THE RAS-RELATED GTP-BINDING PROTEIN, RAB1B, REGULATES EARLY STEPS IN EXOCYTIC TRANSPORT AND PROCESSING OF BETA-AMYLOID PRECURSOR PROTEIN

The role of the Ras-related GTP-binding protein, Rab1B, in intracellular trafficking of beta-amyloid precursor protein (beta APP) was studied in cultured 293 cells, beta APP is processed via one of two alternative routes. In the major secretory pathway, beta APP is cleaved by alpha-secretase within the region comprising the beta-amyloid peptide (A beta), resulting in release of a soluble NH2-terminal exodomain (APP(alpha)) and a 3-kDa peptide (p3) derived from the carboxyl-terminal tail. In the alternative amyloidogenic pathway, beta APP is cleaved by beta-secretase, with the release of a truncated exodomain (APP(beta)) and an intact A beta peptide. When beta APP(751) was coexpressed with Rab1B(wt) or dominant negative Rab1B mutants (Rab1B(N121I) or Rab1B(S22N)) there was a marked decrease in conversion of the immature Endo-H sensitive form of beta APP(751) (108 kDa) to the mature O-glycosylated form of beta APP(751) (130 kDa) in cells expressing the mutant forms of Rab1B. The block in Golgi-dependent processing of beta APP was accompanied by inhibition of secretion of APP(S) (APP(alpha)). A similar decrease in secretion of APP(S) (APP, + APP(beta)) was observed in cells that were coexpressing Rab1B(N121I) with the ''Swedish'' variant of beta APP(751) (i.e. beta APPSW(751)), which undergoes increased amyloidogenic processing. Coincident with the decline in APP(8) secretion, the cells coexpressing beta APPSW(751) with Rab1B(N121I) showed a 90% decrease in AP secretion. The data indicate that Rab1B plays a key role in endoplasmic reticulum --> Golgi transport of beta APP, and that beta APP must pass through a late Golgi compartment before entering either the alpha-secretase or the amyloidogenic beta-secretase pathway. The results also suggest that mutant versions of other Rab proteins that function in different parts of the exocytic and endocytic pathways may be useful in defining the specific routes of beta APP transport involved in the biogenesis of A beta.