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Protein-protein interactions within the neuronal porosome complex using STRING database search. 13 Two clusters of protein-protein interactions are identifiable within the porosome complex. The cluster to the left likely represents the open end of the porosome facing outside, and is composed of cytoskeletal and signaling proteins. The other cluster includes membrane fusion proteins like SNAREs and calcium channels. The right cluster would therefore represent the porosome basal. (A color version of this figure is available in the online journal.)
Source publication
Cup-shaped secretory portals at the cell plasma membrane called porosomes mediate the precision release of intravesicular material from cells. Membrane-bound secretory vesicles transiently dock and fuse at the base of porosomes facing the cytosol to expel pressurized intravesicular contents from the cell during secretion. The structure, isolation,...
Contexts in source publication
Context 1
... central plug similarly would be composed of myosin, actin, and associated proteins required for its vertical movement to establish the fusion pore at the base of the porosome complex. In order to further advance our understanding of the porosome complex, and the possible association of other minor proteins within the complex, STRING 9.0 database search of known protein-protein interactions ( Figure 5) 110 has been implemented. Over five million proteins from 1133 organisms are covered by STRING, which integrates protein interaction data to develop a likely model of protein-protein interactions. ...
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... five million proteins from 1133 organisms are covered by STRING, which integrates protein interaction data to develop a likely model of protein-protein interactions. STRING maps generated on the neuronal porosome proteome identifies two clusters of protein-protein interactions ( Figure 5), 13 one cluster representing cytoskeletal and signaling proteins, while the other cluster contains proteins involved in membrane fusion such as SNAREs, their associated regulatory proteins, and calcium channels. Although the predicted functional interactions using STRING represents >99% confidence, further chemical cross-linking studies followed by mass spectrometry, and structural studies using electron crystallography, SAXS, neutron scattering, and molecular dynamic simulations will provide a molecular arrangement model of the neuronal porosome complex. ...
Context 3
... central plug similarly would be composed of myosin, actin, and associated proteins required for its vertical movement to establish the fusion pore at the base of the porosome complex. In order to further advance our understanding of the porosome complex, and the possible association of other minor proteins within the complex, STRING 9.0 database search of known protein-protein interactions ( Figure 5) 110 has been implemented. Over five million proteins from 1133 organisms are covered by STRING, which integrates protein interaction data to develop a likely model of protein-protein interactions. ...
Context 4
... five million proteins from 1133 organisms are covered by STRING, which integrates protein interaction data to develop a likely model of protein-protein interactions. STRING maps generated on the neuronal porosome proteome identifies two clusters of protein-protein interactions ( Figure 5), 13 one cluster representing cytoskeletal and signaling proteins, while the other cluster contains proteins involved in membrane fusion such as SNAREs, their associated regulatory proteins, and calcium channels. Although the predicted functional interactions using STRING represents >99% confidence, further chemical cross-linking studies followed by mass spectrometry, and structural studies using electron crystallography, SAXS, neutron scattering, and molecular dynamic simulations will provide a molecular arrangement model of the neuronal porosome complex. ...
Citations
... During the pre-fusion docking process, the ribbon structure drives the vesicle to the synapse so that v-SNARE and the t-SNAREs are within striking distance of each other, allowing clamping to initiate [13]. Neuronal 'porosomes' have also been proposed to be present at the clamping site of these excretory cells [22][23][24][25]. The interaction distances between the porosomes and the synaptic vesicles are believed to be less than 2 nm, and this interaction is proposed to be crucial for vesicle fusion. ...
Synaptic vesicle fusion is a crucial step in the neurotransmission process. Neurotransmitter-filled vesicles are pre-docked at the synapse by the mediation of ribbon structures and SNARE proteins at the ribbon synapses. An electrical impulse triggers the fusion process of pre-docked vesicles, leading to the formation of a fusion pore and subsequently resulting in the release of neurotransmitter into the synaptic cleft. In this study, a continuum model of lipid membrane along with lubrication theory is used to determine the traverse time of the synaptic vesicle under the influence of hydrodynamic forces. We find that the traverse time is strongly dependent on how fast the driving force decays or grows with closure of the gap between the vesicle and the plasma membrane. If the correct behaviour is chosen, the traverse time obtained is of the order of a few hundred milliseconds and lies within the experimentally obtained value of approximately 250 ms (Zenisek D, Steyer JA, Almers W. 2000 Nature406, 849-854 (doi:10.1038/35022500)). We hypothesize that there are two different force behaviours, which complies with the experimental findings of pre-fusion docking of synaptic vesicles at the ribbon synapses. The common theme in the proposed force models is that the driving force has to very rapidly increase or decrease with the amount of clamping.
... Similarly, in the past 20 years following discovery of the supramolecular secretory machinery in cells, the "porosome" 8,9 , there has been a flood of papers on new protein being identified to associate with t-SNAREs at the so called "fusion protein complex" or the "fusion active zone" at the cell plasma membrane 10 , completely ignoring the actual porosome structure at the cell plasma membrane where secretory vesicles transiently dock and fuse to release a fraction of their contents in a highly regulated manner, as opposed to an all-or-none mechanism of complete vesicle merger at the cell plasma membrane. Seeing is believing, and the physical existence of the porosome using atomic force microscopy, and confirmed by electron microscopy and X-Ray solution imaging is undeniable. ...
... Porosomes have been immunoisolated from a number of cells including the exocrine pancreas and neurons, biochemically characterized, and functionally reconstituted into artificial lipid membrane 5 . A large body of evidence has accumulated on the role of porosome-associated proteins on cell secretion and secretory defects, including neurotransmission and neurological disorders 6 . In a recent study, the porosome complex in mouse insulinoma Min6 cells was isolated, its proteome determined 7 , and the isolated porosome was functionally reconstituted into live Min6 cells 8 . ...
Porosomes are plasma membrane structures in secretory cells that allow transient docking and/or partial fusion of vesicles during which they release their content then disengage. This is referred to as “kiss and run” exocytosis. During early pregnancy, at the time of receptivity, there is a high level of vesicle activity in uterine epithelial cells (UECs). One of the secretory pathways for these vesicles could be via porosomes, which had yet to be identified in UECs. This study identified porosomes in the apical plasma membrane of UECs for the first time. These structures were present on days 1, 5.5 and 6 of early pregnancy, where they likely facilitate partial secretion via “kiss and run” exocytosis. The porosomes were measured and quantified on days 1, 5.5 and 6, which showed there are significantly more porosomes on day 5.5 (receptive) compared to day 1 (non‐receptive) of pregnancy. This increase in porosome number may reflect the major morphological and molecular changes in the apical plasma membrane at this time such as increased cholesterol and SNARE proteins, as these are structural and functional components of the porosome complex assembly. Porosomes were observed in both resting (inactive) and dilated (active) states on days 1, 5.5 and 6 of early pregnancy. Porosomes on day 5.5 are significantly more active than on day 1 as demonstrated by the dilation of their base diameter. Further two‐way ANOVA analysis of base diameter in resting and dilated states found a significant increase in porosome activity in day 5.5 compared to day 1. This study therefore indicates an increase in the number and activity of porosomes at the time of uterine receptivity in the rat, revealing a mechanism by which the UECs modify the uterine luminal environment at this time. This article is protected by copyright. All rights reserved.
Secretion is a highly regulated fundamental cellular process in living organisms, from yeast to cells in humans. Cellular cargoes such as neurotransmitters in neurons, insulin in beta cells of the endocrine pancreas, or digestive enzymes in the exocrine pancreas are all packaged and stored in membrane-bound secretory vesicles that dock and fuse at the cell plasma membrane to release their contents during secretion. The prevailing view was that secretory vesicles completely merge with the cell plasma membrane, emptying the entire vesicular contents outside the cell during secretion. However, accumulation of partially empty secretory vesicles observed in electron micrographs in cells following a secretory episode suggested fractional release of intra-vesicular contents during cell secretion. Given the high surface tension at the secretory vesicle membrane, fractional intra-vesicular content release during cell secretion could only be possible via a plasma membrane structure capable of preventing the complete merger or collapse of secretory vesicles into the cell plasma membrane. Cup-shaped plasma membrane-embedded lipoprotein structure called porosomes was first discovered in 1996 in live pancreatic acinar cells using atomic force microscopy (AFM) and subsequently confirmed in all cells examined including neurons using AFM, electron microscopy (EM), and solution X-ray. The porosome exhibits dynamics and its chemical composition demonstrates the utilization of energy in the form of both ATP and guanosine triphosphate (GTP), the participation of molecular motors, ion channels, and soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) membrane fusion proteins, among others. Porosomes are composed of nearly 30 proteins, as opposed to the 120 nm nuclear pore complex comprised of nearly 1000 protein molecules. Porosomes range in size from 15 nm in neurons and astrocytes to 100–180 nm in endocrine and exocrine cells. Porosome has been functionally reconstituted into artificial lipid membrane and in live cells. During secretion, secretory vesicles dock at the base of the porosome complex via v-SNARE proteins at the secretory vesicle membrane and t-SNARE proteins at the porosome base. In the presence of calcium, the v-SNARE and t-SNARE proteins in the opposing bilayers interact in a circular array to establish conducting channels or fusion pores. An increase in volume of the docked secretory vesicle via the rapid entry of ions and aquaporin-mediated rapid entry of water molecules results in increased intra-vesicular pressure, enabling the fractional release of vesicular contents from the cell with great precision. Collectively, these observations provide a molecular understanding of the fractional release of intra-vesicular contents via the transient or kiss-and-run mechanism of cell secretion. The discovery of the porosome and the molecular mechanism of its structure–function has resulted in a paradigm shift in our understanding of the secretory process in cells.
