lating natural plant communities, species of Glomales differ in their ability to promotegrowth of particular plant hosts7. The under-lying mechanisms responsible for these differential growth effects are unknown.Bidartondo and colleagues’ study1 raises thetantalizing possibility that asymmetries infungus-mediated transfer of carbohydratesbetween plants could be a factor.

The new work1 is significant because itdemonstrates that Glomales, which pro-duce by far the most common form of mycorrhizae, can be drawn into epiparasiticassociations. It also joins a growing body ofresearch suggesting that mutualisms are notstable endpoints in evolution, but are in-herently unstable and can be disrupted byconflicts of interest among the partners8,9.The breakdown of mutualisms can lead toparasitism10 or even the complete dissolu-tion of the symbiosis11,12. In the groups studied by Bidartondo et al., the plants areparasitic on AM fungi and their associatedgreen plants. At the other extreme, an AMfungal species, Glomus macrocarpum, hasbeen implicated as the cause of stunt diseaseof tobacco plants13.

Clearly, the characterization of AM sym-bioses as benign, stable associations does not

reflect their dynamic nature. Functional andecological studies are now needed to quan-tify the costs and benefits of mycorrhizalsymbioses and to understand what causesthem to shift along the continuum frommutualism to parasitism14. ■

David S. Hibbett is in the Department of Biology,Clark University, 950 Main Street, Worcester,Massachusetts 01610-1477, USA. e-mail: dhibbett@blackntan.clarku.edu

1. Bidartondo, M. I. et al. Nature 419, 389–392 (2002).

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(1996).

3. Taylor, D. L. & Bruns, T. D. Proc. Natl Acad. Sci. USA 94,

4510–4515 (1997).

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51, 923–931 (2001).

5. Bever, J. D., Morton, J. B., Antonovics, J. & Schultz, P. A. J. Ecol.

84, 71–82 (1996).

6. Sanders, I. R. & Fitter, A. H. Mycol. Res. 96, 415–419 (1992).

7. van der Heijden, M. G. A. et al. Nature 396, 69–72 (1998).

8. Herre, E. A., Knowlton, N., Mueller, U. G. & Rehner, S. A.

Trends Ecol. Evol. 14, 49–53 (1999).

9. Bronstein, J. L. Trends Ecol. Evol. 9, 214–217 (1994).

10.Pellmyr, O., Leebens-Mack, J. & Huth, C. J. Nature 380,

155–156 (1996).

11.Hibbett, D. S., Gilbert, L.-B. & Donoghue, M. J. Nature 407,

506–508 (2000).

12.Lutzoni, F., Pagel, M. & Reeb, V. Nature 411, 937–940

(2001).

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(1986).

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reaction and adding as little as 1% chiralalcohol sealed the fate of the product.

Singleton and Vo1 simply asked whetherthe nonlinearity of this reaction was sensi-tive enough to amplify the random, statisti-cal imbalance that occurs naturally in a ‘balanced’ mixture of handed molecules. Totheir surprise, the products were formed inimbalanced proportions, but from reactionto reaction the amount of chiral product didnot vary in a statistical way, indicating a systematic cause for the selectivity. Oddlyenough, Soai had patented the same reac-tion, assuming random behaviour7.

Through a series of careful control anddoping experiments, Singleton and Vo1

deduced that chiral impurities in the reac-tion solvent — at the level of parts per billionand too small to be detected directly — wereat the root of the phenomenon. Their resultscorroborate Soai’s report of the effect ofminor amounts of chiral additives on thisreaction and emphasize the extreme sensi-tivity of the autocatalytic reaction. The highdegree of selectivity and the apparent abilityof so many different additives to instigatethis selectivity demonstrate the need foradditional experimentation and a completekinetic analysis8.

The work highlights a caveat in the studyof the origin of biological homochirality: theneed to reconcile studies of a reaction thathas a highly nonlinear response to devia-tions from the achiral state with the inherentpresence of chiral impurities after a billionyears of life on the planet. In contrast to the worries of biological contamination in a modern context, there remain issues of degradation or re-equilibration of thehanded forms in a prebiotic context, whichSingleton and Vo address.

Indeed, one problem associated with try-ing to explain the prebiotic origin of achiralsymmetry breaking as being due to crystal-lization is that the system would re-equili-brate when crystals dissolved or degraded.But coupling chirality in crystals with chemistry as selective as the diisopropylzinc/pyrimidine carboxaldehyde systemmeans that symmetry breaking during crystallization could be propagated fromone type of compound to another. Clearly,any process capable of shifting the balance inone setting need only leave a trace imbalanceto influence the outcome of an autocatalyticreaction such as the one investigated by Soai.

Many proposals for the origin of biologi-cal homochirality have been predicated onthe idea that a deterministic mechanism isnecessary3 — that is, that the handedness wesee in biomolecules must inherently bedetermined in the laws of physics, like theloss of parity. Singleton and Vo’s study, inconjunction with Soai’s work, supports theidea that abiological autocatalysis could bethe process by which a randomly generatedtrace chiral influence was amplified. If so,

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346 NATURE | VOL 419 | 26 SEPTEMBER 2002 | www.nature.com/nature

Chemistry

Shattered mirrorsJay S. Siegel

How did the preference for ‘single-handedness’ in biological moleculesarise? Amplification of the trace imbalance in a mixture of handedmolecules bolsters the case for chance being the answer.

Some molecules are chiral — they existin two forms that are mirror images ofeach other, right-handed and left-

handed. Thus it seems reasonable that reac-tions that form chiral molecules from purelyachiral precursors should produce equalamounts of each handed form to preserveachiral symmetry. So how did biologicalprocesses develop a preference for one or theother, such as left-handed amino acids, andright-handed sugars? In the Journal of theAmerican Chemical Society, Singleton andVo1 report that, starting from two achiralcompounds, they have prepared a chiralproduct in which one handed form domi-nates over the other. Have they demonstrat-ed a fundamental principle that explains themystery surrounding the predominance of aparticular handedness in biomolecules?

The superstition surrounding theimmutability of the achiral state has led to areverence for molecular phenomena thatbreak such reaction symmetry. However, formany years it has been known that such sym-metry breaking can occur randomly througha variety of different mechanisms2: for exam-

ple, during the crystallization of a solution inwhich both hands are in equilibrium3, or inthe spontaneous formation of chiral liquid-crystal domains4. In such cases, a small devi-ation from the achiral state is coupled to aprocess that is self-perpetuating and expo-nentially self-amplifying — that is, autocat-alytic. As yet, spontaneous achiral symmetrybreaking has not been observed for a reac-tion that occurs completely in homogeneoussolution, and that is what Singleton and Vowere hoping to test.

The authors were following up the workof the Japanese chemist Kenso Soai, whoreported an autocatalytic reaction in which aslight excess of one hand in the product suffices to direct any future product to thatsame handed form5,6. The reaction schemeinvolved batchwise additions of diisopropylzinc to 2-methylpyrimidine-5-carboxalde-hyde, recycling the product as the catalyst forthe next batch. The selectivity (that is, thefraction of handed product formed) wasdetermined as a function of batch number.In Soai’s experiments, numerous alcoholswere claimed to influence the course of the

© 2002 Nature Publishing Group

then in the prebiotic world examples ofdominant molecular handedness werealready likely to have been abundant. Thus,biomolecular handedness is an imperative ofthe evolutionary process downstream frommolecular handedness, and smart moneystill bets on chance over determinism9. ■

Jay S. Siegel is in the Department of Chemistry,University of California at San Diego, La Jolla,California 92093-0358, USA. e-mail: jss@chem.ucsd.edu

1. Singleton, D. A. & Vo, L. K. J. Am. Chem. Soc. 124, 10010–10011

(2002).

2. Siegel, J. S. Chirality 10, 24–27 (1998).

3. Havinga, E. Chem. Weekblad 38, 642–645 (1941).

4. Link, D. R. et al. Science 278, 1924–1927 (1997).

5. Soai, K., Shibata, T., Morioka, H. & Choi, K. Nature 378,

767–768 (1995).

6. Sato, I., Kadowaki, K. & Soai, K. Angew. Chem. Int. Edn 39,

1510–1512 (2000).

7. Soai, K., Shibata, T. & Kowata,Y. Japan Kokai Tokkyo Koh o

(1997) 9-268179 (application date: 1 February 1996 and

18 April 1996).

8. Blackmond, D. G., McMillan, C. R., Ramdeehul, S.,

Schorm, A. & Brown, J. M. J. Am. Chem. Soc. 123,

10103–10104 (2001).

9. Berger, R., Quack, M. & Tschumper, G. S. Helv. Chim. Acta 83,

1919–1950 (2000).

in the formation of clathrin-coated vesicles.Previous structural analyses6,7 of the

ENTH domains of CALM and epsin, in thepresence of the lipid phosphatidylinositol-4,5-bisphosphate (PIP2), revealed a tellingfact: these domains bind PIP2 differently. Infact, the domain in AP180 and CALM is nowreferred to as the ANTH (for ‘AP180-like N-terminal homology’) domain. Ford et al.1

now delve a little deeper. Their structuralstudies show that the PIP2-binding site in theENTH domain of most (but not all) epsin-family members involves positively chargedamino acids that are dispersed through several of the structural units (a-helices) thatbuild up the domain. Upon PIP2 binding, thedomain is reorganized into a deep basicgroove in which the phosphorylated inositolhead group is buried. This is radically differ-ent from the ANTH domains of AP180 andCALM, which bind PIP2 through a small,positively charged stretch of consecutiveamino acids6.

Ford et al. also find that this discrepancyin the lipid-bound structures of the ENTHand ANTH domains relates to functionalspecificity in vesicle budding. They showthat epsin is recruited to artificial PIP2-enriched liposomes (spherical structuresthat, like natural vesicles, are bounded by alipid bilayer), and can bend this templateinto tubules. Amazingly, this also happenswhen the experiment is done with epsin’s

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NATURE | VOL 419 | 26 SEPTEMBER 2002 | www.nature.com/nature 347

If you could look inside your cells youwould see several different compartments,delimited by membranes composed of

lipids and proteins. You would notice thatthe compartments are highly dynamic andundergo a series of changes in morphology,allowing small, sack-like structures to budoff from time to time. These membrane-bounded carriers can differ in molecularcomposition, size and shape, and shuttlecargo from one compartment to another.One of the biggest challenges in cell biology isto unravel the molecular events that shapethese vesicles emerging from a membrane,and that couple this process to the selectionof cargo. On page 361 of this issue, Ford andco-workers1 propose a mechanism of actionfor proteins known as epsins, and suggestthat they bridge the gap between cargo selection and vesicle shaping.

Among the several types of vesicle thatbud off from the plasma membrane — themembrane that surrounds the cell — somecan be seen to be decorated by an electron-dense meshwork when visualized at high res-olution, for example by electron microscopy.This meshwork is referred to as the mem-brane coat and is mainly involved in shapingthe vesicle. It is often composed primarily oftwo protein complexes: clathrin and the AP2adaptor. A whole bunch of accessory pro-teins are also part of the coat and controlmembrane budding, through molecularconnections that involve protein–protein aswell as protein–lipid interactions2 (Fig. 1).

Proteins of the epsin family3, which areunder the spotlight in Ford and colleagues’study1, are among these accessory proteins.Epsins have at one end (the amino, or N, ter-minus) a structurally defined module thatbinds phosphoinositides — lipids bearing ahead group composed of a so-called inositol

ring, modified with phosphate groups(phosphorylated) at several positions. Thismodule, called the ENTH (for ‘epsin N-terminal homology’) domain, is also foundin other proteins involved in membranedynamics4,5. Among these, the brain-specificprotein AP180 and its ubiquitously ex-pressed relative CALM are also implicated

Membrane transport

The making of a vesicleAnne A. Schmidt

The transport of molecules from one cellular compartment to another oftenrequires membrane-bounded carriers. New work gives insight into how theshaping of membrane into such vesicles is linked to the selection of cargo.

Figure 1 Bending membranes. A vesicle is shown budding from a cell’s plasma membrane into thecell. Ford et al.1 propose that epsin proteins induce the membrane curvature associated withbudding, and couple it to loading of the vesicle with cargo. Epsin associates with clathrin and the AP2 adaptor complex, basic components of a budding vesicle’s ‘coat’. Epsin’s ENTH domain alsobinds the phosphorylated head group of the lipid PIP2 in the nearby leaflet of a membrane bilayer(see inset). Ford et al. propose that this disturbs the bilayer’s stability, contributing to membranecurvature. Through its interaction with AP2, which in turn interacts with cargo receptors, epsinmight couple initial membrane invagination to cargo recruitment. Then, while the coated pit isgrowing, epsin might affect membrane curvature at the rim of the coat, where another of its partners,Eps15, has been detected. Note that other proteins can also induce membrane curvature, and futurework should reveal their spatial and temporal roles.

Epsin

ENTH

Eps15

Assembledclathrin

Clathrin-coated pit

Transmembranereceptor

PIP2

AP2adaptorcomplex

Membrane bilayer

Phosphate group

Inside cell

Outside cell

© 2002 Nature Publishing Group