Sterol trafficking between the endoplasmic reticulum and plasma membrane in yeast

We recently showed that transport of ergosterol from the ER (endoplasmic reticulum) to the sterol-enriched PM (plasma membrane) in yeast occurs by a non-vesicular (Sec18p-independent) mechanism that results in the equilibration of sterol pools in the two organelles [Baumann, Sullivan, Ohvo-Rekilä, Simonot, Pottekat, Klaassen, Beh and Menon (2005) Biochemistry 44, 5816-5826]. To explore how this occurs, we tested the role of proteins that might act as sterol transporters. We chose to study oxysterol-binding protein homologues (Osh proteins), a family of seven proteins in yeast, all of which contain a putative sterol-binding pocket. Recent structural analyses of one of the Osh proteins [Im, Raychaudhuri, Prinz and Hurley (2005) Nature (London) 437, 154-158] suggested a possible transport cycle in which Osh proteins could act to equilibrate ER and PM pools of sterol. Our results indicate that the transport of newly synthesized ergosterol from the ER to the PM in an OSH deletion mutant lacking all seven Osh proteins is slowed only 5-fold relative to the isogenic wild-type strain. Our results suggest that the Osh proteins are not sterol transporters themselves, but affect sterol transport in vivo indirectly by affecting the ability of the PM to sequester sterols.