The functions of steryl glycosides come to those who wait: Recent advances in plants, fungi, bacteria and animals

The attachment of a sugar moiety to the 3-hydroxy group of a sterol drastically increases the size of the hydrophilic part of the lipid. It is obvious that the glycosylation of a considerable fraction of membrane-bound free sterols alters the biophysical properties of the membrane. However, the consequences of such changes in the proportions of free sterols and steryl glycosides on the biological functions of a membrane are still unclear. This is the main hurdle in understanding the biological functions of steryl glycosides on a molecular level. The recent cloning of sterol glycosyltransferase genes from plants, fungi and bacteria has enabled genetic approaches to analyze steryl glycoside functions. Down regulation of phytosteryl beta-glycoside biosynthesis in Arabidopsis thaliana causes several dysfunctions in seed development. Ergosteryl beta-glycoside depleted mutants of the yeast Pichia pastoris lose their ability to degrade their peroxisomes by an autophagic mechanism called micropexophagy. In the plant-pathogenic fungus Colletotrichum orbiculare the same defect impairs invasion of the cucumber host plants. Helicobacter pylori, a bacterium colonizing the human stomach, is unable to modulate the host's immune response when the cholesteryl alpha-glycoside biosynthesis of the bacterium is mutated. These mutants with manipulated steryl glycoside metabolism will inspire further studies with cell biological, biophysical and other methods that will provide us with a mechanistic understanding of steryl glycoside functions.