Decorin core protein secretion is regulated by N-linked oligosaccharide and glycosaminoglycan additions.
Expression of decorin using the vaccinia virus/T7 expression system resulted in secretion of two distinct glycoforms: a proteoglycan substituted with a single chondroitin sulfate chain and N-linked oligosaccharides and a core protein glycoform substituted with N-linked glycans but without a glycosaminoglycan chain. In this report, we have addressed two distinct questions. What is the rate-limiting step in glycosaminoglycan synthesis? Is glycosylation with either N-linked oligosaccharides or glycosaminoglycan required for secretion of decorin? N-terminal sequencing of the core protein glycoform, the addition of benzyl-beta-d-xyloside, and a UDP-xylose: core protein beta-d-xylosyltransferase activity assay show that xylosylation is a rate-limiting step in chondroitin sulfate biosynthesis. Decorin can be efficiently secreted with N-linked oligosaccharides alone or with a single chondroitin sulfate chain alone; however, there is severely impaired secretion of core protein devoid of any glycosylation. A decorin core protein mutant devoid of N-linked oligosaccharide attachment sites will not be secreted by Chinese hamster ovary cells deficient in xylosyltransferase or by parental Chinese hamster ovary wild type cells if the xylosyltransferase recognition sequence is disrupted. This finding suggests that quality control mechanisms sensitive to an absence of N-linked oligosaccharides can be abrogated by interaction of the core protein with the glycosaminoglycan synthetic machinery. We propose a model of regulation of decorin secretion that has several components, including appropriate substitution with N-linked oligosaccharides and factors involved in glycosaminoglycan synthesis.