The signalling factor PI3K is a specific regulator of the clathrin-independent dynamin-dependent endocytosis of IL-2 receptors

Abstract

Receptor-mediated endocytosis is an essential process used by eukaryotic cells to internalise many molecules. Several clathrinindependent endocytic routes exist, but the molecular mechanism of each pathway remains to be uncovered. The present study focuses on a clathrin-independent dynamin-dependent pathway used by interleukin 2 receptors (IL-2R), essential players of the immune response. Ras-related C3 botulinum toxin substrate (Rac1) and its targets, the p21-activated kinases (Pak), are specific regulators of this pathway, acting on cortactin and actin polymerization. The present study reveals a dual and specific role of phosphatidylinositol 3-kinase (PI3K) in IL-2R endocytosis. Inhibition of the catalytic activity of PI3K strongly affects IL-2R endocytosis, in contrast to transferrin (Tf) uptake, a marker of the clathrin-mediated pathway. Moreover, Vav2, a GTPase exchange factor (GEF) induced upon PI3K activation, is specifically involved in IL-2R entry. The second action of PI3K is through its regulatory subunit, p85a, which binds to and recruits Rac1 during IL-2R internalisation. Indeed, the overexpression of a p85a mutant missing the Rac1 binding motif leads to the specific inhibition of IL-2R endocytosis. The inhibitory effect of this p85a mutant could be rescued by the overexpression of either Rac1 or the active form of Pak, indicating that p85a acts upstream of the Rac1-Pak cascade. Finally, biochemical and fluorescent microscopy techniques reveal an interaction between p85a, Rac1 and IL-2R that is enhanced by IL-2. In summary, our results indicate a key role of class I PI3K in IL-2R endocytosis that creates a link with IL-2 signalling. © 2013. Published by The Company of Biologists Ltd.