Intracellular retention of procollagen within the endoplasmic reticulum ismediated by prolyl 4-hydroxylase

The correct folding and assembly of proteins within the endoplasmic reticul um (ER) are prerequisites for subsequent transport from this organelle to t he Gels apparatus. The mechanisms underlying the ability of the cell to rec ognize and retain unassembled or mal-folded proteins generally require bind ing to molecular chaperones within the ER. One classic example of this proc ess occurs during the biosynthesis of procollagen. Here partially folded in termediates are retained and prevented from secretion, leading to a build u p of unfolded chains within the cell. The accumulation of these partially f olded intermediates occurs during vitamin C deficiency due to incomplete pr oline hydroxylation, as vitamin C is an essential co-factor of the enzyme p rolyl 4-hydroxylase. in this report we show that this retention is tightly regulated with little or no secretion occurring under conditions preventing proline hydroxylation. me studied the molecular mechanism underlying reten tion by determining which proteins associate with partially folded procolla gen intermediates within the ER. By using a combination of cross-linking an d sucrose gradient analysis, we show that the major protein binding to proc ollagen during its biosynthesis is prolyl 4-hydroxylase, and no binding to other ER resident proteins including Hsp47 was detected. This binding is re gulated by the folding status rather than the extent of hydroxylation of th e chains demonstrating that this enzyme can recognize and retain unfolded p rocollagen chains and can release these chains for further transport once t hey have folded correctly.