The role of O-linked sugars in determining the very low density lipoprotein receptor stability or release from the cell

The very low density lipoprotein receptor is a member of the low density li poprotein receptor supergene family for which two isoforms have been report ed, one lacking and the other containing an O-linked sugar domain. In order to gain insight into their functionality, transient and stable transforman ts separately overexpressing previously cloned bovine variants were analyze d. We report evidence that the variant lacking the O-linked sugar domain pr esented a rapid cleavage from the cell and that a large amino-terminal very low density lipoprotein receptor fragment was released into the culture me dium. As only minor proteolysis,vas involved in the other very low density lipoprotein receptor variant, the clustered O-linked sugar domain may be re sponsible for blocking the access to the protease-sensitive site(s). To tes t this hypothesis, a mutant Chinese hamster ovary cell line, IdlD, with a r eversible defect in the protein O-glycosylation, was used, The instability of the O-linked sugar-deficient very low density lipoprotein receptor on th e cell surface was comparable to that induced by the proteolysis of the var iant lacking the O-linked sugar domain. Moreover, our data suggest that the O-linked sugar domain may also protect the very low density lipoprotein re ceptor against unspecific proteolysis. Taken together, these results indica te that the presence of the O-linked sugar domain may be required for the s table expression of the very low density lipoprotein receptor on the cell s urface and its absence may be required for release of the receptor to the e xtracellular space. The exclusive expression of the variant lacking the O-l inked sugar domain in the bovine aortic endothelium opens new perspectives in the physiological significance of the very low density lipoprotein recep tor. (C) 1999 Federation of European Biochemical Societies.