals identified. However, the complexity raises the issue of how these intricate events are regulated to permit functional working of the cell. As with many issues of regulation, it is becoming apparent that the cell turns to phosphorylation to control its complex nuclear transport pathways. The paper from Engel et al. studies the nucleocytoplasmic distribution of MAP kinase-activated
Centriole construction needs new type of tubulin DUTCHER, S. K. and TRABUCO, E. C. (1998) The UNI3 gene is required for assembly of basal bodies of Chlamydomonas and encodes d-tubulin, a new member of the tubulin superfamily Mol. Biol. Cell 9, 1293–1308 The centriole, which forms the core of the centrosome and also templates flagellar assembly, is a cylindrical barrel of nine triplet microtubules. The components of the centriole are largely unknown, partly owing to a lack of genetic investigation of this structure. One reason for this is that yeast cells do not contain centrioles. Instead, a promising genetic system for studying centrioles is the unicellular alga Chlamydomonas (a.k.a. ‘green yeast’), which has excellent genetic properties (easy transformation, tetrad analysis, etc.) and has centrioles identical to those of higher eukaryotes.
Arterial development This month’s headlines were contributed by Søren Andersen, Volker Haucke, Chung Lau, Wallace Marshall, Robin May, Michael Mishkind, Fiona Townsley, Kirsten Sadler and Gary Whittaker.
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LI, D. Y. et al. (1998) Elastin is an essential determinant of arterial morphogenesis Nature 393, 276–280 The formation of blood vessels has been modelled to include three crucial stages. First, mesenchymal cells produce angiopoietin-1 in order to activate endothelial cells to release a recruiting signal for additional mesenchymal cells; second, transforming growth factor b (TGF-b) is activated by a mesenchymal–endothelial cell interaction; and, third, TGF-b induces differentiation of smooth muscle cells from mesenchymal cells. The report by Li et al. shows that
protein kinase 2 (MK2), a downstream effector of the stress-activated protein kinase p38. Using a green fluorescent protein (GFP) fusion, MK2 is shown to contain both a classical nuclear-localization sequence (NLS), and a leucine-based nuclear-export sequence (NES), but, at steady state, it is localized to the nucleus. Localization is disrupted by stress, a MAPK inhibitor and the mutation-based activation of a putative MAPK phosphorylation site, suggesting that stress-induced nuclear import of activated MAPK induces MK2 nuclear export. MK2 thus acts conversely to the MAP kinases themselves, which are imported into the nucleus upon phosphorylation. Localization of MK2 is also affected by leptomycin B, an inhibitor of exportin-mediated nuclear export. The NES of MK2 is believed to be functional only when activated by phosphorylation, whereas the NLS is constitutively active. In addition to other examples of activation being linked to nuclear import, this coupling of nuclear export to activation of the cell represents an additional mechanism by which signal transduction can occur.
Chlamydomonas swims using two flagella. Because centrioles are required to form these flagella, Dutcher and Trabuco screened for Chlamydomonas mutants with abnormal motility and found one, uni3, in which 25% of cells had just one flagellum and 50% had none. Mutant centrioles have an interesting ultrastructural defect: they are composed of microtubule doublets instead of triplets, implying that the UNI3 gene product is needed to assemble the triplet microtubule structures. The authors cloned the UNI3 gene and found that it encodes a new member of the tubulin superfamily. Uni3p is equally homologous to the Chlamydomonas a-, b- and g-tubulins and contains several conserved motifs characteristic of all tubulins. Phylogenetically, Uni3p constitutes a separate branch equidistant from the other three tubulin clusters. Uni3p is probably not unique to Chlamydomonas as a mouse expressed-sequence tag (EST) was found that encodes a likely homologue. The discovery of this new tubulin provides an entry point for further molecular dissection of the centriole and illustrates the power of Chlamydomonas genetics for studying centriole assembly and function.
there is an additional stage, which is not only crucial for vascular formation but is also necessary in preventing common diseases such as atherosclerosis. The authors generated mice that lack elastin (ELN) and report that mutations in one allele encoding elastin can cause supravalvular aortic stenosis. This resembles a human disease in which arteries are blocked. Elastin expression begins in the last third of gestation and is mainly expressed in the vascular system. From embryonic day 17.5 (E17.5), the outer and inner diameters of the aorta in ELN–/– mice become progressively smaller, and the diameter of the arterial wall in ELN–/– mice also becomes thicker than that of ELN+/+ mice. Furthermore, there is increased cellularity in all systemic and pulmonary arteries. The obliteration of the arterial lumens in ELN–/– mice appears to be caused by subendothelial accumulation and reorientation of smooth muscle because, in contrast to the circumferentially orientated smooth muscle cells in trends in CELL BIOLOGY (Vol. 8) September 1998
Signal transduction meets nuclear export ENGEL, K. et al. (1998) Leptomycin B-sensitive nuclear export of MAPKAP kinase 2 is regulated by phosphorylation EMBO J. 17, 3363–3371 Mechanistic aspects of nuclear transport have progressed rapidly over the past few years, with a plethora of receptors and signals identified. However, the complexity raises the issue of how these intricate events are regulated to permit functional working of the cell. As with many issues of regulation, it is becoming apparent that the cell turns to phosphorylation to control its complex nuclear transport pathways. The paper from Engel et al. studies the nucleocytoplasmic distribution of MAP kinase-activated
Centriole construction needs new type of tubulin DUTCHER, S. K. and TRABUCO, E. C. (1998) The UNI3 gene is required for assembly of basal bodies of Chlamydomonas and encodes d-tubulin, a new member of the tubulin superfamily Mol. Biol. Cell 9, 1293–1308 The centriole, which forms the core of the centrosome and also templates flagellar assembly, is a cylindrical barrel of nine triplet microtubules. The components of the centriole are largely unknown, partly owing to a lack of genetic investigation of this structure. One reason for this is that yeast cells do not contain centrioles. Instead, a promising genetic system for studying centrioles is the unicellular alga Chlamydomonas (a.k.a. ‘green yeast’), which has excellent genetic properties (easy transformation, tetrad analysis, etc.) and has centrioles identical to those of higher eukaryotes.
Arterial development This month’s headlines were contributed by Søren Andersen, Volker Haucke, Chung Lau, Wallace Marshall, Robin May, Michael Mishkind, Fiona Townsley, Kirsten Sadler and Gary Whittaker.
344
LI, D. Y. et al. (1998) Elastin is an essential determinant of arterial morphogenesis Nature 393, 276–280 The formation of blood vessels has been modelled to include three crucial stages. First, mesenchymal cells produce angiopoietin-1 in order to activate endothelial cells to release a recruiting signal for additional mesenchymal cells; second, transforming growth factor b (TGF-b) is activated by a mesenchymal–endothelial cell interaction; and, third, TGF-b induces differentiation of smooth muscle cells from mesenchymal cells. The report by Li et al. shows that
protein kinase 2 (MK2), a downstream effector of the stress-activated protein kinase p38. Using a green fluorescent protein (GFP) fusion, MK2 is shown to contain both a classical nuclear-localization sequence (NLS), and a leucine-based nuclear-export sequence (NES), but, at steady state, it is localized to the nucleus. Localization is disrupted by stress, a MAPK inhibitor and the mutation-based activation of a putative MAPK phosphorylation site, suggesting that stress-induced nuclear import of activated MAPK induces MK2 nuclear export. MK2 thus acts conversely to the MAP kinases themselves, which are imported into the nucleus upon phosphorylation. Localization of MK2 is also affected by leptomycin B, an inhibitor of exportin-mediated nuclear export. The NES of MK2 is believed to be functional only when activated by phosphorylation, whereas the NLS is constitutively active. In addition to other examples of activation being linked to nuclear import, this coupling of nuclear export to activation of the cell represents an additional mechanism by which signal transduction can occur.
Chlamydomonas swims using two flagella. Because centrioles are required to form these flagella, Dutcher and Trabuco screened for Chlamydomonas mutants with abnormal motility and found one, uni3, in which 25% of cells had just one flagellum and 50% had none. Mutant centrioles have an interesting ultrastructural defect: they are composed of microtubule doublets instead of triplets, implying that the UNI3 gene product is needed to assemble the triplet microtubule structures. The authors cloned the UNI3 gene and found that it encodes a new member of the tubulin superfamily. Uni3p is equally homologous to the Chlamydomonas a-, b- and g-tubulins and contains several conserved motifs characteristic of all tubulins. Phylogenetically, Uni3p constitutes a separate branch equidistant from the other three tubulin clusters. Uni3p is probably not unique to Chlamydomonas as a mouse expressed-sequence tag (EST) was found that encodes a likely homologue. The discovery of this new tubulin provides an entry point for further molecular dissection of the centriole and illustrates the power of Chlamydomonas genetics for studying centriole assembly and function.
there is an additional stage, which is not only crucial for vascular formation but is also necessary in preventing common diseases such as atherosclerosis. The authors generated mice that lack elastin (ELN) and report that mutations in one allele encoding elastin can cause supravalvular aortic stenosis. This resembles a human disease in which arteries are blocked. Elastin expression begins in the last third of gestation and is mainly expressed in the vascular system. From embryonic day 17.5 (E17.5), the outer and inner diameters of the aorta in ELN–/– mice become progressively smaller, and the diameter of the arterial wall in ELN–/– mice also becomes thicker than that of ELN+/+ mice. Furthermore, there is increased cellularity in all systemic and pulmonary arteries. The obliteration of the arterial lumens in ELN–/– mice appears to be caused by subendothelial accumulation and reorientation of smooth muscle because, in contrast to the circumferentially orientated smooth muscle cells in trends in CELL BIOLOGY (Vol. 8) September 1998