Chen L, Davis N G
Department of Pharmacology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA.
J Cell Biol. 2000 Oct 30;151(3):731-8. doi: 10.1083/jcb.151.3.731.
The yeast a-factor receptor (Ste3p) is subject to two mechanistically distinct modes of endocytosis: a constitutive, ligand-independent pathway and a ligand-dependent uptake pathway. Whereas the constitutive pathway leads to degradation of the receptor in the vacuole, the present work finds that receptor internalized via the ligand-dependent pathway recycles. With the a-factor ligand continuously present in the culture medium, trafficking of the receptor achieves an equilibrium in which continuing uptake to endosomal compartments is balanced by its recycling return to the plasma membrane. Withdrawal of ligand from the medium leads to a net return of the internalized receptor back to the plasma membrane. Although recycling is demonstrated for receptors that lack the signal for constitutive endocytosis, evidence is provided indicating a participation of recycling in wild-type Ste3p trafficking as well: a-factor treatment both slows wild-type receptor turnover and results in receptor redistribution to intracellular endosomal compartments. Apparently, a-factor acts as a switch, diverting receptor from vacuole-directed endocytosis and degradation, to recycling. A model is presented for how the two Ste3p endocytic modes may collaborate to generate the polarized receptor distribution characteristic of mating cells.
酵母a因子受体（Ste3p）存在两种机制不同的内吞作用模式：一种是组成型、不依赖配体的途径，另一种是依赖配体的摄取途径。组成型途径导致受体在液泡中降解，而目前的研究发现，通过依赖配体途径内化的受体可进行再循环。在培养基中持续存在a因子配体的情况下，受体的运输达到一种平衡，即持续摄取到内体区室的过程与其再循环回到质膜的过程相平衡。从培养基中去除配体导致内化受体净返回质膜。虽然已证明缺乏组成型内吞信号的受体可进行再循环，但也有证据表明再循环也参与野生型Ste3p的运输：a因子处理既减缓野生型受体的周转，又导致受体重新分布到细胞内的内体区室。显然，a因子起到了开关的作用，将受体从导向液泡的内吞作用和降解转向再循环。本文提出了一个模型，说明两种Ste3p内吞模式如何协同作用，以产生交配细胞特有的极化受体分布。
