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, a nd intracellular transport of nascent low density lipoprotein (LDL) re ceptor 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 suppr essor of endogenous Rab1B function. [S-35]Methionine pulse-chase analy sis of immunoprecipitated LDL-receptor indicated that coexpression wit h Rab1B(N121I) but not Rab1B(WT), impaired its conversion from the End o-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 re ceptor. In cells expressing Rab1B(N121I), the newly synthesized LDL-re ceptor was unable to reach the cell surface as evidenced by its inacce ssibility to sulfo-NHS-biotin added to the cultures. These observation s provide a direct demonstration of Rab protein involvement in LDL rec eptor trafficking and lend support to the concept of Rab1B as a univer sal mediator of ER --> Golgi transport of membrane glycoproteins in ma mmalian cells.