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Human genome project – where next?
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Yet another clue to a transmembrane link An elusive component of the integrin signaling cascade is the serine/threonine protein kinase ILK (integrin-linked kinase), which interacts with the cytoplasmic tail of the integrin β1 chain. Initially, some uncertainties about the role of ILK in integrin signaling were caused by lack of good immunological reagents that could verify that integrins and ILK co-distributed within cells. Another problem is the multifunctionality of ILK, which also takes part in events mediated by cell–cell contact. Ishigatsubo and colleagues, in an impressive work, describe a new component in the cascade initiated by integrin–ILK interactions1. They identify by yeast two-hybrid screening a novel ILKbinding protein, affixin, consisting of two tandem calponin homology domains. Detailed studies of the distribution of affixin in spreading cells and in tissues reveal a distribution compatible with affixin serving as an integrin-interacting protein both in dynamic situations, where new contacts are formed, and in more static situations in adult tissues. In vitro studies in spreading cells show that affixin colocalizes with ILK to cell–substrate contacts well before focal adhesion kinase (FAK) and vinculin, whereas, in mature contacts, ILK and affixin colocalize with FAK and vinculin. Interestingly, overexpression of the ILKbinding part of affixin inhibits the development of focal adhesions. In vivo studies show a distribution of affixin in muscle tissue at the interface between the sarcolemma and cytoskeleton, coinciding with the distribution of the β1D integrin splice variant. Studies of the affixin orthologue in Caenorhabditis elegans have showed that affixin is needed for muscle integrity. The mechanism for how affixin defects can cause a muscle phenotype is still unclear. It will now be important to determine in more detail how the ILKinteracting protein affixin mediates integrin-dependent events. 1 Yamaji, S. et al. (2001) A novel integrin-linked kinase binding protein, affixin, is involved in the early stage of cell–substrate interaction. J. Cell Biol. 153, 1251–1264
Donald Gullberg [email protected] http://tcb.trends.com
TRENDS in Cell Biology Vol.11 No.9 September 2001
363
Some don’t like it hot In higher plants, microtubules of cells in interphase are arranged in an array associated with the plasma membrane. This cortical microtubule system controls morphogenesis by determining the alignment of newly synthesized cellulose microfibrils in the cell wall. Higher plants lack defined microtubule-organizing centers (MTOCs), making it particularly difficult to understand how cortical microtubules become organized spatially to serve their function. Novel insights now are provided by the characterization of an Arabidopsis thaliana microtubuleassociated protein (MAP) possessing unique properties. Wasteneys and colleagues1 identified mutants of the MICROTUBULE ORGANIZATION 1 (MOR1) gene, which led to single amino acid substitutions within a putative protein–protein interaction domain of the MOR1 protein. In mutant plants, temperatures above 28°C caused disruption and misalignment of cortical microtubules. Microtubule reorganization commenced as soon as temperatures were lowered again. Mutants grown at restrictive temperature developed severe morphological defects. However, cell patterns remained similar to those in the wild type, indicating that microtubules involved in mitosis and cell plate formation were not affected.
Deformed mutant plants showed normal further development following transfer to permissive temperatures. MOR1 is predicted to possess significant similarity to known MAPs, including human TOGp, Xenopus MAP215 and Drosophila MSPS1. Three conclusions seem outstanding. First, MAPs sharing homology with animal counterparts were not generally expected in plants, given the distinct organization of microtubules in both kingdoms. Second, specific regulatory mechanisms of the plasma-membrane-associated microtubule system can now be scrutinized because MOR1 appears to act exclusively on cortical, but not on cell-division-related, microtubules. Third, the rapid reorganization of cortical microtubules following transfer of mutants to the permissive temperature is intriguing and might hold the key to a long-standing riddle in plant cell development: if cortical microtubules control the alignment of cellulose fibrils, what controls the alignment of cortical microtubules? 1 Whittington, A.T. et al. (2001) MOR1 is essential for organizing cortical microtubules in plants. Nature 411, 610–613
Winfried S. Peters [email protected]
In Brief
Human genome project – where next? The rivalry between the public and privately funded human genome sequencing projects was set aside on 6 June as scientists from the two teams got together in Chevy Chase, Maryland. The goal of the meeting, according to Chad Nussbaum of the Whitehead Institute, was to find the cheapest strategy that can also make sense of a previously unexplored genome. Press reports state that the meeting was conducted in a calm, amicable manner. The groups agreed that both approaches had been useful in sequencing the human genome, and that the best way forward for future projects would depend on the complexity
of the genome being studied and would benefit greatly from more advanced computer software. Two of the main protagonists, Eric Lander and Craig Venter, were not present, and the promise of a paper by Lander critical of Celera’s approach indicates that hostilities might soon recommence (see Science magazine 15 June 2001, p. 292 for further details). Meanwhile, estimates of the number of genes in the human sequence still vary widely. Bill Haseltine, CEO of Human Genome Sciences (HGS), described in the Financial Times as a ‘maverick’, has suggested that there might be as many as 100 000 human genes, despite the fact that most estimates are around 30 000. Haseltine claims that HGS has already identified 60 000 genes through analysis of mRNA transcripts. S.L.
0962-8924/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
Yet another clue to a transmembrane link An elusive component of the integrin signaling cascade is the serine/threonine protein kinase ILK (integrin-linked kinase), which interacts with the cytoplasmic tail of the integrin β1 chain. Initially, some uncertainties about the role of ILK in integrin signaling were caused by lack of good immunological reagents that could verify that integrins and ILK co-distributed within cells. Another problem is the multifunctionality of ILK, which also takes part in events mediated by cell–cell contact. Ishigatsubo and colleagues, in an impressive work, describe a new component in the cascade initiated by integrin–ILK interactions1. They identify by yeast two-hybrid screening a novel ILKbinding protein, affixin, consisting of two tandem calponin homology domains. Detailed studies of the distribution of affixin in spreading cells and in tissues reveal a distribution compatible with affixin serving as an integrin-interacting protein both in dynamic situations, where new contacts are formed, and in more static situations in adult tissues. In vitro studies in spreading cells show that affixin colocalizes with ILK to cell–substrate contacts well before focal adhesion kinase (FAK) and vinculin, whereas, in mature contacts, ILK and affixin colocalize with FAK and vinculin. Interestingly, overexpression of the ILKbinding part of affixin inhibits the development of focal adhesions. In vivo studies show a distribution of affixin in muscle tissue at the interface between the sarcolemma and cytoskeleton, coinciding with the distribution of the β1D integrin splice variant. Studies of the affixin orthologue in Caenorhabditis elegans have showed that affixin is needed for muscle integrity. The mechanism for how affixin defects can cause a muscle phenotype is still unclear. It will now be important to determine in more detail how the ILKinteracting protein affixin mediates integrin-dependent events. 1 Yamaji, S. et al. (2001) A novel integrin-linked kinase binding protein, affixin, is involved in the early stage of cell–substrate interaction. J. Cell Biol. 153, 1251–1264
Donald Gullberg [email protected] http://tcb.trends.com
TRENDS in Cell Biology Vol.11 No.9 September 2001
363
Some don’t like it hot In higher plants, microtubules of cells in interphase are arranged in an array associated with the plasma membrane. This cortical microtubule system controls morphogenesis by determining the alignment of newly synthesized cellulose microfibrils in the cell wall. Higher plants lack defined microtubule-organizing centers (MTOCs), making it particularly difficult to understand how cortical microtubules become organized spatially to serve their function. Novel insights now are provided by the characterization of an Arabidopsis thaliana microtubuleassociated protein (MAP) possessing unique properties. Wasteneys and colleagues1 identified mutants of the MICROTUBULE ORGANIZATION 1 (MOR1) gene, which led to single amino acid substitutions within a putative protein–protein interaction domain of the MOR1 protein. In mutant plants, temperatures above 28°C caused disruption and misalignment of cortical microtubules. Microtubule reorganization commenced as soon as temperatures were lowered again. Mutants grown at restrictive temperature developed severe morphological defects. However, cell patterns remained similar to those in the wild type, indicating that microtubules involved in mitosis and cell plate formation were not affected.
Deformed mutant plants showed normal further development following transfer to permissive temperatures. MOR1 is predicted to possess significant similarity to known MAPs, including human TOGp, Xenopus MAP215 and Drosophila MSPS1. Three conclusions seem outstanding. First, MAPs sharing homology with animal counterparts were not generally expected in plants, given the distinct organization of microtubules in both kingdoms. Second, specific regulatory mechanisms of the plasma-membrane-associated microtubule system can now be scrutinized because MOR1 appears to act exclusively on cortical, but not on cell-division-related, microtubules. Third, the rapid reorganization of cortical microtubules following transfer of mutants to the permissive temperature is intriguing and might hold the key to a long-standing riddle in plant cell development: if cortical microtubules control the alignment of cellulose fibrils, what controls the alignment of cortical microtubules? 1 Whittington, A.T. et al. (2001) MOR1 is essential for organizing cortical microtubules in plants. Nature 411, 610–613
Winfried S. Peters [email protected]
In Brief
Human genome project – where next? The rivalry between the public and privately funded human genome sequencing projects was set aside on 6 June as scientists from the two teams got together in Chevy Chase, Maryland. The goal of the meeting, according to Chad Nussbaum of the Whitehead Institute, was to find the cheapest strategy that can also make sense of a previously unexplored genome. Press reports state that the meeting was conducted in a calm, amicable manner. The groups agreed that both approaches had been useful in sequencing the human genome, and that the best way forward for future projects would depend on the complexity
of the genome being studied and would benefit greatly from more advanced computer software. Two of the main protagonists, Eric Lander and Craig Venter, were not present, and the promise of a paper by Lander critical of Celera’s approach indicates that hostilities might soon recommence (see Science magazine 15 June 2001, p. 292 for further details). Meanwhile, estimates of the number of genes in the human sequence still vary widely. Bill Haseltine, CEO of Human Genome Sciences (HGS), described in the Financial Times as a ‘maverick’, has suggested that there might be as many as 100 000 human genes, despite the fact that most estimates are around 30 000. Haseltine claims that HGS has already identified 60 000 genes through analysis of mRNA transcripts. S.L.
0962-8924/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.