|Institution:||Texas Tech University|
|Keywords:||Ergosterol; Cycloartenol; Green algae; Sterol; Chlamydomonas reinhardtii; Sterol C-24 methyltransferase; Sterol evolution|
|Full text PDF:||http://hdl.handle.net/2346/58853|
The green algae Chlamydomonas reinhardtii synthesizes the protosterol cycloartenol and converts it to ergosterol (C24-methyl) and 7-dehydroporiferasterol (C24-ethyl) through a highly conserved sterol C24- methylation-C25-reduction (25(27)-olefin) pathway that is distinct from the well described acetate-mevalonate pathway to fungal lanosterol and its conversion to ergosterol by the 24(28)-olefin pathway. 23 sterols were isolated and characterized by a combination of GC-MS and proton nuclear magnetic resonance spectroscopy analysis from a set of mutant, wild-type, and 25-thialanosterol-treated cells. The structure and stereochemistry of the final C24-alkyl sterol side chains possessed different combinations of 24-methyl/ethyl groups and 22(23)E and 25(27)-double bond constructions. When incubated with [methyl-2H3]methionine, cells incorporated three (into ergosterol) or five (into 7-dehydroporiferasterol) deuterium atoms into the newly biosynthesized 24-alkyl sterols, consistent only with a 25(27)-olefin pathway. Thus, our findings demonstrate that two separate isoprenoid-24-alkyl sterol pathways evolved in fungi and green algae, both of which converge to yield a common membrane insert ergosterol.