Institut de Pharmacologie Moléculaire et Cellulaire, Université de Nice Sophia-Antipolis and Centre National de la Recherche Scientifique, 06560 Valbonne, France.
J Cell Biol. 2011 Dec 12;195(6):965-78. doi: 10.1083/jcb.201104062.
Osh/Orp proteins transport sterols between organelles and are involved in phosphoinositide metabolism. The link between these two aspects remains elusive. Using novel assays, we address the influence of membrane composition on the ability of Osh4p/Kes1p to extract, deliver, or transport dehydroergosterol (DHE). Surprisingly, phosphatidylinositol 4-phosphate (PI(4)P) specifically inhibited DHE extraction because PI(4)P was itself efficiently extracted by Osh4p. We solve the structure of the Osh4p-PI(4)P complex and reveal how Osh4p selectively substitutes PI(4)P for sterol. Last, we show that Osh4p quickly exchanges DHE for PI(4)P and, thereby, can transport these two lipids between membranes along opposite routes. These results suggest a model in which Osh4p transports sterol from the ER to late compartments pinpointed by PI(4)P and, in turn, transports PI(4)P backward. Coupled to PI(4)P metabolism, this transport cycle would create sterol gradients. Because the residues that recognize PI(4)P are conserved in Osh4p homologues, other Osh/Orp are potential sterol/phosphoinositol phosphate exchangers.
Osh/Orp 蛋白在细胞器之间运输固醇，并参与磷酸肌醇代谢。这两个方面之间的联系仍然难以捉摸。我们使用新的测定方法来研究膜成分对 Osh4p/Kes1p 提取、传递或运输去氢麦角固醇 (DHE) 的能力的影响。令人惊讶的是，磷脂酰肌醇 4-磷酸 (PI(4)P) 特异性抑制 DHE 提取，因为 PI(4)P 本身被 Osh4p 有效地提取。我们解决了 Osh4p-PI(4)P 复合物的结构，并揭示了 Osh4p 如何选择性地用固醇替代 PI(4)P。最后，我们表明 Osh4p 可以快速将 DHE 交换为 PI(4)P，并沿相反的路径在膜之间运输这两种脂质。这些结果表明了一个模型，其中 Osh4p 将固醇从 ER 运输到被 PI(4)P 标记的晚期隔室，并反过来将 PI(4)P 向后运输。与 PI(4)P 代谢偶联，这种运输循环将产生固醇梯度。因为识别 PI(4)P 的残基在 Osh4p 同源物中是保守的，所以其他 Osh/Orp 可能是潜在的固醇/磷酸肌醇磷酸交换器。
