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Department of Cell Biology and Anatomy and Department of Pediatrics, Steele Memorial Children's Research Center,
University of Arizona, Tucson, Arizona 85724
Polarized cells such as epithelial cells and
neurons have distinct endosomal compartments associated with different plasma membrane domains. The endosomes of the neuronal cell body and the basolateral
cytoplasm of epithelial cells are thought to perform cellular "housekeeping" functions such as the uptake of
nutrients and metabolites, while the endosomes in the
apical cytoplasm or axons are thought to be specialized
for the sorting and transcytosis of cell type-specific
ligands and receptors. However, it is not known if nonpolarized cells such as fibroblasts contain a specialized endosomal compartment analogous to the specialized
endosomes found in neurons and epithelia. We have
expressed a protein that is normally found in the apical
early endosomes of developing intestinal epithelial cells
in normal rat kidney fibroblasts. This apical endosomal
marker, called endotubin, is targeted to early endosomes in transfected fibroblasts, and is present in peripheral as well as perinuclear endosomes. The peripheral endosomes that contain endotubin appear to
exclude transferrin, fluid phase markers, and the mannose-6-phosphate receptor, although in the perinuclear region colocalization of endotubin and these markers is
present. In addition, endotubin positive structures do
not tubulate in response to brefeldin A and instead redistribute to a diffuse perinuclear location. Since this
endosomal compartment has many of the characteristics of an apical or axonal endosomal compartment, our
results indicate that nonpolarized cells also contain a
specialized early endosomal compartment.
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