| Secreted proteoglycans probably undergo surveillance by molecular chaperones in the endoplasmic reticulum (ER); the chaperones determine if proper conformation is present, serving as gatekeepers for proteoglycan core protein entry into the Golgi stacks. Such entry requires proper conformation of at least one core protein globular domain. Meanwhile, non-globular structures may not be subject to chaperone surveillance. We postulate that the C-terminal G3 domain of proteoglycans is a major conformer for quality control surveillance by chaperones. This postulate is based upon the following considerations: (1) a subdomain of G3 is a common motif in secreted and cell surface proteins; (2) lack of G3 causes proteoglycan core protein retention in the ER and its non-entry into Golgi stacks; (3) G3 seems expendable following proteoglycan secretion. We compared the secretory behavior of the globular ends of aggrecan, namely G1 (N-terminus) and G3 (C-terminus), in separate constructs containing endogenous consensus sites for glycosaminoglycan (GAG) chain addition. The consensus sites served as reporter groups for Golgi transit. Each construct was separately transfected into Chinese hamster ovary K1 (CHO) cells. The transfection system had been previously optimized using chloramphenicol acetyltransferase instead of G1 or G3; expression was... |
