VEGFR2 plays a fundamental role in blood vessel formation and in life threatening diseases, such as cancer angiogenesis and cardiovascular disorders. Although inactive growth factor receptors are mainly localized at the plasma membrane, VEGFR2 undergoes constitutive endocytosis (in the absence of ligand) and recycling. Intriguingly, the significance of these futile transport cycles of VEGFR2 remains unclear. Here we found that, unexpectedly, the function of constitutive endocytosis of VEGFR2 is to protect the receptor against plasma membrane cleavage (shedding), thereby preserving the functional state of the receptor until the time of activation by VEGF. Inhibition of constitutive endocytosis of VEGFR2, by interference with the function of clathrin, dynamin, or Rab5, increases dramatically the cleavage/shedding of VEGFR2. Shedding of VEGFR2 produces an N-terminal soluble fragment (100 kDa, s100), which is released in the extracellular space, and a residual C-terminal part (130 kDa, p130) that remains integrated at the plasma membrane. The released soluble fragment (s100) co-immunoprecipitates with VEGF, in line with the topology of the VEGF-binding domain at the N terminus of VEGFR2. Increased shedding of VEGFR2 (via inhibition of constitutive endocytosis) results in reduced response to VEGF, consistently with the loss of the VEGF-binding domain from the membrane remnant of VEGFR2. These data suggest that constitutive internalization of VEGFR2 protects the receptor against shedding and provides evidence for an unprecedented mechanism via which endocytosis can regulate the fate and activity of growth factor receptors.Growth factor receptors are synthesized and processed in the ER/Golgi biosynthetic network. Subsequently, they are delivered to the plasma membrane to become available for binding to their extracellular cognate ligands. In principle, in the absence of ligand, quiescent receptors are preferentially localized at the plasma membrane, whereas only a small percentage of the surface molecules are constitutively internalized (1-3). On the other hand, ligand binding and subsequent receptor activation increases dramatically the efficiency of receptor internalization (3, 4). Internalized receptors explore the different endocytic itineraries and the diverse endocytic compartments to finetune the intensity, nature, and duration of the signaling events (2, 4, 5). Subsequently, growth factor receptors are either recycled back to plasma membrane, for another round of activation, or they are sorted to lysosomes or the proteasome for degradation, leading to irreversible termination of the receptor function (6). In fact, in general, the degradative role of endocytosis is considered to be one of the major housekeeping functions of the cell that eventually down-regulates the levels of the internalized, extracellular or plasma membrane, molecules (7-9).VEGFR2 is a receptor-tyrosine kinase that plays a critical role in blood vessel formation and pathological angiogenesis (10,11). Similarly to other receptor-tyrosine kinas...