How human FGFR1 localizes to the PM is unknown. Currently, it is assumed that newly synthesized FGFR1 is continuously delivered to the PM. However, evidence indicates that FGFR1 is mostly sequestered in intracellular post-Golgi vesicles (PGVs) under normal conditions. In this report, live-cell imaging and total internal reflection fluorescence microscopy (TIRFM) were employed to study the dynamics of these FGFR1-positive vesicles. We designed recombinant proteins to target different transport components to and from the FGFR1 vesicles. Mouse embryoid bodies (mEBs) were used as a 3D model system to confirm major findings. Briefly, we found that Rab2a, Rab6a, Rab8a, RalA and caveolins are integral components of FGFR1-positive vesicles, representing a novel compartment. While intracellular sequestration prevented FGFR1 activation, serum starvation and hypoxia stimulated PM localization of FGFR1. Under these conditions, FGFR1 C-terminus acts as a scaffold to assemble proteins to (i) inactivate Rab2a and release sequestration, and (ii) assemble Rab6a for localized activation of Rab8a and RalA-exocyst to deliver the receptor to the PM. This novel pathway is named Regulated Anterograde RTK Transport (RART). This is the first instance of RTK regulated through control of PM delivery.
人类成纤维细胞生长因子受体1（FGFR1）如何定位至质膜（PM）的机制尚不明确。目前普遍认为新合成的FGFR1会持续运输至质膜。然而，有证据表明在正常生理条件下，FGFR1主要被隔离在高尔基体后囊泡（PGVs）中。本研究采用活细胞成像与全内反射荧光显微镜（TIRFM）技术，系统观测了这些FGFR1阳性囊泡的动态行为。我们设计重组蛋白靶向FGFR1囊泡运输过程中的不同组分，并利用小鼠胚胎体（mEBs）作为三维模型系统验证主要发现。研究结果表明：Rab2a、Rab6a、Rab8a、RalA和小窝蛋白是FGFR1阳性囊泡的核心组成元件，共同构成一种新型细胞区室。虽然细胞内隔离机制抑制了FGFR1的激活，但血清饥饿和缺氧条件能显著促进FGFR1向质膜的定位。在此过程中，FGFR1的C末端发挥支架作用，通过双重机制调控受体运输：（1）使Rab2a失活从而解除囊泡隔离；（2）募集Rab6a实现Rab8a与RalA-外囊泡复合体的局部激活，最终将受体递送至质膜。该新发现的运输途径被命名为"受调控的受体酪氨酸激酶顺向运输"（RART）。本研究首次揭示了通过质膜递送调控实现受体酪氨酸激酶功能调节的新范式。
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