Your friend, who is an engineering major, is interested in the mechanical properties of extracellular matrix and wants to study how secreted collagen molecules assemble into a collagen fiber. As part of his studies, he has been trying to speed up the rate at which cells can produce the collagen protein and is considering reducing the size of the propeptide that is attached to the collagen chain such that it contains only the ER targeting sequence as well as the site for cleavage of the propeptide. To his surprise, this modification causes collagen to be degraded inside the cell. Can you explain why?