ORGANIZATION OF PREDATOR-PREY COMMUNITIES AS AN EVOLUTIONARY GAME

We consider a simple predator-prey model of coevolution. By allowing coevolution both within and between trophic levels the model breaks the traditional dichotomy between coevolution among competitors and coevolution between a prey and its predator. By allowing the diversity of prey and predator species to emerge as a property of the evolutionarily stable strategies (ESS), the model breaks another constraint of most approaches to coevolution that consider as fixed the number of coevolving species. The number of species comprising the ESS is influenced by a parameter that determines the predator's niche breadth. Depending upon the parameter's value the ESS may contain: 1) one prey and one predator species, 2) two prey and one predator, 3) two prey and two predators, 4) three prey and two predators, 5) three prey and three predators, etc. Evolutionarily, these different ESSs all emerge from the same model. Ecologically, however, these ESSs result in very different patterns of community organization. In some communities the predator species are ecologically keystone in that their removal results in extinctions among the prey species. In others, the removal of a predator species has no significant impact on the prey community. These varied ecological roles for the predator species contrasts sharply with the essential evolutionary role of the predators in promoting prey species diversity. The ghost of predation past in which a predator's insignificant ecological role obscures its essential evolutionary role may be a frequent property of communities of predator and prey.     

Single-Step Purification of Two Functional Human Apolipoprotein E Variants Hyperexpressed inEscherichia coli

We have cloned, from total human liver RNA, the cDNA encoding apolipoprotein E3 (apoE3). Site-directed mutagenesis was used to obtain the cDNA encoding the apoE4 isoform, a major variant of this apolipoprotein in man. These two cDNAs were subcloned into the procaryotic expression vector pAHRS. A polyhistidine tag was added at the NH₂-termini of the recombinant proteins (apoE3 and apoE4) to enable rapid purification. The resulting plasmids (pAHRS-apoE3 and pAHRS-apoE4) were introduced into theEscherichia colistrain BL21(DE3). Recombinant strains were grown at 37°C in a Luria and Bertani medium and the addition of isopropyl β-thiogalactoside resulted in the expression of large amounts of apoE protein (40.5 kDa), representing at least 15% of cellular proteins. The recombinant apoE isoforms were purified, under denaturating conditions, in one step by affinity chromatography on a Ni-chelated agarose column, yielding to about 20 mg of 96% pure protein per liter of culture. Compared to plasma apoE3 purified from human very low density lipoproteins, the two renatured recombinant apoE isoforms have the same secondary structure content, as revealed by circular dichroism measurement. Moreover, the recombinant apoE3 isoform shares similar properties for the association with lipids, compared to the human protein, indicating that the addition of the amino-terminal polyhistidine peptide does not influence the structure and the lipid binding properties of this recombinant apoE isoform. No differences in the secondary structure of recombinant apoE4 were detected, whereas this isoform presents specific reactivity with lipids. This simple and rapid procedure for the expression and the purification of functional recombinant apoE should therefore enable structural and physiological studies requiring large amounts of these apolipoproteins.     

Import of a new chloroplast inner envelope protein is greatly stimulated by potassium phosphate

A cDNA clone encoding a major chloroplast inner envelope membrane protein of 96 kDa (IEP96) was isolated and characterized. The protein is synthesized as a larger-molecular-weight precursor (pIEP96) which contains a cleavable N-terminal transit sequence of 50 amino acids. The transit peptide exhibits typical stromal targeting information. It is cleaved in vitro by the stromal processing peptidase, though the mature protein is clearly localized in the inner envelope membrane. Translocation of pIEP96 into chloroplasts is greatly stimulated in the presence of 80 mM potassium phosphate which results in an import efficiency of about 90%. This effect is specific for potassium and phosphate, but cannot be ascribed to a membrane potential across the inner envelope membrane. Protein sequence analysis reveals five stretches of repeats of 26 amino acids in length. The N-terminal 300 amino acids are 45% identical (76% similarity) to the 35 kDa -subunit of acetyl-CoA carboxyl-transferase from Escherichia coli. The C-terminal 500 amino acids share significant similarity (69%) with USOI, a component of the cytoskeleton in yeast.Abbreviations P_i phosphate - IEP inner envelope membrane protein - pIEP precursor form of IEP - SSU small subunit of ribulose-1,5-bisphosphate carboxylase oxygenase - IEP96_pep peptide specific antiserum to IEP96 - IEP96_pol polyspecific antiserum to IEP96     

Human medulloblastoma gangliosides

To establish a model system for the study of ganglioside metabolismof the human brain tumor, medulloblastoma, we have chemicallycharacterized the gangliosides of the Daoy cell line. Thesecells contain a high concentration of gangliosides (143 ±13 nmol LBSA/10⁸ cells). The major species have been structurallyconfirmed to be G_M2 (65.9%), G_M3 (13.0%), and G_Dla (10.3%).Isolation of individual gangliosides homogeneous in both carbohydrateand ceramide moieties by reversed-phase HPLC and analysis bynegative-ion fast atom bombardment collisionally activated dissociationtandem mass spectrometry have allowed us to unequivocally characterizeceramide structures. In the case of G_M2, 10 major ceramide subspecieswere identified: d18:1-hC16:0, d18:1-C16:0, d18:0-C16:0, d18:1-C18:0,d18:1-C20:0, d18:1-C22:0, d18:2-C24:1, d18: 1-C23:1, d18:1-C24:1,and d18:1-C24:0. Taken together with previous studies, thesefindings in human medullo-blastoma cells support the view thathigh expression and marked heterogeneity of ceramide structureare general characteristics of tumor gangliosides, moleculeswhich are shed by the tumor cells and which are biologicallyactive in vivo. medulloblastoma gangliosides ceramide structure HPLC mass spectrometry