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In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell membrane, resulting in their containment within a small vesicle inside the cell. These pinocytotic vesicles then typically fuse with early endosomes to hydrolyze (break down) the particles. Pinocytosis is variably subdivided into categories depending on the molecular mechanism and the fate of the internalized molecules. In humans, this process occurs primarily for absorption of fat droplets. In endocytosis the cell plasma membrane extends and folds around desired extracellular material, forming a pouch that pinches off creating an internalized vesicle. The invaginated pinocytosis vesicles are much smaller than those generated by phagocytosis. The vesicles eventually fuse with the lysosome, whereupon the vesicle contents are digested. Pinocytosis involves a considerable investment of cellular energy in the form of ATP. Pinocytosis is used primarily for clearing extracellular fluids (ECF) and as part of immune surveillance. In contrast to phagocytosis, it generates very small amounts of ATP from the wastes of alternative substances such as lipids (fat). Unlike receptor-mediated endocytosis, pinocytosis is nonspecific in the substances that it transport: the cell takes in surrounding fluids, including all solutes present. The word pinocytosis (ˌpɪnəsaɪˈtoʊsᵻs,ˌpaɪ-,-noʊ-,_-sə-) uses combining forms of pino- + cyto- + -osis, all Neo-Latin from Greek, reflecting píno, to drink, and cytosis. The term was proposed by W. H. Lewis in 1931. Non-specific, adsorptive pinocytosis is a form of endocytosis, a process in which small particles are taken in by a cell by splitting off small vesicles from the cell membrane. Cationic proteins bind to the negative cell surface and are taken up via the clathrin-mediated system, thus the uptake is intermediate between receptor-mediated endocytosis and non-specific, non-adsorptive pinocytosis.

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Analysis of a specific modular serine protease acting in the Drosophila Toll pathway

Valentin Sottas

In Drosophila, the Toll pathway plays an important role in the immune defense against Gram-positive bacteria and fungi. Molecular determinants coming from those pathogens are directly detected by pattern recognition receptors (PPRs) circulating in the hemolymph. It has been proposed that several serine protease cascades are activated by PRRs, leading to the activation of a cleaved form of the cytokine-like molecule Spätzle, the ligand of the Toll receptor. The results obtained during my master project demonstrate an essential role for ModSP, a modular serine protease acting in the activation of the Toll pathway upon Gram-positive and fungal infections. We demonstrate that ModSP integrates signals coming from GNBP3 and PGRP-SA recognition molecules and that ModSP sends this signal to Grass, a serine protease already known to activate SPE and thereby Spätzle. Further biochemical experiments show the interaction between ModSP and GNBP1 demonstrating an apical role of this serine protease in the proteolytic cascades leading to Toll pathway activation. We also find that ModSP is expressed in specific vesicles released from the fat bodies into the hemolymph. Also, preliminary studies suggest that ModSP does not participate in the melanization reaction, a secondary but important insect immune mechanism. Biochemical analysis done by some collaborators indicates that ModSP does not cleave Grass and that ModSP exhibits a high level of auto-proteolysis when this molecule is expressed. Our data reveal a conserved role of modular serine protease in the regulation of immune proteolytic cascade in insects

2009

Pinocytosis

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