Cellular Function Of The ADP-ribosylation Factor 6

Arf6 GTP酶调节质膜(PM)和内小体室之间的膜运输，并影响皮质肌动蛋白细胞骨架的动力学。在许多细胞中，这种内吞途径是PM蛋白缺乏网状蛋白定位序列(如MHCI，主要组织相容性复合体I类)所遵循的路线，因此被内吞到细胞内而不依赖于网状蛋白。在内化后，这些膜小泡与含有依赖于笼蛋白的货物蛋白的早期内体融合。从这里，像MHCI这样的蛋白质可以被运输到晚期内体和溶酶体进行降解，或者沿着缺乏网状蛋白货物的管状内体循环回到PM。我们发现，一些脂筏相关蛋白，那些由糖基磷脂酰肌醇(GPI)部分锚定在膜上的蛋白，也通过这种不依赖于笼蛋白的机制进入细胞，并遵循一条类似于MHCI的降解或循环途径。我们发现，这些GPI锚定蛋白和MHCI的内吞作用由活性Arf6-GTP刺激，依赖于PM胆固醇，但不依赖于Dynamin，Dynamin是依赖于笼蛋白的内吞所需的GTP酶。在我们对MHCI回收到PM的研究中，我们发现了Rab GTP结合蛋白在这一过程中的复杂协调。转铁蛋白受体是一种典型的依赖于笼蛋白的货物蛋白，也可以循环使用，它的循环需要Rab11的功能。MHCI沿不同的管状结构循环需要Rab11和Rab22a的功能。Rab22a特别需要将不依赖于笼蛋白的货物重新分离到这些不同的管状内小体中，而Rab11可能在PM融合之前的较晚步骤中发挥作用。细胞分别调节依赖于和不依赖于笼蛋白的货物蛋白的内化和再循环的能力使细胞能够不同地调节这些过程。由于这种不依赖于笼蛋白的内体系统携带着影响免疫功能和细胞黏附的重要膜蛋白，如MHCI和整合素，了解这一途径是如何调控的可能在病原体侵袭和肿瘤细胞转移等疾病状态下至关重要。

The Arf6 GTPase regulates membrane traffic between the plasma membrane (PM) and an endosomal compartment and influences the dynamics of the cortical actin cytoskeleton. In many cells this endocytic pathway is the route followed by PM proteins that lack clathrin localization sequences (such as MHCI, the major histocompatibility complex class I) and hence are endocytosed into cells independently of clathrin. After internalization, these membrane vesicles fuse with early endosomes containing clathrin-dependent cargo proteins. From here, proteins such as MHCI can either be trafficked on to late endosomes and lysosomes for degradation or recycled back to the PM along tubular endosomes that lack clathrin cargo. We have found that some lipid raft-associated proteins, those anchored to the membrane by a glycosylphosphatidyl inositol (GPI) moiety, also enter cells via this clathrin-independent mechanism and follow a pathway, either to degradation or recycling, similar to MHCI. We found that endocytosis of these GPI anchored proteins and MHCI is stimulated by active Arf6-GTP, is dependent upon PM cholesterol but is not dependent upon dynamin, a GTPase required for clathrin-dependent endocytosis. During our studies on recycling of MHCI back to the PM, we have uncovered an intricate coordination of Rab GTP-binding proteins in this process. Recycling of transferrin receptor, a typical clathrin-dependent cargo protein that is also recycled, requires the function of Rab11. The recycling of MHCI along distinct tubular structures requires the function of Rab11 and Rab22a. Rab22a is specifically required for the re-segregation of clathrin-independent cargo into these distinct tubular endosomes whereas Rab11 may function at a later step prior to PM fusion. The ability of cells to regulate separately the internalization and recycling of clathrin-dependent vs. clathrin-independent cargo proteins enables cells to differentially regulate these processes. Since this clathrin-independent endosomal system carries important membrane proteins that influence immune function and cell adhesion, such as MHCI and integrins, understanding how this pathway is regulated may be critical in disease states such as pathogen invasion and tumor cell metastasis.