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Intracellular Translocation of Procollagen and Extrusion into the Extracellular Space

The procollagen molecules, now detached from the ribosome, emerge from the endoplasmic reticulum (ER) and move toward the Golgi apparatus (G) through the microsomal lumen. In the Golgi, the C-terminal mannose-rich carbohydrate extensions are remodeled, the molecules are packaged in vesicles, and subsequently carried towards the cellular membrane.

The small aggregates of oriented procollagen molecules are probably trimmed of their nonhelical amino and carboxyl extensions by specific peptidases when they reach the extracellular space. In the case of Type I collagen, the first peptidase to act seems to be the amino protease; this is followed by a carboxyprotease. In Type III collagen the sequence of removal may be reversed.

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