Processing of Collagen Does Not End with Secretion

Recall that collagen is an extracellular matrix protein that serves as a major constituent of many connective tissues (see figs. 4.10 to 4.13). Collagen fibrils have a distinctive banded pattern with a periodicity of 680 A. Individual fibrils are composed of three polypeptide chains wound around one another in a right-handed helix with a total length of 3,000 A. Each of the polypeptide chains in the triple helix has a repetitious tripeptide sequence, Gly-X-Y, where X is frequently a proline and Y is frequently a hy-droxyproline. The latter amino acid is not one of the 20 that are specified genetically, so it must be formed posttransla-tionally by a modification of some of the prolines. 

Since collagen is a secreted protein, we know that it must follow the route of synthesis that starts on a ribosome bound to the endoplasmic reticulum (fig. 29.23). This is where the fun begins, as we find that the nascent collagen polypeptide has extensive N and C termini (150 and 250 

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