Distinct reading of different structural determinants modulates the dileucine-mediated transport steps of the lysosomal membrane protein LIMPII and the insulin-sensitive glucose transporter GLUT4

Leucine-based motifs mediate the sorting of membrane proteins at such cellu lar sites as the trans-Golgi network, endosomes, and plasma membrane. A Leu paired with a second Leu, He, or Met, while itself lacking the ability to mediate transport, is the key structural feature in these motifs, Here we h ave studied the structural differences between the leucine-based motifs con tained in the COOH tails of LIMPII and GLUT4, two membrane proteins that ar e transported through the secretory pathway and are targeted to lysosomes ( 1-3) and to a perinuclear compartment adjacent to the Golgi complex (4), re spectively. LIMPII and GLUT4 display negatively (Asp(470)/Glu(471)) and pos itively (Arg(484)/Ar-485) charged residues, respectively, at positions -4 a nd -5 upstream from the critical Leu residue. The change in the charge sign of residues -4 and -5 results in missorting of LIMPII and GLUT4. We note t hat the acidic Glu residue at position -4 is critical for efficient intrace llular sorting of LIMPII to lysosomes, but is dispensable for its surface i nternalization by endocytosis. Efficient intracellular sorting and endocyto sis of GLUT4 require an Arg pair between positions -4 and -7. These results are consistent with the existence of distinct leucine-based motifs and pro vide evidence of their different readings at different cellular sites.