Figure 219

Lysosome biogenesis. This diagram shows regulated and constitutive pathways for delivery of lysosomal specific membrane proteins into early and late endosomes. The lysosomal membrane possesses highly glycosylated specific membrane proteins that protect the membrane from digestion by lysosomal enzymes. These lysosome-specific proteins are synthesized in the rER, transported to the Golgi apparatus, and reach their destination by two pathways. Blue arrows indicate the constitutive secretory pathway in which certain lysosomal membrane proteins exit the Golgi apparatus and are delivered to the cell surface. From there they are endocytosed and, via the early and late endosomal compartments, finally reach lysosomes. Green arrows indicate the endosomal Golgi-derived coated vesicle secretory pathway. Here, other lysosomal proteins, after sorting and packaging, exit the Golgi apparatus in clathrin-coated vesicles. These vesicles are delivered to the early and/or late endosome by use of the M-6-P targeting mechanism.

• Intracellular particles such as entire organelles, cytoplasmic proteins, and other cellular components are isolated from the cytoplasmic matrix by endoplasmic reticulum membranes, transported to lysosomes, and degraded in the process called autophagy (see below).

In addition, some cells (e.g., osteoclasts involved in bone resorption and neutrophils involved in acute inflammation) may release lysosomal enzymes directly into the extracellular space to digest components of the extracellular matrix.

bacterium bacterium

■smooth endoplasmic \ endocytosis ^reticulum early Xendosome

■smooth endoplasmic \ endocytosis ^reticulum mitochondrion 1 endosome autophagosome

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