Pentacyclic triterpenoids of the hopane family were found in about half of some 100 strains of prokaryotes belonging to diverse taxonomic groups, such a wide distribution indicating the biological significance of these compounds. Hopanoids were found in almost all the cyanobacteria and obligate methylotrophs examined, in all the purple non-sulphur bacteria studied and in many taxonomically diverse Gram-negative or Gram-positive chemoheterotrophs. They were absent in all archaebacteria and purple sulphur bacteria examined as well as in various other Gram-positive or Gram-negative genera. The C30 hopanoids, diploptene and diplopterol, are present in almost all hopanoid-containing prokaryotes. The major compounds are always the C35 bacteriohopanepolyols, which are present at a level of 0-1-2 mg per g dry weight, the most common one being bacteriohopanetetrol. Because of their structural characteristics and their influence on the properties of biological membrane models, these compounds might be the structural equivalents of the sterols found in eukaryotes. 0022-1287/84/0001-1523 $02.00 0 1984 SGM Chlorobium limicola Chlorobium limicola var. 'thiosulfatophilum' 6230 Chromatium vinosum D Ectothiorhodospira mobilis 8 1 15 Ectothiorhodospira shaposhnikovii Moskau N 1 Thiocapsa roseopersicina 63 1 1 Thiocystis violacea 231 1 C. Gram-negative chemoautotrophs and c hemohe terotrop hs Agrobacterium tumefaciens CIP 671 Caulobacter crescentus CIP 771 5 Erwinia herbicola NCIB 9680 Escherichia coli CIP K 1212000 Flexithrix QQ-1 'Methylomonas Clara' Hoechst 'Methylophilus rnethylotrophus' ICI 'Moraxella displex non liquefaciens' CIP 5545 'Moraxella lwoffi' CIP 5382 Paracoccus denitrificans DSM 38 1 -65b Proteus mirabilis CIP A235 Pseudomonas acidovorans ATCC 17046 Pseudomonas aeruginosa ATCC 15692 Pseudomonas chlororaphis ATCC 9446 Pseudomonas diminuta CIP 7 129 Pseudomonas jlmrescens 'Pseudomonas maltophilia' ATCC 17445 Pseudomonas stutzeri ATCC 17588 Rhizobium lupini CIP 6357 Thiobacillus A2 Thiobacillus thioparus NCIB 8370 Xanthomonas campestris CIP 7423 D. Gram-positive bacteria Actinoplanes brasiliensis ATCC 25844 Bacillus subtilist Brevibacterium linens CIP 6372 Clostridium paraputrificum ATCC 25780 Desulfovibrio desulfuricans NCIB 8310 'Micrococcus jlavus' CIP 53 160 Micromonospora sp. Roche 2207-85 Streptococcus faecalis CIP 761 17 'Sporosarcina lutea' CIP 5345 Source and growth conditions

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More than 100 individual derivatives of hopane have been isolated from sedim~ntary organic matter of most varied origin.

Derivatives of one triterpene family, the hopane family, are widely distributed in prokaryotes; they may be localized in membranes, playing there the same role as sterols play in eukaryotes, as a result of their similar size, rigidity, and amphiphilic character. Their biosynthesis embodies many primitive features compared to that of sterols and could have evolved toward the latter once aerobic conditions had been established. Membrane reinforcement appears to be achieved in other prokaryotes by other mechanisms, involving either approximately 40-A-long rigid hydrocarbon chains terminated by one polar group acting like a peg through the double-layer or similar chains terminated by two polar groups acting like tie-bars across the membrane. These inserts can be tetraterpenes (e.g., carotenoids). The biophysical function of membrane optimizers appears to have evolved toward sterols by changes limited to only a few enzymatic steps of the same fundamental biosynthetic processes.

To gain some insight into the structural and functional roles of sterols in higher plant cells, various plant sterols have been incorporated into soybean phosphatidyicholine (PtdCho) bilayers and tested for their ability to regulate water permeability and acyl chain ordering. Sitosterol was the most efficient sterol in reducing the water permeability of these vesicles and stigmasterol appeared to have no significant effect.Vesicles containing 24fmethylcholesterol exhibited an intermediate behavior, similar to that of vesicles containing cholesterol. Cycloartenol, the first cyclic biosynthetic precursor of plant sterols, reduced the water permeability in a very effective way. Oftwo unusual plant sterols, 246-methylpollinastanol and 14a,24Udimethylcholest-8-en-3fi-ol, the former was found to be functionally equivalent to sitosterol and the latter was found to be relatively inefficient. 2H NMR experiments have been performed with oriented bilayers consisting of soybean PtdCho with sitosterol, stigmasterol, or 24-methylpoll nastanol. The results provided clear evidence that sitosterol and 24k-methylpolinastanol exhibit a high efficiency to order PtdCho acyl chains that closely parallels their ability to reduce water permeability. By contrast, stigmasterol shows a low efficiency for both functions. These results show that sitosterol and stigmasterol, two major 24-ethylsterols differing only by the absence or presence of the A22 double bond in the side chain, probably play different roles in regulating plant membrane properties; they also may explain why 9P,19-cyclopropylsterols behave as good surrogates of sitosterol.In mammalian cells, the bulk of free cholesterol is localized in the plasma membrane where it participates in the regulation of membrane fluidity and in the activity of many membrane-bound enzymes (1). Changes in cholesterol content have been shown to induce modifications in cellular functions, which depend on the dynamics of the cell-surface membrane such as the recognition of an external signal (1). In higher plant cells, the three typical phytosterols: sitosterol, and 24F-methylcholesterol [the mixture of 24R (i.e., campesterol) and 24S (i.e., 22-dihydrobrassicasterol) epimers] are also mainly concentrated in the plasma membrane (2). To investigate whether the different plant sterols play similar roles in higher plant cells, we started a study with well-defined model membrane systems prepared from soybean phosphatidylcholine (PtdCho), a major representative plant phospholipid, and various sterols in different molar ratios. Because of their high content in polyunsaturated fatty acyl chains (3), soybean PtdCho bilayers can be considered a valuable model of higher plant membrane. The present work deals with the effect of some plant sterols on water permeability and acyl chain ordering of these bilayers. Permeability changes were monitored by measuring the swelling rates of large unilamellar vesicles (LUVs) following an osmotic shock in a stopped-flow spectrophotometer. This method had been develo...