Department of Cell, Developmental, and Integrative Biology, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA.
J Biol Chem. 2013 Apr 19;288(16):11532-45. doi: 10.1074/jbc.M112.438481. Epub 2013 Feb 5.
Three Sec7 guanine nucleotide exchange factors (GEFs) activate ADP-ribosylation factors (ARFs) to facilitate coating of transport vesicles within the secretory and endosomal pathways. GBF1 recruits COPI to pre-Golgi and Golgi compartments, whereas BIG1 and BIG2 recruit AP1 and GGA clathrin adaptors to the trans-Golgi network (TGN) and endosomes. Here, we report a functional cascade between these GEFs by showing that GBF1-activated ARFs (ARF4 and ARF5, but not ARF3) facilitate BIG1 and BIG2 recruitment to the TGN. We localize GBF1 ultrastructurally to the pre-Golgi, the Golgi, and also the TGN. Our findings suggest a model in which GBF1 localized within pre-Golgi and Golgi compartments mediates ARF activation to facilitate recruitment of COPI to membranes, whereas GBF1 localized at the TGN mediates ARF activation that leads to the recruitment of BIG1 and BIG2 to the TGN. Membrane-associated BIG1/2 then activates ARFs that recruit clathrin adaptors. In this cascade, an early acting GEF (GBF1) activates ARFs that mediate recruitment of late acting GEFs (BIG1/2) to coordinate coating events within the pre-Golgi/Golgi/TGN continuum. Such coordination may optimize the efficiency and/or selectivity of cargo trafficking through the compartments of the secretory pathway.
三种 Sec7 鸟嘌呤核苷酸交换因子（GEFs）激活 ADP-核糖基化因子（ARFs）以促进分泌和内体途径中的运输小泡被涂层。GBF1 将 COPI 募集到前高尔基体和高尔基体区室，而 BIG1 和 BIG2 将 AP1 和 GGA 网格蛋白衔接蛋白募集到反式高尔基体网络（TGN）和内体。在这里，我们通过显示 GBF1 激活的 ARFs（ARF4 和 ARF5，但不是 ARF3）促进 BIG1 和 BIG2 募集到 TGN 来报告这些 GEF 之间的功能级联。我们将 GBF1 超微结构定位到前高尔基体、高尔基体，也定位到 TGN。我们的发现表明，定位于前高尔基体和高尔基体区室的 GBF1 介导 ARF 激活以促进 COPI 募集到膜上，而定位于 TGN 的 GBF1 介导 ARF 激活导致 BIG1 和 BIG2 募集到 TGN。然后，膜相关的 BIG1/2 激活 ARFs，招募网格蛋白衔接蛋白。在这个级联中，早期作用的 GEF（GBF1）激活 ARFs，介导晚期作用的 GEF（BIG1/2）募集到前高尔基体/高尔基体/TGN 连续体，以协调涂层事件。这种协调可能优化货物通过分泌途径的区室运输的效率和/或选择性。
