Figure 214

Electron micrograph of an early endosome. This deep-etch electron micrograph shows the structure of an early endosome in Dic-tyostelium. Early endosomes are located near the plasma membrane and have a typical tubulovesical structure. The lumen of the endosome is subdivided into multiple compartments or cisternae by the invagination of its membrane and undergoes frequent change in shape, x 15,000. (Courtesy of Dr. John E. Heuser, Washington University School of Medicine.)

nal patch on a protein destined for a lysosome is then modified by several enzymes that attach mannose-6-pbospbate (M-6-P) to the enzyme surface. M-6-P acts as a target for specific proteins possessing a M-6-P receptor. M-6-P receptors are present in early and late endosomes, lysosomes, and the Golgi apparatus that is involved in sorting and retrieving secreted hydrolyses destined for transport to endosomes (Fig. 2.15).

Early and late endosomes differ in their cellular localization, morphology, and state of acidification and function

Early and late endosomes are localized in different areas of the cell. Early endosomes can be found in the more peripheral cytoplasm, whereas late endosomes are often positioned near the Golgi apparatus and the nucleus. Early endosomes have a tubulovesicular structure: The lumen is subdivided into cisternae that are separated by invagination of its membrane. They exhibit only a receptor

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