Binds to the C-terminal propeptide of types I and II procollagens and may enhance the cleavage of that propeptide by BMP1. Interacts with heparin with high affinity, and type I or II collagen. Highly expressed in the heart, trabecular meshwork, pituitary gland, bladder, mammary gland, trachea and placenta and weakly expressed in the brain. Expressed in cartilage. O-glycosylated; contains sialic acid.