Microtubule Dependent Transport and Fusion of Phagosomes with the Endocytic Pathway
Cells internalize material from the extracellular fluid by several different mechanisms (for a recent review on the diversity of mechanisms see Watts and Marsh, 1992). The two best characterized mechanisms are the clathrin-dependent pathway that functions for most receptor-mediated uptake as well as a variable proportion of the bulk fluid uptake, and phagocytosis. For a number of years our group has focused on the organization and function of the “classical” clathrin mediated endocytic pathway, both in fibroblasts and in polarized MDCK cells (see Griffiths and Gruenberg, 1991). Collectively, our results argue that, following the initial clathrin coated pit/vesicle uptake, material destined for degradation will traverse four distinct cellular organelles, the early endosome, the multivesicular body-like endosomal carrier vesicle (ECV), the late endosome (or prelysosomal compartment) and the terminal lysosome (Fig. 1). In our view the early and late endosomes represent true cellular compartments that are pre-existing in the cell and whose passage to daughter cells during mitosis is, we propose, essential for cell viability. In contrast, we propose that the ECV and the terminal lysosomes are transient vesicles (although these may be very long lived) which bud off the early and late endosomes, respectively. In line with this prediction we put forward the hypothesis that the ECV and dense lysosomes are not essential for daughter cell viability since they can be formed by budding from pre-existing compartments. While this view of the structures we now refer to as lysosomes may seem heretical, an increasing list of data can be put forward to argue that it is the late endosomes or prelysosomes that are the key functional degradation compartment in the cell (see Griffiths, 1990).
KeywordsMotor Protein Early Endosome Late Endosome Endocytic Pathway Latex Bead
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