LEUCINE-BASED RECEPTOR SORTING MOTIFS ARE DEPENDENT ON THE SPACING RELATIVE TO THE PLASMA-MEMBRANE

Many integral membrane proteins contain leucine-based motifs within th eir cytoplasmic domains that mediate internalization and intracellular sorting. Two types of leucine-based motifs have been identified. One type is dependent on phosphorylation, whereas the other type, which in cludes an acidic amino acid, is constitutively active. In this study, we have investigated how the spacing relative to the plasma membrane a ffects the function of both types of leucine-based motifs, For phospho rylation-dependent leucine-based motifs, a minimal spacing of 7 residu es between the plasma membrane and the phospho-acceptor was required f or phosphorylation and thereby activation of the motifs. For constitut ively active leucine-based motifs, a minimal spacing of 6 residues bet ween the plasma membrane and the acidic residue was required for optim al activity of the motifs. In addition, we found that the acidic resid ue of leucine-based motifs must be located amino-terminal to the dileu cine sequence for proper function of the motifs and that residues surr ounding the motifs affect the activity of the motifs, Thus, our observ ations suggest that the position, the exact sequence, and surrounding residues are major determinants of the function of leucine-based recep tor sorting motifs.