A Specific Dileucine Motif Is Required for the GGA-dependent Entry of Newly Synthesized Insulin-responsive Aminopeptidase into the Insulin-responsive Compartment

Abstract

In muscle and adipose cells, the insulin-responsive aminopeptidase (IRAP) is localized to intracellular storage sites and undergoes insulin-dependent redistribution to the cell surface. Following expression, the newly synthesized IRAP protein traffics to the perinuclear insulin-sensitive compartment and acquires insulin sensitivity 6-9 h following biosynthesis. Knockdown of GGA1 by RNA interference prevented IRAP from entering, but not exiting, the insulin-responsive compartment. Mutation of the dileucine motif at positions 76 and 77 (EGFP-IRAP/AA(76,77)), but not the dileucine motif at positions 53 and 54, resulted in the rapid default of the reporter to the cell surface beginning at 3 h following biosynthesis. Alanine substitution of 9 residues amino- or carboxyl-terminal to LL(76,77) did not perturb basal intracellular sequestration or abrogate insulin-stimulated IRAP translocation. Moreover, a dominant interfering GGA mutant (VHS-GAT) potently inhibited insulin-stimulated translocation of EGFP-IRAP/WT but did not block the constitutive exocytotic trafficking of EGFP-IRAP/AA(76,77). In addition, the EGFP-IRAP/WT and EGFP-IRAP/AA(76,77) constructs occupied morphologically distinct tubulovesicular compartments in the perinuclear region. Taken together, these data indicate that LL(76,77) functions during the GGA-dependent sorting of newly made IRAP into the insulin-responsive storage compartment.