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9 Membrane Traffic in the Endocytic Pathway of Eukaryotic Cells, Page 1 of 2
< Previous page Next page > /docserver/preview/fulltext/10.1128/9781555817633/9781555813024_Chap09-1.gif /docserver/preview/fulltext/10.1128/9781555817633/9781555813024_Chap09-2.gifAbstract:
Eukaryotic cells need to be in constant communication with their environment in order to perform most of their functions, such as the transmission of neuronal, metabolic, and proliferative signals and the uptake of nutrients or to protect the organism from microbial invasion, to name only a few. Macropinocytosis involves remodeling of the actin cytoskeleton, a process regulated by the small GTPases Ras, Rac, and Cdc42, and leads to massive membrane internalization. Phagocytosis requires important actin rearrangements and pseudopod extension under the control of Rho GTPases. Several pathways have been proposed to mediate membrane traffic between endosomes and the biosynthetic pathway. In particular, several lines of evidence support the view that direct transport routes mediate anterograde and retrograde transport between early endosomes and the trans-Golgi network (TGN). PI3K activity and, hence, PI(3)P are required for autophagy in yeast, where Vps34 has been shown to coimmunoprecipitate with proteins required for autophagy. Early endosomes are important sorting stations along the endocytic pathway. After receptor-ligand uncoupling at the mildly acidic pH (pH 6.2), housekeeping receptors are transported along the recycling route, whereas ligands follow the degradation pathway together with downregulated receptors and fluid phase markers. Phosphorylation of the inositol ring of phosphatidylinositol in the 3, 4, and 5 positions generates seven different phosphoinositides at the cytosolic face of cellular membranes. During the past few years, phosphoinositides have emerged as regulators of membrane traffic by regulating the localization and/or activity of effector proteins.