LYSOSOME BIOGENESIS REQUIRES RAB9 FUNCTION AND RECEPTOR RECYCLING FROM ENDOSOMES TO THE TRANS-GOLGI NETWORK

Newly synthesized lysosomal enzymes bind to mannose 6-phosphate recept ors (MPRs) in the TGN, and are carried to prelysosomes, where they are released. MPRs then return to the TGN for another round of transport. Rab9 is a ras-like GTPase which facilitates MPR recycling to the TGN in vitro. We show here that a dominant negative form of rab9, rab9 S21 N, strongly inhibited MPR recycling in living cells. The block, was sp ecific in that the rates of biosynthetic protein transport, fluid phas e endocytosis and receptor-mediated endocytosis were unchanged. Expres sion of rab9 S21N was accompanied by a decrease in the efficiency of l ysosomal enzyme sorting. Cells compensated for the presence of the mut ant protein by inducing the synthesis of both soluble and membrane-ass ociated lysosomal enzymes, and by internalizing lysosomal enzymes that were secreted by default. These data show that MPRs are limiting in t he secretory pathway of cells expressing rab9 S21N and document the im portance of MPR recycling and the rab9 GTPase for efficient lysosomal enzyme delivery.