EXPRESSION AND CHARACTERIZATION OF A VERY-LOW-DENSITY LIPOPROTEIN RECEPTOR VARIANT LACKING THE O-LINKED SUGAR REGION GENERATED BY ALTERNATIVE SPLICING

The very low density lipoprotein receptor (VLDLR) gene contains an exo n encoding a region of clustered serine and threonine residues immedia tely outside the membrane-spanning sequence, and this region has been proposed to be the site of clustered O-linked carbohydrate chains, Two forms of VLDLR transcripts, with and without the O-linked sugar regio n, are generated through alternative splicing. Reverse transcription p olymerase chain reaction with RNAs from various rabbit tissues reveale d that the VLDLR transcript with the O-linked sugar region (type-1 VLD LR) is the major transcript in heart and muscle, while the VLDLR trans cript without the O-linked sugar region (type-2 VLDLR) predominates in non-muscle tissues, including cerebrum, cerebellum, kidney, spleen, a drenal gland, testis, ovary, and uterus, Hamster fibroblasts expressin g type-a VLDLR bound with relatively low affinity to beta-migrating ve ry low density lipoprotein compared with type-1 VLDLR-transfected cell s, In contrast, the internalization, dissociation, and degradation of the ligand were not significantly impaired in either type of VLDLR-tra nsfected cell. The receptor proteins in type-2 VLDLR-transfected cells underwent rapid degradation and accumulated in the culture medium, wh ile those in type-1 VLDLR-transfected cells were stable and resistant to proteolytic cleavage. Analysis of the O-linked sugars of both types of transfected cells suggested that the O-linked sugar region is the major site for O-glycosylation.