Procollagen polypeptides

Collagen chains are synthesized as longer precursors, called procollagens, with globular extensions—propeptides of about 200 residues—at both ends. These procollagen polypeptide chains are transported into the lumen of the rough endoplasmic reticulum where they undergo hydroxylation and other chemical modifications before they are assembled into triple chain molecules. The terminal propeptides are essential for proper formation of triple... 

The first evidence for PDI having a different role came from Pihla-janiemi etal. (1987) during their study of the enzyme prolyl 4-hydroxylase (P4H). This enzyme is localized in the ER and catalyzes the hydroxylation of proline residues in nascent procollagen polypeptides. The enzyme is a tetramer composed of two a and two jS subunits (Kivirikko et al., 1989). Antibodies raised against the holoenzyme were used to screen a human gtll expression library and clones were isolated that coded for the /3 subunit. After sequencing it was revealed that the human /3 subunit was 94% identical with the rat PDI sequence. Southern blot analysis identified... 

Type XVII collagen is composed of three identical procollagen polypeptides, each about 1,500 amino acids in length. It has a tadpole-like shape under physiological conditions (deduced from rotary shadowing electron microscopy of bovine cell lines or the pure protein). The protein is one of very few proteins whose N- and C-termini are inverted with respect to the membrane. In nearly all transmembrane proteins, for example integrins, the C-terminus is cytosolic and the N-terminus is extracellular. In type XVII collagen, the cytosolic N-terminal domain comprises about a third of the amino acid residues... 

Folding of procollagen polypeptides into a triple helix. 

The propeptide regions of procollagen polypeptides contain cysteine residues that can form both intra- and interchain disulfide linkages (Figure 25-19). As is the case for other proteins that contain disulfide linkages, it is not known whether synthesis of these linkages requires an enzyme or whether it occurs spontaneously. The assembly of procollagen polypeptides into a triple helix appears to have two requirements ... 

After the procollagen polypeptides are assembled into a triple helix, they are secreted by the classical route. They pass through the smooth endoplasmic reticulum and the Golgi complex, where they are packaged into membranous vesicles and secreted into the extracellular space by exo-cytosis. This process requires ATP and may involve microtubules and microfilaments. The conformation of procollagen markedly affects the rate of secretion. Prevention of the formation of a triple helix (e.g., lack of 4-hydroxylation due to vitamin C deficiency) leads to the accumulation of nonhelical propolypeptides within the cistemae of the rough endoplasmic reticulum and a delayed rate in its secretion. 

Proline is incorporated into polypeptide precursors of collagen. In the procollagen polypeptide (precursor to the mature collagen), proline is converted to hydroxyproline by the enzyme procollagen proline hydroxylase (Figure 21.4). As seen in the figure, the reaction requires ascorbic acid (vitamin... 

The incorporation of ciJ-4-hydroxy-L-proline into procollagen polypeptides, which occurs on incubation with matrix-free chick embryo tendon cells, prevents the polypeptides from folding into a triple-helical formation. Secretion of the non-helical polypeptides is delayed, and contrary to previous observations they are not degraded intracellularly, but instead are secreted as non-helical pro-y chains which contain increased amounts of glycosylated L-hydroxylysine. 

A procollagen triple helix is assembled in the endoplasmic reticulum helix formation is aided by disulfide bonds between N- and C-terminal propeptides, which align the polypeptide chains. Post-translational modification of procollagen is crucial in allowing for collagen fibril formation. For example, in cells deprived of ascorbate, as in the disease scurvy, the procollagen chains are not hydroxylated sufficiently to form stable triple helices at normal body temperature (hydrox-ylation is through the activity of prolyl hydroxylase, which requires the cofactor ascorbic acid). 

---