Intracellular transport and maturation of nascent low density lipoprotein receptor is blocked by mutation in the Ras-related GTP-binding protein, Rab1B

The relationship between the Ras-related GTP-binding protein, Rab1B, and intracellular transport of nascent low density lipoprotein (LDL) receptor was studied in cultured human embryonic kidney cells (Line 293) cotransfected with plasmids encoding the LDL-receptor and either wild-type Rab1B or a Rab1B mutant (N121I) known to act as a dominant suppressor of endogenous Rab1B function. [S-35]Methionine pulse-chase analysis of immunoprecipitated LDL-receptor indicated that coexpression with Rab1B(N121I) but not Rab1B(WT), impaired its conversion from the Endo-H-sensitive 120-125 kDa form to the O-glycosylated 160-170 kDa form, consistent with a block in ER --> Golgi trafficking of the nascent receptor. In cells expressing Rab1B(N121I), the newly synthesized LDL-receptor was unable to reach the cell surface as evidenced by its inaccessibility to sulfo-NHS-biotin added to the cultures. These observations provide a direct demonstration of Rab protein involvement in LDL receptor trafficking and lend support to the concept of Rab1B as a universal mediator of ER --> Golgi transport of membrane glycoproteins in mammalian cells.