Riederer M A, Soldati T, Shapiro A D, Lin J, Pfeffer S R
Department of Biochemistry, Stanford University School of Medicine, CA 94305-5307.
J Cell Biol. 1994 May;125(3):573-82. doi: 10.1083/jcb.125.3.573.
Newly synthesized lysosomal enzymes bind to mannose 6-phosphate receptors (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 S21N, strongly inhibited MPR recycling in living cells. The block was specific in that the rates of biosynthetic protein transport, fluid phase endocytosis and receptor-mediated endocytosis were unchanged. Expression of rab9 S21N was accompanied by a decrease in the efficiency of lysosomal enzyme sorting. Cells compensated for the presence of the mutant protein by inducing the synthesis of both soluble and membrane-associated lysosomal enzymes, and by internalizing lysosomal enzymes that were secreted by default. These data show that MPRs are limiting in the secretory pathway of cells expressing rab9 S21N and document the importance of MPR recycling and the rab9 GTPase for efficient lysosomal enzyme delivery.
新合成的溶酶体酶在反式高尔基体网络（TGN）中与甘露糖6-磷酸受体（MPR）结合，并被转运至前溶酶体，然后在那里被释放。MPR随后返回TGN进行新一轮转运。Rab9是一种类Ras GTP酶，在体外促进MPR循环回到TGN。我们在此表明，Rab9的显性负性形式Rab9 S21N在活细胞中强烈抑制MPR循环。这种阻断具有特异性，因为生物合成性蛋白质转运、液相内吞作用和受体介导的内吞作用的速率未发生改变。Rab9 S21N的表达伴随着溶酶体酶分选效率的降低。细胞通过诱导可溶性和膜相关溶酶体酶的合成，以及内化默认分泌的溶酶体酶来补偿突变蛋白的存在。这些数据表明，MPR在表达Rab9 S21N的细胞分泌途径中是有限的，并证明了MPR循环和Rab9 GTP酶对于高效溶酶体酶递送的重要性。
