Membrane phosphatidylserine regulates surface charge and protein localization
11 Jan 2008-Science (American Association for the Advancement of Science)-Vol. 319, Iss: 5860, pp 210-213
TL;DR: A biosensor developed to study the subcellular distribution of phosphatidylserine found that it binds the cytosolic leaflets of the plasma membrane, as well as endosomes and lysosomes.
Abstract: Electrostatic interactions with negatively charged membranes contribute to the subcellular targeting of proteins with polybasic clusters or cationic domains. Although the anionic phospholipid phosphatidylserine is comparatively abundant, its contribution to the surface charge of individual cellular membranes is unknown, partly because of the lack of reagents to analyze its distribution in intact cells. We developed a biosensor to study the subcellular distribution of phosphatidylserine and found that it binds the cytosolic leaflets of the plasma membrane, as well as endosomes and lysosomes. The negative charge associated with the presence of phosphatidylserine directed proteins with moderately positive charge to the endocytic pathway. More strongly cationic proteins, normally associated with the plasma membrane, relocalized to endocytic compartments when the plasma membrane surface charge decreased on calcium influx.
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TL;DR: It is discussed how restriction of intercellular transport to the interface of adjacent phloem cells may be an effective mechanism to limit the availability of photosynthetic carbon in the leaf apoplasm in order to prevent pathogen infections.
Abstract: Plants transport fixed carbon predominantly as sucrose, which is produced in mesophyll cells and imported into phloem cells for translocation throughout the plant. It is not known how sucrose migrates from sites of synthesis in the mesophyll to the phloem, or which cells mediate efflux into the apoplasm as a prerequisite for phloem loading by the SUT sucrose-H(+) (proton) cotransporters. Using optical sucrose sensors, we identified a subfamily of SWEET sucrose efflux transporters. AtSWEET11 and 12 localize to the plasma membrane of the phloem. Mutant plants carrying insertions in AtSWEET11 and 12 are defective in phloem loading, thus revealing a two-step mechanism of SWEET-mediated export from parenchyma cells feeding H(+)-coupled import into the sieve element-companion cell complex. We discuss how restriction of intercellular transport to the interface of adjacent phloem cells may be an effective mechanism to limit the availability of photosynthetic carbon in the leaf apoplasm in order to prevent pathogen infections.
981 citations
TL;DR: The determinants and functional implications of the subcellular distribution and membrane topology of the most abundant negatively charged phospholipid in eukaryotic membranes are discussed.
Abstract: Phosphatidylserine (PS) is the most abundant negatively charged phospholipid in eukaryotic membranes. PS directs the binding of proteins that bear C2 or gamma-carboxyglutamic domains and contributes to the electrostatic association of polycationic ligands with cellular membranes. Rather than being evenly distributed, PS is found preferentially in the inner leaflet of the plasma membrane and in endocytic membranes. The loss of PS asymmetry is an early indicator of apoptosis and serves as a signal to initiate blood clotting. This review discusses the determinants and functional implications of the subcellular distribution and membrane topology of PS.
793 citations
TL;DR: Inhibitors of Na+/H+ exchange proteins block macropinocytosis by lowering the pH near the plasma membrane, which in turn inhibits actin remodeling by Rho family GTPases.
Abstract: Macropinocytosis is differentiated from other types of endocytosis by its unique susceptibility to inhibitors of Na+/H+ exchange. Yet, the functional relationship between Na+/H+ exchange and macropinosome formation remains obscure. In A431 cells, stimulation by EGF simultaneously activated macropinocytosis and Na+/H+ exchange, elevating cytosolic pH and stimulating Na+ influx. Remarkably, although inhibition of Na+/H+ exchange by amiloride or HOE-694 obliterated macropinocytosis, neither cytosolic alkalinization nor Na+ influx were required. Instead, using novel probes of submembranous pH, we detected the accumulation of metabolically generated acid at sites of macropinocytosis, an effect counteracted by Na+/H+ exchange and greatly magnified when amiloride or HOE-694 were present. The acidification observed in the presence of the inhibitors did not alter receptor engagement or phosphorylation, nor did it significantly depress phosphatidylinositol-3-kinase stimulation. However, activation of the GTPases that promote actin remodelling was found to be exquisitely sensitive to the submembranous pH. This sensitivity confers to macropinocytosis its unique susceptibility to inhibitors of Na+/H+ exchange.
746 citations
TL;DR: It is proposed that endosomal microautophagy shares molecular components with both the endocytic and autophagic pathways and is distinct from chaperone-mediated autophagy that occurs in lysosomes.
739 citations
Cites background from "Membrane phosphatidylserine regulat..."
...Although PS is distributed in all cellular membranes, it only confers a negative charge to the plasma and the endosomal membranes, likely because the PS present in mitochondria, Golgi, and ER is confined to their luminal leaflets (Yeung et al., 2008)....
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...It has been shown that PS directs proteins with strong positive charge to the cytosolic leaflet of the plasma membrane, and proteins with moderate positive charge, such as hsc70, to the cytosolic leaflet of the endosomal membrane (Yeung et al., 2008)....
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...This amphiphilic (cationic/hydrophobic) strategy is used by several biological structures to insert into anionic membranes, and it was previously shown to be used by PS to recruit proteins to the endosomal-limiting membrane (Yeung et al., 2008)....
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...Although hsc70 does not bear any of the canonical protein lipid-binding domains, its C-terminal region (aa 526–539) presents one cluster of basic residues that could potentially bind to acidic phospholipids through electrostatic interaction (Figure 4G) (Yeung et al., 2008)....
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TL;DR: The endocytic pathways are introduced, new technologies to confirm the specific endocytical pathways are presented and factors for pathway selection are discussed, which may provide new thoughts for the design of novel multifunctional nanomedicines.
479 citations
References
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TL;DR: Within the hypervariable domain of p21K-ras(B), which is not palmitoylated, a novel plasma membrane targeting signal consisting of a polybasic domain that also acts in combination with the CAAX motif is identified.
1,064 citations
TL;DR: Recent studies suggest that membrane proteins concentrate PIP2 and, in response to local increases in intracellular calcium concentration, release it to interact with other biologically important molecules.
Abstract: Phosphatidylinositol 4,5-bisphosphate (PIP2), which comprises only about 1% of the phospholipids in the cytoplasmic leaflet of the plasma membrane, is the source of three second messengers, activates many ion channels and enzymes, is involved in both endocytosis and exocytosis, anchors proteins to the membrane through several structured domains and has other roles. How can a single lipid in a fluid bilayer regulate so many distinct physiological processes? Spatial organization might be the key to this. Recent studies suggest that membrane proteins concentrate PIP2 and, in response to local increases in intracellular calcium concentration, release it to interact with other biologically important molecules.
856 citations
TL;DR: This work surveyed PM-targeting mechanisms by imaging the subcellular localization of 125 fluorescent protein–conjugated Ras, Rab, Arf, and Rho proteins and found that proteins with polybasic clusters dissociated from the PM only when both PI(4,5)P2 and phosphatidylinositol 3,4, 5-trisphosphate were depleted.
Abstract: Many signaling, cytoskeletal, and transport proteins have to be localized to the plasma membrane (PM) in order to carry out their function. We surveyed PM-targeting mechanisms by imaging the subcellular localization of 125 fluorescent protein–conjugated Ras, Rab, Arf, and Rho proteins. Out of 48 proteins that were PM-localized, 37 contained clusters of positively charged amino acids. To test whether these polybasic clusters bind negatively charged phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] lipids, we developed a chemical phosphatase activation method to deplete PM PI(4,5)P2. Unexpectedly, proteins with polybasic clusters dissociated from the PM only when both PI(4,5)P2 and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] were depleted, arguing that both lipid second messengers jointly regulate PM targeting.
740 citations
TL;DR: For MARCKS, and perhaps other proteins, phosphorylation of serines within its basic cluster reduces the electrostatic attraction, producing translocation of the protein from the membrane to the cytosol by a simple 'electrostatic switch' mechanism.
667 citations
TL;DR: By deciphering the interplay between endocytosis and signalling, this work will be able to gain a more sophisticated level of understanding of signal transduction mechanisms.
550 citations