Leucine-based receptor sorting motifs are dependent on the spacing relative to the plasma membrane

Many integral membrane proteins contain leucine-based motifs within their 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 includes an acidic amino acid, is constitutively active. In this study, we have investigated how the spacing relative to the plasma membrane affects the function of both types of leucine-based motifs. For phosphorylation-dependent leucine-based motifs, a minimal spacing of 7 residues between the plasma membrane and the phospho-acceptor was required for phosphorylation and thereby activation of the motifs. For constitutively active leucine-based motifs, a minimal spacing of 6 residues between the plasma membrane and the acidic residue was required for optimal activity of the motifs. In addition, we found that the acidic residue of leucine-based motifs must be located amino-terminal to the dileucine sequence for proper function of the motifs and that residues surrounding the motifs affect the activity of the motifs. Thus, our observations suggest that the position, the exact sequence, and surrounding residues are major determinants of the function of leucine-based receptor sorting motifs.