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Initial entry of IRAP into the insulin-responsive storage compartment occurs prior to basal or insulin-stimulated plasma membrane recycling

¹Department of Pediatric and Adolescent Medicine and Department of Biochemistry and Molecular Biology, Mayo Clinic College of Medicine, Rochester, Minnesota; and ²Department of Pharmacological Sciences, Stony Brook University, Stony Brook, New York

Submitted 22 April 2005 ; accepted in final form 25 May 2005

To examine the acquisition of insulin sensitivity after the initial biosynthesis of the insulin-responsive aminopeptidase (IRAP), 3T3-L1 adipocytes were transfected with an enhanced green fluorescent protein-IRAP (EGFP-IRAP) fusion protein. In the absence of insulin, IRAP was rapidly localized (1–3 h) to secretory membranes and retained in these intracellular membrane compartments with little accumulation at the plasma membrane. However, insulin was unable to induce translocation to the plasma membrane until 6–9 h after biosynthesis. This was in marked contrast to another type II membrane protein (syntaxin 3) that rapidly defaulted to the plasma membrane 3 h after expression. In parallel with the time-dependent acquisition of insulin responsiveness, the newly synthesized IRAP protein converted from a brefeldin A-sensitive to a brefeldin A-insensitive state. The initial trafficking of IRAP to the insulin-responsive compartment was independent of plasma membrane endocytosis, as expression of a dominant-interfering dynamin mutant (Dyn/K44A) inhibited transferrin receptor endocytosis but had no effect on the insulin-stimulated translocation of the newly synthesized IRAP protein.

insulin-responsive aminopeptidase; trafficking; insulin; cargo selection; biosynthesis

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