Protein kinase A regulates chloride conductance in endocytic vesicles from proximal tubule.

Regulation of ion transport by phosphorylation and G proteins occurs in several epithelial and non-epithelial cell plasma membranes1-5. It is not known whether transporters on intracellular membranes are target sites for second messengers. Here we present direct evidence that a chloride conductance in endocytic vesicles from rabbit proximal tubule is activated by phosphorylation through a cyclic AMP-dependent protein kinase. To measure chloride transport, endocytic vesicles were labelled in vivo with a Cl(-)-sensitive fluorescent indicator6-8. It was found that labelled endosomes contained an inward proton pump and a chloride conductance, but no ion-coupled chloride transport, and that the chloride conductance was regulated by protein kinase A. These results, taken together with measurements of chloride effects on ATP-dependent acidification, suggest that endosomal pH can be controlled by phosphorylation of a stilbene-sensitive conductive chloride transporter.     

Purified inositol 1,4,5-trisphosphate receptor mediates calcium flux in reconstituted lipid vesicles

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), a second messenger molecule involved in actions of neurotransmitters, hormones and growth factors, releases calcium from vesicular non-mitochondrial intracellular stores. An Ins(1,4,5)P3 binding protein, purified from brain membranes, has been shown to be phosphorylated by cyclic-AMP-dependent protein kinase and localized by immunohistochemical techniques to intracellular particles associated with the endoplasmic reticulum. Although the specificity of the Ins(1,4,5)P3 binding protein for inositol phosphates and the high affinity of the protein for Ins(1,4,5)P3 indicate that it is a physiological Ins(1,4,5)P3 receptor mediating calcium release, direct evidence for this has been difficult to obtain. Also, it is unclear whether a single protein mediates both the recognition of Ins(1,4,5)P3 and calcium transport or whether these two functions involve two or more distinct proteins. In the present study we report reconstitution of the purified Ins(1,4,5)P3 binding protein into lipid vesicles. We show that Ins(1,4,5)P3 and other inositol phosphates stimulate calcium flux in the reconstituted vesicles with potencies and specificities that match the calcium releasing actions of Ins(1,4,5)P3. These results indicate that the purified Ins(1,4,5)P3 binding protein is a physiological receptor responsible for calcium release.     

Voltage-dependent InsP3-insensitive calcium channels in membranes of pancreatic endoplasmic reticulum vesicles.

Stimulus-secretion coupling in exocrine glands involves Ca2+ release from intracellular stores. In endoplasmic reticulum vesicle preparations from rat exocrine pancreas, an inositol 1,4,5-trisphosphate(InsP3)-sensitive, as well as an InsP3-insensitive, Ca2+ pool has been characterized. But Ca2+ channels in the endoplasmic reticulum of rat exocrine pancreas have not been demonstrated at the level of single-channel current. We have now used the patch-clamp technique on endoplasmic reticulum vesicles fused by means of the dehydration-rehydration method. In excised patches, single Ba2(+)- and Ca2(+)-selective channels were recorded. The channel activity was markedly voltage-dependent. Caffeine increased channel open-state probability, whereas ruthenium red and Cd2+ blocked single-channel currents. Ryanodine, nifedipine and heparin had no effect on channel activity. The channel activity was not dependent on the free Ca2+ concentration, the presence of InsP3, or pH. We conclude that this calcium channel mediates Ca2+ release from an intracellular store through an InsP3-insensitive mechanism.     

Compartmentalization of S-RNase and HT-B degradation in self-incompatible Nicotiana

Pollen-pistil interactions are crucial for controlling plant mating. For example, S-RNase-based self-incompatibility prevents inbreeding in diverse angiosperm species. S-RNases are thought to function as specific cytotoxins that inhibit pollen that has an S-haplotype that matches one of those in the pistil. Thus, pollen and pistil factors interact to prevent mating between closely related individuals. Other pistil factors, such as HT-B, 4936-factor and the 120 kDa glycoprotein, are also required for pollen rejection but do not contribute to S-haplotype-specificity per se. Here we show that S-RNase is taken up and sorted to a vacuolar compartment in the pollen tubes. Antibodies to the 120 kDa glycoprotein label the compartment membrane. When the pistil does not express HT-B or 4936-factor, S-RNase remains sequestered, unable to cause rejection. Similarly, in wild-type pistils, compatible pollen tubes degrade HT-B and sequester S-RNase. We suggest that S-RNase trafficking and the stability of HT-B are central to S-specific pollen rejection.     

上海市岸滩冲淤等级划分及其在风险评估中的应用

海岸滩涂是沿海地区经济发展和生态系统平衡等的重要组成部分,虽一直受到国际学术界的极大关注,但其等级评估研究目前仍处于起步阶段,缺乏统一标准,不利于对岸滩的综合开发和治理．本在综合考虑了目前长江河口区和杭州湾北岸水动力机制、沉积、地貌的基础上,将上海市目前重要的岸滩按冲淤强度分为强淤涨型、弱淤涨型、侵蚀型和稳定型四个等级,并在重点分析海平面上升和人类活动对岸滩冲淤的影响后,预测至2030年上海市岸滩冲淤强度可分为五个等级,即强淤涨型、弱淤涨型、强侵蚀型、弱侵蚀型和稳定型,并进而讨论了这种分级在风险评估中的应用。     

Co-localization of molecules involved in antigen processing and presentation in an early endocytic compartment

The pathways of intracellular traffic involved in antigen processing and presentation have been defined by immunoelectron microscopy. The export pathway for class II histocompatibility molecules and the antigen import pathway meet in a peripheral endocytic compartment having all the molecular machinery believed to be required for antigen processing and presentation, including internalized surface immunoglobulins, proteolytic enzymes and invariant chains. This compartment defines a site where peptides from endocytosed antigen can bind class II molecules en route to the cell surface for presentation to T cells.     

A single motif responsible for ubiquitin recognition and monoubiquitination in endocytic proteins

Ubiquitination is a post-translation modification in which ubiquitin chains or single ubiquitin molecules are appended to target proteins, giving rise to poly- or monoubiquitination, respectively. Polyubiquitination targets proteins for destruction by the proteasome. The role of monoubiquitination is less understood, although a function in membrane trafficking is emerging, at least in yeast. Here we report that a short amino-acid stretch at the carboxy-termini of the monoubiquitinated endocytic proteins Eps15 and eps15R is indispensable for their monoubiquitination. A similar sequence, also required for this modification, is found in other cytosolic endocytic proteins, such as epsins and Hrs. These sequences comprise a protein motif, UIM (ref. 6), which has been proposed to bind to ubiquitin. We confirm this for the UIMs of eps15, eps15R, epsins and Hrs. Thus, the same motif in several endocytic proteins is responsible for ubiquitin recognition and monoubiquitination. Our results predict the existence of a UIM:ubiquitin-based intracellular network. Eps15/eps15R, epsins and Hrs may function as adaptors between ubiquitinated membrane cargo and either the clathrin coat or other endocytic scaffolds. In addition, through their own ubiquitination, they may further contribute to the amplification of this network in the endocytic pathway.     

Compartmentalization of a haematopoietic growth factor (GM-CSF) by glycosaminoglycans in the bone marrow microenvironment

Haematopoietic progenitor cells proliferate and mature in semisolid media when stimulated by exogenous haematopoietic cell growth factors (HCGFs) such as granulocyte-macrophage colony-stimulating factor (GM-CSF). They also proliferate in association with marrow-derived stromal cells although biologically active amounts of HCGFs cannot be detected in stromal culture supernatants. It is possible that HCGFs are synthesized in small amounts by stromal cells but remain bound to the stromal cells and/or their extracellular matrix (ECM). This interpretation accords with haematopoietic progenitor cell proliferation in close association with stromal layers in long-term cultures. Glycosaminoglycans (GAGs) are found in the ECM produced by stromal cells. They are prime candidates for selectively retaining HCGFs in the stromal layer; they influence embryonic morphogenesis and cyto-differentiation and they may regulate haematopoiesis. We now report that granulocyte-macrophage colony-stimulating activity can be eluted from cultured stromal layers and that exogenous GM-CSF binds to GAGs from bone marrow stromal ECM. Selective compartmentalization of HCGFs in this manner may be an important function of the marrow microenvironment and may be involved in haematopoietic cell regulation.     

Inhibition of endocytic vesicle fusion in vitro by the cell-cycle control protein kinase cdc2

Membrane transport between the endoplasmic reticulum and the plasma membrane, which involves the budding and fusion of carrier vesicles, is inhibited during mitosis in animal cells. At the same time, the Golgi complex and the nuclear envelope, as well as the endoplasmic reticulum in some cell types, become fragmented. Fragmentation of the Golgi is believed to facilitate its equal partitioning between daughter cells. In fact, it has been postulated that both the inhibition of membrane traffic and Golgi fragmentation during mitosis are due to an inhibition of vesicle fusion, while vesicle budding continues. Although less is known about the endocytic pathway, internalization and receptor recycling are also arrested during mitosis. We have now used a cell-free assay to show that the fusion of endocytic vesicles from baby hamster kidney cells is reduced in Xenopus mitotic cytosol when compared with interphase cytosol. We reconstituted this inhibition in interphase cytosol by adding a preparation enriched in the starfish homologue of the cdc2 protein kinase. Inhibition was greater than or equal to 90% when the added cdc2 activity was in the range estimated for that in mitotic Xenopus eggs, which indicates that during mitosis the cdc2 kinase mediates an inhibition of endocytic vesicle fusion, and possibly other fusion events in membrane traffic.