Mycological Mystery Tour
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NEWS AND VIEWS are thought to undergo receptor-mediated the virus surface, and its possible connec multitude of new questions. It also shows, endocytosis. Under conditions of low pH, tions to the M protein and to the inner once again, the resourcefulness of viruses the E dimer reorganizes itself to form a nucleocapsid protein, are also unclear. when attacking their cellular prey. D trimer9 and activates the fusion process. The size of sE is too big for it to have T=4 The residues reponsible for fusion, the symmetry but it might be consistent with Richard J. Kuhn and Michael G. Rossmann so-called fusion domain, map to a highly T=3 symmetry. For example, if the glyco are in the Department of Biological Sci conserved hydrophobic loop in domain II protein dimer axes were placed coincident ences, Purdue University, West Lafayette, which has been implicated in the fusogenic with the icosahedral and quasi two-fold Indiana 4 790 7, USA. 10 activity of the virus . Exactly how the E axes, a rearrangement of domain II in dimer reorganizes to form a trimer re duced by changes in pH would bring these 1. Wilson, I .. Skehei,J. &Wiley. D. Nature289. 366--373 mains obscure, but from data derived domains together to form trimers and (1981). from antibody probes it seems that there is abrogate the dimer contacts (see figure). 2. Wilson, I. A., Wiley, D. C. &Skehel, J. J. Nature289, 373-378 (1981). a significant reorganization of domain II At the same time this would expose the 3. Weis, W. eta/. Nature333, 426-431 (1988). following the conformational change. The fusogenic peptide previously hidden in a 4. Bullough, P. A., Hughson. F. M .. Skehel, J. J. &Wiley, soluble E dimer used by Rey et al. fails to pocket under the attached carbohydrate. D.C. Nature371, 37-43 (1994). 5. Rey, F. A., Heinz, F. X., Mandl, C., Kunz. C. & Harrison, undergo the conformational change, sug Future studies of TBE could include S.C. Nature375, 291-298 (1995). gesting that the additional E sequences, or cryoelectron microscopy analysis which 6. Heinz. F. X. etai.J. Viral. 65, 5579-5583 (1991). 7. Cheng, R. H. eta/. Ce/180, 621-630 (1995). perhaps the M protein, may be required would be useful to identify the symmetry 8. Leahy, D. J., Hendrickson. W. A., Aukhil, I. & Erickson. for inducing the remarkable change neces of the virion. H. P. Science258, 987-991 (1992). 9. Allison, S. L. etai.J. Viral. 69,695-700 (1995). sary to bring about membrane fusion. The structure of the major envelope 10. Roehrig, J. T.. Hunt. A. R.. Johnson, A. J. & Hawkes, R. A. Exactly how the E dimer is arranged on protein of a flavivirus provides us with a Virology171, 49-60 (1989). -PARASITISM--------------------------------, Mycological mystery tour FROM the mossy depths of the spruce cellar and showed that they sprouted example of a Squamanita protocarpic forests of the Olympic peninsula in mushrooms in the absence of a support· tuber. Washington state comes an eldritch tale. ing mycelium. It was this second occur· It was then that suspicions of parasit· For within these ancient forests are renee which inspired the creation of ism arose, as examples of this odd spe· mushrooms growing on the living tissues the genus Squamanita, based largely on cies were collected from within colonies of yet other mushrooms: the hosts may the presence of the basal bulb. of otherwise normal examples of Cys remain fertile but become deformed Several decades of mycological dis· toderma. "It seems as if another agaric almost beyond recognition. quiet set in, as collectors and resear· has been grafted on to the stem of The pictured specimen of a Galerina chers suspected that Squamanita was Cystoderma amianthinum", observed species, from which grow several one collector. Mycologists began fruiting bodies of the parasitic to view Squamanita as a change· Squamanita, is unusual in that it ling, parasitizing Cystoderma to still looks like a mushroom (for the extent of perverting the tis· scale, the cap is 6 mm across). It sues destined to be the upper was this specimen, in part, which stalk and cap of the mushroom to enabled S. A. Redhead and col· its own image - giving the leagues to resolve a thorny tax· appearance of a Squamanita onomic tangle which has long growing from a tuber with an been the bane of mycologists uncanny resemblance to Cys (Canadian Journal of Botany 72, toderma. 1812-1824; 1994). The swollen, Support for this idea came oddly coloured basal 'protocar· from Japan, with the discovery in pic tubers' from which Squama· 1990 of an example of Squama· nita fruiting bodies were seen to nita with a protocarpic tuber emerge were so distorted that bearing characteristic features they were long thought to be parts of more than a fungal curio. Persistent of another fungus, Phaerolepiota aurea. Squamanita itself, rather than the tissues reports of the varied character of the The clear-cut parasitism of Squamanita of hosts made shapeless by possession. 'protocarpic tubers' suggested that they on specimens of Galerina from the Olym· Redhead and colleagues now number were less constant than taxonomic pic peninsula now unequivocally ex· Squamanita as one of a small number of propriety demanded. More worrying poses the relationship between host and fungal genera which are obligate para· were the noted resemblances between unwelcome guest. sites of other fungi. these tubers and the body parts of other The solution raises another problem. The story begins as long ago as 1918, fungi. One such was the agaric Cys· The genus Squamanita is traditionally in the Netherlands. Botanists were struck toderma paradoxum, the stem of which defined on the presence of the basal by the unexpected appearance, in sever· seemed to be made of two different tuber. But this is invalid if these tubers allocations, of a remarkable new species tissues, separated in the middle by an are really the shattered wrecks of other of fungus notable for the odour and annular constriction. The tissues of the fungi. Redhead and colleagues pick their colour of grapes, and its curiously swol· thicker bottom half were typical of its way out of the impasse to justify the len base. A second species, odourless but genus, but these tissues graded upwards existence of Squamanita as a cadre of bright yellow- and again with a swollen, into the thinner stalk and cap of a obligate parasites of fungi, and then bulb-like base - was found in Swit· Squamanita. This unwholesome chi· review all other similar instances. The zerland in the mid-1930s. The botanist maera was removed to the genus result is an enjoyable snapshot of L. Schreier, who described this species, Squamanita, the peculiar Cystoderma· some 80 years of thinking about the nurtured the isolated bulbs in a darkened like base being taken as just another phenomenon. Henry Gee 276 NATURE · VOL 375 · 25 MAY 1995 .