Cell Biology

... from active transport to vesicles


Exocytosis is the process by which cells pass macromolecules through their membranes. In multicellular organisms, exocytosis serves regulatory functions and participates in signaling, while monocellular organisms such as protozoa employ exocytosis for the elimination of waste products. The exosome complex consists almost exclusively of exoribonucleolytic proteins.

In multicellular organisms, exocytosis is 'c-non-c':
constitutive, calcium independent – release of molecules into the ECM, cell membrane turnover following vesicle transport and fusion
non-constitutive, calcium ion-triggered – neurotransmitter release from presynaptic neurons, secretion of antibodies, enzymes, biochemical mediators such as cytokines, peptide and steroid hormones, antigen presentation, etc.

1. vesicle trafficking
2. vesicle tethering
3. vesicle docking
4. vesicle priming
5. vesicle fusion

[] endocytosis & exocytosis animation [] Inner Life of the Cell animation (lo-res) and explanation []

The exosome complex consists almost exclusively of exoribonucleolytic proteins – 10 different proteins important for 3' → 5' degradation of ARE-containing mRNAs in mammalian cells. Although exosomes accumulate in the nucleolus, they also localize in the cytoplasm and in neoplasm. The exosome membrane is organized as a lipid bilayer with a random distribution of phosphatidylethanolamines. Exosome membranes display a similar content of the major phospholipids and cholesterol, but an enrichment in sphingomyelin when compared to the parent cell membrane.

Review Exosome & Models of exosome complex.

Edinburgh Research Archive : Item 1842/734: "The exosome complex of 3' → 5' exoribonucleases functions in both the precise processing of 3' extended precursor molecules to mature stable RNAs and the complete degradation of other RNAs. Both processing and degradative activities of the exosome depend on additional cofactors, notably the putative RNA helicases Mtr4p and Ski2p. "

• A • adhesion • C • cell membranescellular adhesion moleculescellular signal transductioncentrioleschemotaxischloroplastciliacommunicationconcentration gradientscytokine receptorscytoplasmcytoskeleton • E • energy transducersendoplasmic reticulumendosomesexosome • G • Golgi apparatusGPCRs • H • hormones • I • ion channels • L • lysosome • M • meiosismicrotubulesmitosismitochondrion • N • Nitric Oxideneurotransmissionneuronal interconnectionsnuclear membranenuclear pore • P • pinocytosisproteasomepumps • R •receptor proteinsreceptor-mediated endocytosis • S • second messengerssignaling gradientssignal transductionspindlestructure • T • transporttwo-component systems • V • vacuolevesicle

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