Plant ER-PM Contact Sites in Endocytosis and Autophagy: Does the Local Composition of Membrane Phospholipid Play a Role?

摘要

Plant EPCS Resident Proteins Direct interaction between the ER and PM was identified in plants in many early studies (Staehelin and Chapman, 1987 ; Staehelin, 1997 ; Perico and Sparkes, 2018 ), where two membranes of distinct organelles are in close apposition without fusing. However, proteins localized to these sites have recently been identified in plants and these include VAP27, NET3C, and SYT1 (or SYTA) (Wang et al., 2014 , 2016 ; Pérez-Sancho et al., 2015 , 2016 ; Siao et al., 2016 ; McFarlane et al., 2017 ). Of these three proteins, VAP27 is the most well-known (as Scs2 in yeast and as VAP in animals). VAP-like proteins have been shown to be involved in the formation of different MCS as well as EPCS (Salvador-Gallego et al., 2017 ). The VAP27 family is expanded in plants; for example, the Arabidopsis genome encodes 10 VAP27-related proteins (Wang et al., 2016 ), while the human and S. cerevisiae genomes encode three and two VAP27-related proteins, respectively. Therefore, the function of the plant VAP27 proteins are likely to be more diverse, and they may fulfill some of the functions of those animal EPCS proteins that are missing in plants. The extended-synaptotagmins (E-SYTs) have also been shown to be essential for the formation of EPCS. The plant SYT family contains five members, all of which are similar in peptide sequence to both synaptotagmins (SYTs) and extended-synaptotagmins (E-SYTs) in metazoans (Manford et al., 2012 ; Malmersj? and Meyer, 2013 ). However, plant SYT1 is more functionally equivalent to E-SYTs as knock-out expression affects both ER morphology and PM tethering (Levy et al., 2015 ; Siao et al., 2016 ; McFarlane et al., 2017 ). NET3C is a member of the NETWORKED family which is unique to plants (Deeks et al., 2012 ). NET3C localizes to the EPCS and interacts with VAP27 (Wang et al., 2014 ). Different members of the NETWORKED family bind to the actin cytoskeleton localizing the network to different membranes structures where they act as membrane-cytoskeleton adaptors or linkers. Some other Arabidopsis NET proteins also localize to stationary foci at the PM (e.g., NET2A), these structures may also represent EPCS (Duckney et al., 2017 ) but further studies are required for this to be confirmed. In summary, the ER-PM connection is a conserved link observed across phylogeny, with some features that appear to be specific for plants. Because of this conservation, it is likely that many of the known functions of EPCS are likely to be conserved, such as the regulation of phospholipid homeostasis, endocytosis, and autophagosome formation, but the differences may also reflect some plant specific additions/adaptations to their function. EPCS in regulating Lipid Transport, Composition, and Homeostasis Early studies demonstrated that the yeast EPCS resident protein, VAP, binds to lipid binding proteins and recruits these proteins (e.g., Osh2, 3) to the ER-PM interface, where they regulate lipid metabolism (Loewen and Levine, 2005 ). In addition, many other lipid-synthesizing enzymes are enriched at these contact sites and these include OPI3 (a phosphatidylethanolamine N -methyltransferase) which synthesizes phosphatidylcholines (PC). Disrupting the ER-PM connection by knocking-out VAP (Scs2) gene expression reduces PC levels, indicating that an intact EPCS structure is important for the function of OPI3 (Tavassoli et al., 2013 ). Moreover, lipid synthesis enzymes are not only localized to the EPCS as they may also regulate the formation of this connection, for example, PAH1 (a phosphatidic acid phosphatase enzyme) whose over-expression can restore the ER-PM connection in the Scs2 mutant (Tavassoli et al., 2013 ). In Hela cells, on the other hand, EPCS localized lipid binding protein, TMEM24, regulates the transport of phosphatidylinositol between ER and PM (Lees et al., 2017 ). Similar activities have been reported for many lipid binding proteins in other cell types; such as ORPs/Osh proteins which bind to oxysterol as well as to phosphoinositide (Saheki and De Camilli, 2017 ; Sohn et al., 2018 ), and Aster proteins which are recruited to the PM in response to cholesterol accumulation and these proteins transport the excess cholesterol back to the ER (Sandhu et al., 2018 ). In addition, another key function of EPCS is to regulate local lipid composition and homeostasis. Among all lipid molecules, phosphatidylinositol serves an essential role in signaling and cytoskeleton re-organization. One of the most well-known regulators of PI-signaling is Sac1, an ER localized PI phosphatase that could covert PI monophosphates (such as PI3P, PI4P) to PI (Nemoto et al., 2000 ). When activated, Sac1 can be recruited to the PM, where it dephosphorylates PI4P and consequently reduces the level of PI(4,5)P2 in order to maintain steady state (Stefan et al., 2011 ; Dickson et al., 2016 ). However, the spatial organization of Sac1 is controlled by EPCS resident proteins, such as E-Syts and the Scs2/Osh3 comple