TRG_DiLeu_BaEn_2

Adaptin-binding acidic dileucine motifs and variants thereof occur almost exclusively on the cytosolic side of membrane proteins, mostly integral (transmembrane) proteins. In the latter, they are frequently located near the protein N- or C-termini, with relative proximity (within 10-100aa) to a transmembrane segment. These motifs bind directly to a highly conserved site located on the sigma subunits of adaptin complexes (adaptins AP1-4; Doray,2007; Kelly,2008). They serve to initiate clathrin-mediated endocytosis or protein sorting and can work synergistically with the adaptin mu subunit binding YxxPhi-type motifs (TRG_ENDOCYTIC_2). Sigma subunits of AP complexes differ slightly in their surface charge densities and binding groove geometry, allowing for both generic and selective interactions with protein partners.

In multicellular animals, AP1 targets its ligands from the trans-Golgi network to the cell membrane, mainly to the basolateral surface of polarized epithelial cells or somato-dendritic compartment of neurons (Nakatsu,2014). AP2 is chiefly involved in endocytosis of cell surface proteins and their trafficking to early or late endosomes. AP3 targets its ligands to the lysosome, late endosome or melanosome (or less commonly, to the axonal compartment of neurons), while the biological function of AP4 remains mostly unknown. In fungi and plants, dileucine motifs are often responsible for the vacuolar or tonoplast localization of proteins carrying these motifs.

Due to the similarity of the adaptin sigma subunits, variant dileucine motifs may have overlapping specificities, being capable of binding multiple adaptins. In many eukaryotes, AP3 appears to be a dominant partner, that drives permanent intracellular localization of ligands it can interact with, regardless of their binding to other adaptins. Unfortunately, the similarity of this motif to the GGA-binding dileucine motifs (that also target certain proteins to the late endosome or lysosome) has been the source of considerable confusion in the past.

The name of classical dileucine motifs stems from their preferred hydrophobic amino acids, although it is somewhat of a misnomer. In addition to the idealized ExxPL[LI] sequence, a multitude of relaxed motif variations are reported to exist, many of them still poorly characterized. The degree of relaxation seems to heavily influence the targeting properties of dileucine-like motifs (Sitaram,2012). Motifs that do not satisfy the optimal consensus tend to prefer adaptins other than AP3, hence they are more likely to be trafficked to the cell surface.