- Email: [email protected]
Life still a secret
MISCELLANEA
Terrone Rosenberty
1.
Dept of Pharmacology, Case Western Reserve, University School of Medicine, Cleveland, OH 44106-4965, USA.
protein) or binding of a second messenger such as Ca2+ (e.g. retinal recoverin) or CTP (e.g. ARF) and that the released myristoyl group then targets the protein to a membrane. Thus, as for prenylated proteins, specific protein-lipid interactions may be essential for function. Section 5 covers a range of topics involving glycosylphosphatidylinositol (Q&anchored proteins. Post-translational modification by GPI differs from lipid modifications described in the earlier sections in several important ways. First, the modification occurs in the lumen of the endoplasmic reticulum (ER) and, thus, only involves proteins destined for the ceil surface. Second, the CPI anchor provides the only attachment of these proteins to the cell membrane, and cleavage of the CPI by addition of phosphatidylinositolspecific phospholipase C releases the proteins from the cell. Third, the GPI is assembled as a glycolipid precursor in a biosynthetic pathway of six to eight steps prior to transfer to the protein. The chapters in this section include established procedures for the determination of GPI structures, many
Life still a secret What is Life? The Next Fifty Years edited by M. P. Murphy and 1. A. I. 0’ Neil, Cambridge University Press, 7 995. f 7 7.95/lJS$24.95 (7 9 7 pages) ISBN 0527 45509 X This is a pretty odd collection of essays (as opposed to a collection of pretty odd essays, although some of them are that too) to expect to sell between one pair of covers. Also, contributors to ‘proceedings’ of brainstormingstyle conferences often have difficulty, after the stimulating ‘rencontre’ with other wide-ranging thinkers (and perhaps alcoholic catalysis), once back in front of their word processors and amid the pressures of earning their livings. With such an august line-up of authors as here, this last factor should have been minimal, but I did sense
446
0 1996 Elsevier Science
Ltd
of which were introduced by M. A. J. Ferguson and his colleagues, and the isolation of GPI-specific phospholipases C and D, which are important diagnostic and structural tools. Protocols for assaying individual steps in the biosynthetic pathway with cell lysates and subcellularfractions have also been found to be useful in many laboratories. However, in contrast to the intracellular lipid modifications described in earlier sections, it has not yet been possible to extract and isolate the biosynthetic enzymes. Expression-cloning studies are described that have used GPI-deficient mutant cell lines to isolate four mammalian genes involved in the biosynthetic pathway, and the protein sequences deduced from these genes offer some insight into the structures of the enzymes. The unique core glycan structure of GPls is highly conserved from yeast to mammals, and this has prompted speculation that CPls are recognized as a cell signal in addition to their role in tethering proteins to the cell surface. Moreover, several intermediates in the proposed mammalian biosynthetic pathway can escape the ER and proceed to the
plasma membrane as free GPls, further contributing to speculation about their functional roles. GPI-anchored proteins appear enriched in plasma membrane invaginations called caveolae, and, in some polarized epithelial cells, a GPI anchor serves as a potent signal for directing proteins to the apical surface. These observations suggest that GPls may be recognized as a targeting signal and segregated to specific organelles on their way to caveolae on the cell surface. One chapter describes a simple procedure for isolating caveolae based on the insolubility of GPI-anchored proteins in the non-ionic detergent Triton X-l 00 at 4°C. Complexes that include GPI-anchored proteins may be isolated as membrane microdomains that are buoyant in sucrose density gradients. However, techniques to fractionate these buoyant complexes and assess their heterogeneity have not been developed yet, and caution must be taken in equating them with caveolae. Nevertheless, the isolation and characterization of caveolae is of great interest in exploring potential GPI function.
constraint, and hurriedness of presentation relative to their normal written styles, by some of them. The occasion was the marking, in Trinity College, Dublin, of the fiftieth anniversary of the then refugee Schrodinger’s lectures that formed the basis of his book What is Life? That book is marked not solely or even mainly by the originality of its prophetic insights, but by the immense stimulating effect it had on research programmes of intellectual biologists of the time. But, even half a century ago, the whole world of knowledge was so relatively small and undeveloped that not only Schrodinger, but those he immediately stimulated, could claim (whether validly or not) to interrelate most of the areas of biological science in their thoughts, from the origin of life itself to that of culture, via how the brain might work. The equivalent ‘forefront’ thoughts now are had by an immensely diverse set of specialized and recondite scientific intellectuals, and only some of these possess the gift of - or even entertain thoughts remotely amenable to popularization. By ‘popularization’, I mean, without intending to insult either readers or myself, the rendering
of ideas comprehensible to a typical cell- and molecularly-trained working biologist, so long as he or she is at least willing to work at their continuing education rather than looking for an enlivening bedtime book or a substitute for ‘ER’ after the lab on Wednesday evenings. How many of us does that include, even? The nonpopularizers, meanwhile, discourse on several avant-garde topics, to do with minds, brains and the need for new physics, that are surely not currently comprehensible to anyone other than (one hopes!) members of research communities who indulge in their own special books. Inevitably, this journal’s readers will mainly be those better equipped for the chapters addressing origin-of-life/evolution issues, but the brain-and-perception ones are there too and are pretty technical at that. This makes for a book without a market beyond libraries, I suspect (even at its modest price). Given this diversity, is there a thread or debate to be discerned within these pages? In their orienting introduction, very useful especially for younger readers, the editors point out that Schrijdinger gave distinctive expression to two ideas about ‘life’. One, his
trends in CELL BIOLOGY
(Vol.
6) November
1996
MISCELLANEA
order-from-disorder principle, is nowadays the province of students of metabolism, especially energy metabolism, and it is implied that his original way of formulating it has not stood the critical test of time for its usefulness. The other, order-from-order (i.e. heredity), they suggest to have been an incontestably striking insight. He proposed ‘aperiodic crystals’, which we now see to be the sequence of DNA, as the sole vehicle of its propagation. Although we live in an era where scientists are encouraged to go around schools giving talks like ‘Genes are Us’, and undergraduates describe development as ‘a programme of gene activity’ as if quoting a litany, it is not incontestable that gene sequences are the prime and only necessary source of biological organization. The chapters that protest against this orthodoxy, while almost certain to be dismissed as misguided, should be the
A mixed membrane medley Biochemistry of Cell Membranes (A Compendium of Selected Topics) edited
by S. Papa and). M. Tager, Birkhtiuser Verlag, 1995. $149.OO/DM 188.00 (376 pages) ISBN 3 7643 5056 3
Advances in our understanding of the many facets to membrane biology at the molecular level continue to progress at an exciting pace. This monograph presents a collection of invited reviews of selected topics on various aspects of membrane biology, chosen, according to the editors, in areas where progress has been extremely rapid. Furthermore, the statement in the preface that this ‘monograph should serve as a benchmark for indicating the most important lines for future research’ serves to titillate the reader’s interest. The seven major topics selected depict the diversity of the subject matter. Since the individual reader cannot be expected to be trends
in CELL BIOLOGY
(Vol.
6) November
most interesting to young biologists. Stuart Kauffman makes his protest archly and politely, as befits a guest disagreeing with his ‘host’. His heterodox ideas about self-organization at the edge of chaos are worthy of popularization, reminding me personally of a heady era about a quarter(as opposed to half-) century ago, when thought in certain biological circles possessed an open-endedness currently lost. Stephen J. Gould demurs more convolutedly, but for essentially opposite reasons. Rather than plugging for the self-shaping and constraining forces of self-organization to add to DNA sequence information, he wishes more status within core biological thought be accorded to historicity, the ‘just-so-ness’ of the way things happen to have panned out from among all possible worlds in evolution. He has developed this idea eloquently in his book WonderfulLife. But,
in the present chapter, he asks for the recognition of general ‘laws’ of evolution on the large scale, that cannot be deduced from molecular or nearmolecular levels of organization. I find these two claims for the importance of palaeontologyand allied disciplines to biology theory incompatible, but the exercise of judging such things for themselves is one of the better things readers might get from the book. The collection proceeds, via sensible and informative discussions of such things as the origin of language and the ‘computability’ or otherwise of develooment. to the aforementioned discussions related in various ways to the mind-bodv. or at least the molecule-perception, problem. It is here that, however willing the spirit initially, most trends in CELL BIOLOGY readers’ flesh will probably weaken. Life is getting tough for would-be renaissance men and women!
expert in all of these areas, it is incumbent on the authors to present a rationale for the material under review. Unfortunately, some chapters dive into the deep end without providing this key background information. Although it would be easy to criticize the editors and their advisory board for omissions from a volume of this nature, one must recognize that choices have to be made. However, it is essential to guard against the use of this forum as a vehicle for the promotion of weak hypotheses without the benefit of peer review. In this regard, inclusion of specific items in this monograph is questionable. For example, in the chapter on casein kinase 2 (CK2), the authors state that ‘an intriguing outcome of the studies on CK2 is the discrepancy between the abundance of circumstantial data on the role of this enzyme in cellular regulation and the paucity of direct information about the molecular mechanism by which CK2 could accomplish such a putative function within the cell’. Undaunted by their conclusion, the reviewers proceed without any suggestions for the resolution of this ‘intriguing outcome’. Many of the remaining reviews focus almost exclusively on the authors’ studies, which may in part be due to presentation of a narrow topic. A flagrant offence is the article by Defilippi et a/. who have taken this opportunity to publish a research paper without apparent peer
review. It is somewhat disturbing that this appears to have escaped the editors’ attention. In addition, one cannot ignore the fact that an important section on membranes and disease is limited to a single contribution when exciting developments are being made on membrane-associated diseases such as cystic fibrosis. Recent studies on membrane protein structure and developments in unravelling the intricacies of intracellular protein transport serve as examples of the rapid growth of knowledge in these areas since preparation of this monograph. The three-dimensional X-ray structure of cytochrome c oxidase has been reported independently by the groups of Yoshikawa and coworkers in Japan and Michel and coworkers in Germany. This is the largest membrane protein to be crystallized and analysed at atomic resolution, thereby representing a significant major advance in the field. However, one must not overlook the exciting studies that employ two-dimensional crystals of membrane proteins, which are beginning to provide structural information on a variety of proteins including Band 3 from the human red cell. Independent structural studies from the laboratories of Sigler and Sprang on components of the membrane-attached heterotrimeric C protein are providing molecular insights to their function in signal transduction. Progress on intracellular protein
1996
0 1996 Elsevier Science Ltd
,onathan
co0ke
National
Institute
for Medical Research, The Ridgeway, Mill Hill, London, UK NW7 144.
447
Terrone Rosenberty
1.
Dept of Pharmacology, Case Western Reserve, University School of Medicine, Cleveland, OH 44106-4965, USA.
protein) or binding of a second messenger such as Ca2+ (e.g. retinal recoverin) or CTP (e.g. ARF) and that the released myristoyl group then targets the protein to a membrane. Thus, as for prenylated proteins, specific protein-lipid interactions may be essential for function. Section 5 covers a range of topics involving glycosylphosphatidylinositol (Q&anchored proteins. Post-translational modification by GPI differs from lipid modifications described in the earlier sections in several important ways. First, the modification occurs in the lumen of the endoplasmic reticulum (ER) and, thus, only involves proteins destined for the ceil surface. Second, the CPI anchor provides the only attachment of these proteins to the cell membrane, and cleavage of the CPI by addition of phosphatidylinositolspecific phospholipase C releases the proteins from the cell. Third, the GPI is assembled as a glycolipid precursor in a biosynthetic pathway of six to eight steps prior to transfer to the protein. The chapters in this section include established procedures for the determination of GPI structures, many
Life still a secret What is Life? The Next Fifty Years edited by M. P. Murphy and 1. A. I. 0’ Neil, Cambridge University Press, 7 995. f 7 7.95/lJS$24.95 (7 9 7 pages) ISBN 0527 45509 X This is a pretty odd collection of essays (as opposed to a collection of pretty odd essays, although some of them are that too) to expect to sell between one pair of covers. Also, contributors to ‘proceedings’ of brainstormingstyle conferences often have difficulty, after the stimulating ‘rencontre’ with other wide-ranging thinkers (and perhaps alcoholic catalysis), once back in front of their word processors and amid the pressures of earning their livings. With such an august line-up of authors as here, this last factor should have been minimal, but I did sense
446
0 1996 Elsevier Science
Ltd
of which were introduced by M. A. J. Ferguson and his colleagues, and the isolation of GPI-specific phospholipases C and D, which are important diagnostic and structural tools. Protocols for assaying individual steps in the biosynthetic pathway with cell lysates and subcellularfractions have also been found to be useful in many laboratories. However, in contrast to the intracellular lipid modifications described in earlier sections, it has not yet been possible to extract and isolate the biosynthetic enzymes. Expression-cloning studies are described that have used GPI-deficient mutant cell lines to isolate four mammalian genes involved in the biosynthetic pathway, and the protein sequences deduced from these genes offer some insight into the structures of the enzymes. The unique core glycan structure of GPls is highly conserved from yeast to mammals, and this has prompted speculation that CPls are recognized as a cell signal in addition to their role in tethering proteins to the cell surface. Moreover, several intermediates in the proposed mammalian biosynthetic pathway can escape the ER and proceed to the
plasma membrane as free GPls, further contributing to speculation about their functional roles. GPI-anchored proteins appear enriched in plasma membrane invaginations called caveolae, and, in some polarized epithelial cells, a GPI anchor serves as a potent signal for directing proteins to the apical surface. These observations suggest that GPls may be recognized as a targeting signal and segregated to specific organelles on their way to caveolae on the cell surface. One chapter describes a simple procedure for isolating caveolae based on the insolubility of GPI-anchored proteins in the non-ionic detergent Triton X-l 00 at 4°C. Complexes that include GPI-anchored proteins may be isolated as membrane microdomains that are buoyant in sucrose density gradients. However, techniques to fractionate these buoyant complexes and assess their heterogeneity have not been developed yet, and caution must be taken in equating them with caveolae. Nevertheless, the isolation and characterization of caveolae is of great interest in exploring potential GPI function.
constraint, and hurriedness of presentation relative to their normal written styles, by some of them. The occasion was the marking, in Trinity College, Dublin, of the fiftieth anniversary of the then refugee Schrodinger’s lectures that formed the basis of his book What is Life? That book is marked not solely or even mainly by the originality of its prophetic insights, but by the immense stimulating effect it had on research programmes of intellectual biologists of the time. But, even half a century ago, the whole world of knowledge was so relatively small and undeveloped that not only Schrodinger, but those he immediately stimulated, could claim (whether validly or not) to interrelate most of the areas of biological science in their thoughts, from the origin of life itself to that of culture, via how the brain might work. The equivalent ‘forefront’ thoughts now are had by an immensely diverse set of specialized and recondite scientific intellectuals, and only some of these possess the gift of - or even entertain thoughts remotely amenable to popularization. By ‘popularization’, I mean, without intending to insult either readers or myself, the rendering
of ideas comprehensible to a typical cell- and molecularly-trained working biologist, so long as he or she is at least willing to work at their continuing education rather than looking for an enlivening bedtime book or a substitute for ‘ER’ after the lab on Wednesday evenings. How many of us does that include, even? The nonpopularizers, meanwhile, discourse on several avant-garde topics, to do with minds, brains and the need for new physics, that are surely not currently comprehensible to anyone other than (one hopes!) members of research communities who indulge in their own special books. Inevitably, this journal’s readers will mainly be those better equipped for the chapters addressing origin-of-life/evolution issues, but the brain-and-perception ones are there too and are pretty technical at that. This makes for a book without a market beyond libraries, I suspect (even at its modest price). Given this diversity, is there a thread or debate to be discerned within these pages? In their orienting introduction, very useful especially for younger readers, the editors point out that Schrijdinger gave distinctive expression to two ideas about ‘life’. One, his
trends in CELL BIOLOGY
(Vol.
6) November
1996
MISCELLANEA
order-from-disorder principle, is nowadays the province of students of metabolism, especially energy metabolism, and it is implied that his original way of formulating it has not stood the critical test of time for its usefulness. The other, order-from-order (i.e. heredity), they suggest to have been an incontestably striking insight. He proposed ‘aperiodic crystals’, which we now see to be the sequence of DNA, as the sole vehicle of its propagation. Although we live in an era where scientists are encouraged to go around schools giving talks like ‘Genes are Us’, and undergraduates describe development as ‘a programme of gene activity’ as if quoting a litany, it is not incontestable that gene sequences are the prime and only necessary source of biological organization. The chapters that protest against this orthodoxy, while almost certain to be dismissed as misguided, should be the
A mixed membrane medley Biochemistry of Cell Membranes (A Compendium of Selected Topics) edited
by S. Papa and). M. Tager, Birkhtiuser Verlag, 1995. $149.OO/DM 188.00 (376 pages) ISBN 3 7643 5056 3
Advances in our understanding of the many facets to membrane biology at the molecular level continue to progress at an exciting pace. This monograph presents a collection of invited reviews of selected topics on various aspects of membrane biology, chosen, according to the editors, in areas where progress has been extremely rapid. Furthermore, the statement in the preface that this ‘monograph should serve as a benchmark for indicating the most important lines for future research’ serves to titillate the reader’s interest. The seven major topics selected depict the diversity of the subject matter. Since the individual reader cannot be expected to be trends
in CELL BIOLOGY
(Vol.
6) November
most interesting to young biologists. Stuart Kauffman makes his protest archly and politely, as befits a guest disagreeing with his ‘host’. His heterodox ideas about self-organization at the edge of chaos are worthy of popularization, reminding me personally of a heady era about a quarter(as opposed to half-) century ago, when thought in certain biological circles possessed an open-endedness currently lost. Stephen J. Gould demurs more convolutedly, but for essentially opposite reasons. Rather than plugging for the self-shaping and constraining forces of self-organization to add to DNA sequence information, he wishes more status within core biological thought be accorded to historicity, the ‘just-so-ness’ of the way things happen to have panned out from among all possible worlds in evolution. He has developed this idea eloquently in his book WonderfulLife. But,
in the present chapter, he asks for the recognition of general ‘laws’ of evolution on the large scale, that cannot be deduced from molecular or nearmolecular levels of organization. I find these two claims for the importance of palaeontologyand allied disciplines to biology theory incompatible, but the exercise of judging such things for themselves is one of the better things readers might get from the book. The collection proceeds, via sensible and informative discussions of such things as the origin of language and the ‘computability’ or otherwise of develooment. to the aforementioned discussions related in various ways to the mind-bodv. or at least the molecule-perception, problem. It is here that, however willing the spirit initially, most trends in CELL BIOLOGY readers’ flesh will probably weaken. Life is getting tough for would-be renaissance men and women!
expert in all of these areas, it is incumbent on the authors to present a rationale for the material under review. Unfortunately, some chapters dive into the deep end without providing this key background information. Although it would be easy to criticize the editors and their advisory board for omissions from a volume of this nature, one must recognize that choices have to be made. However, it is essential to guard against the use of this forum as a vehicle for the promotion of weak hypotheses without the benefit of peer review. In this regard, inclusion of specific items in this monograph is questionable. For example, in the chapter on casein kinase 2 (CK2), the authors state that ‘an intriguing outcome of the studies on CK2 is the discrepancy between the abundance of circumstantial data on the role of this enzyme in cellular regulation and the paucity of direct information about the molecular mechanism by which CK2 could accomplish such a putative function within the cell’. Undaunted by their conclusion, the reviewers proceed without any suggestions for the resolution of this ‘intriguing outcome’. Many of the remaining reviews focus almost exclusively on the authors’ studies, which may in part be due to presentation of a narrow topic. A flagrant offence is the article by Defilippi et a/. who have taken this opportunity to publish a research paper without apparent peer
review. It is somewhat disturbing that this appears to have escaped the editors’ attention. In addition, one cannot ignore the fact that an important section on membranes and disease is limited to a single contribution when exciting developments are being made on membrane-associated diseases such as cystic fibrosis. Recent studies on membrane protein structure and developments in unravelling the intricacies of intracellular protein transport serve as examples of the rapid growth of knowledge in these areas since preparation of this monograph. The three-dimensional X-ray structure of cytochrome c oxidase has been reported independently by the groups of Yoshikawa and coworkers in Japan and Michel and coworkers in Germany. This is the largest membrane protein to be crystallized and analysed at atomic resolution, thereby representing a significant major advance in the field. However, one must not overlook the exciting studies that employ two-dimensional crystals of membrane proteins, which are beginning to provide structural information on a variety of proteins including Band 3 from the human red cell. Independent structural studies from the laboratories of Sigler and Sprang on components of the membrane-attached heterotrimeric C protein are providing molecular insights to their function in signal transduction. Progress on intracellular protein
1996
0 1996 Elsevier Science Ltd
,onathan
co0ke
National
Institute
for Medical Research, The Ridgeway, Mill Hill, London, UK NW7 144.
447