Vesicle-associated membrane protein (VAMP)-associated proteins (VAPs) are conserved constituents of membrane contact sites that involve ER membranes. VAPs are ER-integral proteins that interact with oxysterol-binding protein and its homologs, which bind to specific lipids in target membranes. VAP27 proteins are the VAPs in plants, and the Arabidopsis genome encodes ten homologues. VAP27-1 binds to OPR2A at contact sites between the ER membrane and either plasma membrane or autophagosome membranes. Whether VAP27 proteins and OPR2A are also involved in formation of contact sites between the ER and chloroplast membranes was previously unknown.

The researchers find that VAP27-1 and VAP27-3 accumulate in ER membranes that are in physical proximity to chloroplasts. The half time of fluorescence recovery after photobleaching is longer in these regions close to chloroplasts, suggesting reduced protein movement there compared to in ER tubules. Fluorescence complementation could also be shown between VAP27-1 or VAP27-3 and the outer chloroplast envelope protein 7 (OEP7), showing physical proximity. VAP27-1 and VAP27-3 interact with OPR2A, and in vitro translated OPR2A as well as VAP27-1 and VAP27-3 lacking transmembrane domains could be co-isolated with intact chloroplasts and are sensitive to trypsin digestion, which is characteristic for proteins associated with the outer envelope membrane. OPR2A and the two VAP27 paralogues can interact with monogalactosyldiacylglycerol, a typical lipid of chloroplast membranes. Interestingly, loss of VAP27 and OPR2A has only minor effects on total acyl composition but leads to accumulation of β-sitosterol and campesterol in chloroplast membranes, suggesting that these proteins may be involved in regulation of sterol homeostasis in chloroplasts.

囊泡相关膜蛋白 (VAMP) 相关蛋白 (VAP) 是涉及 ER 膜的膜接触位点的保守成分。 VAP 是内质网整合蛋白，与氧甾醇结合蛋白及其同系物相互作用，后者与靶膜中的特定脂质结合。 VAP27蛋白是植物中的VAP，拟南芥基因组编码10个同源物。 VAP27-1 在内质网膜与质膜或自噬体膜之间的接触位点与 OPR2A 结合。 VAP27 蛋白和 OPR2A 是否也参与 ER 和叶绿体膜之间接触位点的形成此前尚不清楚。

研究人员发现，VAP27-1 和 VAP27-3 积聚在物理上接近叶绿体的内质网膜中。在靠近叶绿体的这些区域中，光漂白后荧光恢复的半衰期较长，表明与内质网小管相比，那里的蛋白质运动减少。 VAP27-1 或 VAP27-3 与外叶绿体包膜蛋白 7 (OEP7) 之间也显示出荧光互补，显示出物理接近性。 VAP27-1和VAP27-3与OPR2A相互作用，体外翻译的OPR2A以及缺乏跨膜结构域的VAP27-1和VAP27-3可以与完整叶绿体共分离，并且对胰蛋白酶消化敏感，这是相关蛋白质的特征与外层包膜。 OPR2A 和两个 VAP27 旁系同源物可以与单半乳糖基二酰基甘油（叶绿体膜的典型脂质）相互作用。有趣的是，VAP27和OPR2A的丢失对总酰基组成只有很小的影响，但会导致叶绿体膜中β-谷甾醇和菜油甾醇的积累，表明这些蛋白质可能参与叶绿体中甾醇稳态的调节。