Rab4 and Rab11 coordinately regulate the recycling of angiotensin II type I receptor as demonstrated by fluorescence resonance energy transfer microscopy.

The recycling of G-protein-coupled receptors (GPCR) to the cell surface after internalization plays an important role in the regulation of overall GPCR activity. The angiotensin II type I receptor (AT1R) belongs to class B GPCRs that recycle slowly back to the cell surface. Previous studies have proposed that Rab11 controls the recycling of AT1R; however, recent reports show that Rab4, a rapid recycling regulator, co-localizes also with internalized AT1R. Different from the subcellular co-localization provided by fluorescence microscopy, fluorescence resonance energy transfer (FRET) microscopy provided the spatial relationship of AT1R with Rab4 and Rab11 in the nanometer-range proximity during the entire course of AT1R recycling. During the early recycling stage, internalized AT1Rs were mainly associated with Rab4 in the cytoplasm. During the mid-recycling stage, AT1Rs were associated with both Rab4 and Rab11 in the perinuclear compartments. However, during the late-recycling stage, AT1Rs were mainly associated with Rab11, both in the perinuclear compartments and the plasma membrane. Co-immunoprecipitation data confirmed these dynamic associations, which were disrupted by silencing of either the Rab4 or Rab11 gene. Based on these observations, we propose a Rab4 and Rab11 coordinated model for AT1R recycling.