Considerations on the evolution of steroids as membrane components

Some Like It Hot: The Effect of Sterols and Hopanoids on Lipid Ordering at High Temperature

Sterols and hopanoids have been suggested to reinforce membranes and protect against unfavorable environmental conditions. In particular, hopanoids are found in high concentrations in membranes of thermotolerant and thermophilic bacteria. However, the mechanism whereby sterols and hopanoids stabilize membranes at elevated temperatures is poorly understood. Here, the effect of temperature on the ordering of lipids in bilayers containing cholesterol or the hopanoids bacteriohopanetetrol and diplopterol was explored using molecular dynamics simulations. It is shown that cholesterol induces a high level of ordering over a wide range of temperatures. Bacteriohopanetetrol promotes order within the lipid tails but enhances fluid-like properties of the head groups at high temperatures. In contrast, diplopterol partitions in the midplane of the bilayer. This suggests that individual hopanoids fulfill distinct functions in membranes, with the ordering properties of bacteriohopanetetrol being particularly well suited to maintain the integrity of membranes at temperatures preferred by thermotolerant and thermophilic bacteria.

Plasma membrane events associated with the meiotic divisions in the amphibian oocyte: insights into the evolution of insulin transduction systems and cell signaling

BACKGROUND

Insulin and its plasma membrane receptor constitute an ancient response system critical to cell growth and differentiation. Studies using intact Rana pipiens oocytes have shown that insulin can act at receptors on the oocyte surface to initiate resumption of the first meiotic division. We have reexamined the insulin-induced cascade of electrical and ion transport-related plasma membrane events using both oocytes and intact plasma membranes in order to characterize the insulin receptor-steroid response system associated with the meiotic divisions.

RESULTS

[(125)I]Insulin binding (K(d) = 54 ± 6 nM) at the oocyte plasma membrane activates membrane serine protease(s), followed by the loss of low affinity ouabain binding sites, with a concomitant 3-4 fold increase in high affinity ouabain binding sites. The changes in protease activity and ouabain binding are associated with increased Na(+)/Ca2(+) exchange, increased endocytosis, decreased Na(+) conductance resulting in membrane hyperpolarization, increased 2-deoxy-D-glucose uptake and a sustained elevation of intracellular pH (pHi). Hyperpolarization is largely due to Na(+)-channel inactivation and is the main driving force for glucose uptake by the oocyte via Na(+)/glucose cotransport. The Na(+) sym- and antiporter systems are driven by the Na(+) free energy gradient generated by Na(+)/K(+)-ATPase. Shifts in α and/or β Na(+)-pump subunits to caveolar (lipid raft) membrane regions may activate Na/K-ATPase and contribute to the Na(+) free energy gradient and the increase in both Na(+)/glucose co-transport and pHi.

CONCLUSIONS

Under physiological conditions, resumption of meiosis results from the concerted action of insulin and progesterone at the cell membrane. Insulin inactivates Na(+) channels and mobilizes fully functional Na(+)-pumps, generating a Na(+) free energy gradient which serves as the energy source for several membrane anti- and symporter systems.

Functional reconstitution of a maltose ATP-binding cassette transporter from the thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius

The thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius grows at 60 degrees C and pH 2-3. The organism can utilize maltose and maltodextrins as energy source that are taken up by an ATP-binding cassette (ABC) import system. Genes encoding a maltose binding protein, MalE, and two membrane-integral subunits, MalF and MalG, are clustered on the chromosome but a malK gene translating into a cognate ATPase subunit is lacking. Here we report the cloning of malK from genomic DNA by using the msiK gene of Streptomyces lividans as a probe. Purified MalK exhibited a spontaneous ATPase activity with a Vmax of 0.13 micromol Pi/min/mg and a Km of 330 microM that was optimal at the growth temperature of the organism. Coexpression of malK, malF and malG in Escherichia coli resulted in the formation of a complex that could be coeluted from an affinity matrix after solubilization of membranes with dodecylmaltoside. Proteoliposomes prepared from the MalFGK complex and preformed phospholipid vesicles of A. acidocaldarius displayed a low intrinsic ATPase activity that was stimulated sevenfold by maltose-loaded MalE, thereby indicating coupling of ATP hydrolysis to substrate translocation. These results provide evidence for MalK being the physiological ATPase subunit of the A. acidocaldarius maltose transporter. Moreover, to our knowledge, this is the first report on the functional reconstitution of an ABC transport system from a thermophilic microorganism.

Purification and some properties of the squalene-tetrahymanol cyclase from Tetrahymena thermophila

The membrane-bound enzyme from Tetrahymena thermophila responsible for the conversion of squalene into the quasi-hopanoid tetrahymanol was purified 297-fold to near homogeneity. Purification involved solubilization by octylthioglucoside, chromatography on DEAE-trisacryl, hydroxyapatite and FPLC ion-exchange on Mono Q. The apparent KM was found to be 18 microM. 2,3-Iminosqualene and N,N-dimethyldodecylamine-N-oxide are effective inhibitors of the cyclase with I50 values of 50 and 30 nM, respectively. The cyclase has a molecular mass of 72 kDa as judged by electrophoresis in polyacrylamide gels under denaturating conditions. The optimal enzymatic activity was obtained at pH 7.0 and 30 degrees C. The solubilized enzyme needs the presence of detergent for maintaining activity. The influence of different detergents on cyclase activity was studied. Triton X-100 proved to be a strong inactivator of the enzyme. Solubilization of the cyclase in Tween 80 and digitonin inactivates the enzyme. However, its activity can be recovered by complementation of the assay buffer with octylthioglucoside above its critical micellar concentration. We suggest that this approach might be applicable to other membrane-bound proteins.

Properties of purified squalene-hopene cyclase from Bacillus acidocaldarius

The squalene-hopene cyclase from Bacillus acidocaldarius cytoplasmic membrane, was purified to homogeneity by solubilization with Triton X-100, chromatography on DEAE-cellulose, phenyl Sepharose and two gel-filtration columns. The enzyme monomer had a molecular mass of 75 kDa. The sequence of the first 23 amino acids was determined by Edman degradation. The enzyme activity was efficiently inhibited by n-alkyldimethylammonium halides with alkyl chain lengths between 12 and 18 C atoms. Inhibition was also observed with (5-hydroxycarvacryl)trimethylammonium chloride 1-piperidine carboxylate, dodecyldimethylamine N-oxide, azasqualene and farnesol. Competitive inhibition with dodecyltrimethylammonium bromide, (5-hydroxycarvacryl)trimethylammonium chloride 1-piperidine carboxylate and dodecyldimethylamine N-oxide was demonstrated by Lineweaver-Burk plots.

Estradiol membrane binding sites on human breast cancer cell lines. Use of a fluorescent estradiol conjugate to demonstrate plasma membrane binding systems

A fluorescent estradiol macromolecular complex was used to study and to characterize steroid binding to membranes of living target cells. Ligand binding to plasma membranes was quantitated with a sensitivity of 0.1 nM. In this way, we found two types of estradiol-binding sites on hormone sensitive MCF-7 cells. Type A sites (8000-16000 sites per cell) were rapidly saturated at low concentrations of the estradiol-bovine serum albumin-fluorescein isothiocyanate macromolecular complex (E2-BSA-FITC). They had a greater affinity for the complex than did the type B sites for which a phenomenon of cooperative fixation was shown. The complex binding was displaced by estrogenic molecules, but not by non-estrogenic compounds, such as cortisol or progesterone. We also studied complex binding on another breast cancer cell line, MDA-MB-231 (MDA), without intracellular estrogen receptors. These cells showed a specific plasma membrane binding system for estrogen, but lacked the high affinity type A binding site. Then, we report the effects of enzyme treatments (trypsin, phospholipase A2 and neuraminidase) on E2-BSA-FITC binding to MCF-7 cell membranes. The quantity of complex bound to membranes decreased after phospholipase and neuraminidase treatments and increased after trypsin. But, in the three cases, the binding was no longer specific because it could not be displaced by E2-BSA or by estradiol. The enzymatic effects were reversible and specific binding was totally restored within 24 h. However, in the presence of the protein synthesis inhibitor, cycloheximide, no restoration of specific binding occurred on trypsin-treated cells. Estrogen binding to MCF-7 and MDA cell plasma membranes thus possesses the three characteristics of all mediated transport processes across biological membranes: saturability, substrate specificity, and specific inhibition. However, the high affinity type A binding site was found only on the estrogen-sensitive cell line, MCF-7.

Allylamine derivatives: new class of synthetic antifungal agents inhibiting fungal squalene epoxidase

A new class of synthetic antifungal agents, the allylamines , has been developed by modification of naftifine , a topical antimycotic. SF 86-327, the most effective of these compounds so far, is highly active in vitro against a wide range of fungi and exceeds clinical standards in the oral and topical treatment of guinea pig dermatophytoses. SF 86-327 is a powerful specific inhibitor of fungal squalene epoxidase, a key enzyme in sterol biosynthesis.

Interaction of hopanoids with phosphatidylcholines containing oleic and omega-cyclohexyldodecanoic acid in lipid bilayer membranes

Lipid bilayer membranes were made from hopanoid phosphatidylcholine mixtures dissolved in decane. The specific capacity of the mixed membranes was found to increase with increasing hopanoid content. This indicates an interaction between hopanoids and lipids which leads to a reduction of the chemical potential of the solvent in the membranes. The structural properties of mixtures of hopanoids and phosphatidylcholines were investigated using charged probe molecules, the negatively charged lipophilic ions dipicrylamine (DPA) and tetraphenylborate (TphiB) and the positively charged potassium complex PV-K+ (PV, cyclo (D-Val-L-Pro-L-Val-D-Pro)3). The transport properties of the lipophilic ions in the mixed membranes indicate that the electrical properties like dipolar potential and surface potentials of phosphatidylcholine membranes are not changed by the insertion of the hopanoids. The translocation rate constant K of the PV-K+ complex is drastically reduced in the hopanoid phosphatidylcholine membranes with increasing hopanoid content. This effect is discussed on the basis of an alteration of the microviscosity in the mixed membranes. There exists a close analogy between the action of cholesterol and hopanoids in bilayer membranes from phosphatidylcholines. A bilayer membrane composed of di-omega-cyclohexyldodecanoyl-phosphatidylcholine (DCPC) was found to possess a higher specific capacity as compared to other phosphatidylcholines. Also a lower translocation rate constant for PV-K+ was observed which may be caused by the relative high microviscosity of this lipid even above the phase transition temperature.