Direct imaging reveals stable, micrometer-scale lipid domains that segregate proteins in live cells

It has been proposed that membrane rafts, which are sterol- and sphingolipid-enriched liquid ordered (L_o) domains, segregate proteins in membranes and play critical roles in numerous processes in cells. However, rafts remain controversial because they are difficult to observe in cells without invasive methods and seem to be very small (nanoscale) and short lived, leading many to question whether they exist or are physiologically relevant. In this paper, we show that micrometer-scale, stable lipid domains formed in the yeast vacuole membrane in response to nutrient deprivation, changes in the pH of the growth medium, and other stresses. All vacuolar membrane proteins tested segregated to one of two domains. These domains formed quasi-symmetrical patterns strikingly similar to those found in liposomes containing coexisting L_o and liquid-disordered regions. Indeed, we found that one of these domains is probably sterol enriched and L_o. Domain formation was shown to be regulated by the pH-responsive Rim101 signaling pathway and may also require vesicular trafficking to vacuoles.