Ubiquitously expressed secretory carrier membrane proteins (SCAMPs) 1-4 mark different pathways and exhibit limited constitutive trafficking to and from the cell surface

Identification of woolliness response genes in peach fruit after post-harvest treatments

Woolliness is a physiological disorder of peaches and nectarines that becomes apparent when fruit are ripened after prolonged periods of cold storage. This disorder is of commercial importance since shipping of peaches to distant markets and storage before selling require low temperature. However, knowledge about the molecular basis of peach woolliness is still incomplete. To address this issue, a nylon macroarray containing 847 non-redundant expressed sequence tags (ESTs) from a ripe peach fruit cDNA library was developed and used. Gene expression changes of peach fruit (Prunus persica cv. O'Henry) ripened for 7 d at 21 degrees C (juicy fruit) were compared with those of fruit stored for 15 d at 4 degrees C and then ripened for 7 d at 21 degrees C (woolly fruit). A total of 106 genes were found to be differentially expressed between juicy and woolly fruit. Data analysis indicated that the activity of most of these genes (>90%) was repressed in the woolly fruit. In cold-stored peaches (cv. O'Henry), the expression level of selected genes (cobra, endopolygalacturonase, cinnamoyl-CoA-reductase, and rab11) was lower than in the juicy fruit, and it remained low in woolly peaches after ripening, a pattern that was conserved in woolly fruit from two other commercial cultivars (cv. Flamekist and cv. Elegant Lady). In addition, the results of this study indicate that molecular changes during fruit woolliness involve changes in the expression of genes associated with cell wall metabolism and endomembrane trafficking. Overall, the results reported here provide an initial characterization of the transcriptome activity of peach fruit under different post-harvest treatments.

Secretory Carrier Membrane Protein SCAMP2 and Phosphatidylinositol 4,5-Bisphosphate Interactions in the Regulation of Dense Core Vesicle Exocytosis

Secretory carrier membrane protein 2 (SCAMP2) functions in late steps of membrane fusion in calcium-dependent granule exocytosis. A basic/hydrophobic peptide segment within SCAMP2 (SCAMP2 E: CWYRPIYKAFR) has been implicated in this function and shown to bind and sequester phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2 or PIP2] within membranes through an electrostatic mechanism. We now show that alanine substitution of tryptophan W2 within SCAMP2 E substantially weakens peptide binding to negatively charged liposomes; other substitutions for arginine R4 and lysine K8 have only limited effects on binding. Electron paramagnetic resonance analysis of liposomes containing spin-labeled PIP2 shows that R4 but not K8 is critical for SCAMP E binding to PIP2. The interfacial locations of SCAMP E and its structural variants within lipid bicelles measured by oxygen enhancement of nuclear relaxation are all similar. Corresponding point mutations within full-length SCAMP2 (SC2-R204A, SC2-K208A, and SC2-W202A) have been analyzed for biological effects on dense core vesicle exocytosis in neuroendocrine PC12 cells. With the same level of overexpression, SC2-R204A but not SC2-K208A inhibited secretion of cotransfected human growth hormone and of noradrenalin. Inhibition by SC2-R204A was the same as or greater than previously observed for SC2-W202A. Analysis of noradrenalin secretion by amperometry showed that inhibitory mutants of SCAMP2 decrease the probability of fusion pore opening and the stability of initially opened but not yet expanded fusion pores. The strong correlation between SCAMP2 E interactions with PIP2 and inhibition of exocytosis, particularly by SC2-R204A, led us to propose that SCAMP2 interaction with PIP2 within the membrane interface regulates fusion pore formation during exocytosis.

Emerging roles of alkali cation/proton exchangers in organellar homeostasis

The regulated movement of monovalent cations such as H(+), Li(+), Na(+) and K(+) across biological membranes influences a myriad of cellular processes and is fundamental to all living organisms. This is accomplished by a multiplicity of ion channels, pumps and transporters. Our insight into their molecular, cellular and physiological diversity has increased greatly in the past few years with the advent of genome sequencing, genetic manipulation and sophisticated imaging techniques. One of the revelations from these studies is the emergence of novel alkali cation/protons exchangers that are present in endomembranes, where they function to regulate not only intraorganellar pH but also vesicular biogenesis, trafficking and other aspects of cellular homeostasis.

The synaptic vesicle proteome

Synaptic vesicles are key organelles in neurotransmission. Vesicle integral or membrane-associated proteins mediate the various functions the organelle fulfills during its life cycle. These include organelle transport, interaction with the nerve terminal cytoskeleton, uptake and storage of low molecular weight constituents, and the regulated interaction with the pre-synaptic plasma membrane during exo- and endocytosis. Within the past two decades, converging work from several laboratories resulted in the molecular and functional characterization of the proteinaceous inventory of the synaptic vesicle compartment. However, up until recently and due to technical difficulties, it was impossible to screen the entire organelle thoroughly. Recent advances in membrane protein identification and mass spectrometry (MS) have dramatically promoted this field. A comparison of different techniques for elucidating the proteinaceous composition of synaptic vesicles revealed numerous overlaps but also remarkable differences in the protein constituents of the synaptic vesicle compartment, indicating that several protein separation techniques in combination with differing MS approaches are required to identify and characterize the synaptic vesicle proteome. This review highlights the power of various gel separation techniques and MS analyses for the characterization of the proteome of highly purified synaptic vesicles. Furthermore, the newly detected protein assignments to synaptic vesicles, especially those proteins which are new to the inventory of the synaptic vesicle proteome, are critically discussed.

Rice SCAMP1 defines clathrin-coated, trans-golgi-located tubular-vesicular structures as an early endosome in tobacco BY-2 cells

We recently identified multivesicular bodies (MVBs) as prevacuolar compartments (PVCs) in the secretory and endocytic pathways to the lytic vacuole in tobacco (Nicotiana tabacum) BY-2 cells. Secretory carrier membrane proteins (SCAMPs) are post-Golgi, integral membrane proteins mediating endocytosis in animal cells. To define the endocytic pathway in plants, we cloned the rice (Oryza sativa) homolog of animal SCAMP1 and generated transgenic tobacco BY-2 cells expressing yellow fluorescent protein (YFP)-SCAMP1 or SCAMP1-YFP fusions. Confocal immunofluorescence and immunogold electron microscopy studies demonstrated that YFP-SCAMP1 fusions and native SCAMP1 localize to the plasma membrane and mobile structures in the cytoplasm of transgenic BY-2 cells. Drug treatments and confocal immunofluorescence studies demonstrated that the punctate cytosolic organelles labeled by YFP-SCAMP1 or SCAMP1 were distinct from the Golgi apparatus and PVCs. SCAMP1-labeled organelles may represent an early endosome because the internalized endocytic markers FM4-64 and AM4-64 reached these organelles before PVCs. In addition, wortmannin caused the redistribution of SCAMP1 from the early endosomes to PVCs, probably as a result of fusions between the two compartments. Immunogold electron microscopy with high-pressure frozen/freeze-substituted samples identified the SCAMP1-positive organelles as tubular-vesicular structures at the trans-Golgi with clathrin coats. These early endosomal compartments resemble the previously described partially coated reticulum and trans-Golgi network in plant cells.

Analysis of the synaptic vesicle proteome using three gel-based protein separation techniques

Synaptic vesicles are key organelles in neurotransmission. Their functions are governed by a unique set of integral and peripherally associated proteins. To obtain a complete protein inventory, we immunoisolated synaptic vesicles from rat brain to high purity and performed a gel-based analysis of the synaptic vesicle proteome. Since the high hydrophobicity of integral membrane proteins hampers their resolution by gel electrophoretic techniques, we applied in parallel three different gel electrophoretic methods for protein separation prior to MS. Synaptic vesicle proteins were subjected to either 1-D SDS-PAGE along with nano-LC ESI-MS/MS or to the 2-D gel electrophoretic techniques benzyldimethyl-n-hexadecylammonium chloride (BAC)/SDS-PAGE, and double SDS (dSDS)-PAGE in combination with MALDI-TOF-MS. We demonstrate that the combination of all three methods provides a comprehensive survey of the proteinaceous inventory of the synaptic vesicle membrane compartment. The identified synaptic vesicle proteins include transporters, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), synapsins, rab and rab-interacting proteins, additional guanine nucleotide triphosphate (GTP) binding proteins, cytoskeletal proteins, and proteins modulating synaptic vesicle exo- and endocytosis. In addition, we identified novel proteins of unknown function. Our results demonstrate that the parallel application of three different gel-based approaches in combination with mass spectrometry permits a comprehensive analysis of the synaptic vesicle proteome that is considerably more complex than previously anticipated.

Subcellular redistribution of the serotonin transporter by secretory carrier membrane protein 2

The serotonin transporter (SERT) belongs to the SLC6 family of sodium- and chloride-dependent neurotransmitter transporters responsible for uptake of amino acids and biogenic amines from extracellular spaces. Their activities and subcellular distributions are regulated by various cellular mechanisms, including interactions with other proteins. Using the yeast two-hybrid approach we screened a human brain cDNA library and identified secretory carrier membrane protein 2 (SCAMP2) as a novel SERT-interacting protein. GST-pulldown assays confirmed the physical interaction between SCAMP2 and the N-terminal domain of SERT. In addition, SERT was found to form a complex with SCAMP2 as demonstrated by co-immunoprecipitation from a heterologous expression system and from rat brain homogenate. Co-expression of SERT and SCAMP2 in mammalian cells results in the subcellular redistribution of SERT with a decrease in cell surface SERT and a concomitant reduction in 5-HT uptake activity. Using confocal microscopy we show that in neuronal cells endogenous SERT co-localizes with SCAMP2 in discrete structures also containing the lipid raft marker flotillin-1 and the SNARE protein syntaxin 1A. In contrast, SERT immunoreactivity is clearly segregated from transferrin receptor-containing endosomes. A single amino acid mutation, cysteine-201 to alanine, within the conserved cytoplasmic E peptide of SCAMP2, abolished SCAMP2-mediated down-regulation of SERT, although this mutation had no effect on the physical interaction between SERT and SCAMP2. Taken together, our results suggest that SCAMP2 plays an important role in the regulation of the subcellular distribution of SERT.