Re-routing of the invariant chain to the direct sorting pathway by introduction of an AP3-binding motif from LIMP II.

AP3 is a heteromeric adaptor protein complex involved in the biogenesis of late endosomal/lysosomal structures. It recognizes tyrosine- and leucine-based sorting signals present in the cytoplasmic tails or loops of a number of proteins and is thought to be responsible for the direct transport of these proteins from the Golgi network to late endosomal/lysosomal structures. We have previously reported (Rodionov, Höning, Silye, Kongsvik, von Figura, Bakke, 2002.

Structural requirements for interactions between leucine-sorting signals and clathrin-associated adaptor protein complex AP3.

Cytoplasmic tails of LIMPII and the invariant chain contain similar leucine-based sorting signals, but the invariant chain interacts only with AP1 and AP2, whereas LIMPII interacts strongly with AP3. In a series of in vitro experiments, we investigated the effect of residues upstream of the leucine pairs and demonstrated that these residues determine adapter binding, and certain residues favor interactions with AP3. Furthermore, constructs that interacted stronger with AP3 interacted weakly with AP1 and vice versa.

Mechanism of interaction between leucine-based sorting signals from the invariant chain and clathrin-associated adaptor protein complexes AP1 and AP2.

The cytoplasmic tail of the invariant chain contains two leucine-based sorting signals, and each of those seems sufficient to route the invariant chain to its intracellular destination in either normal or polarized cells. It is believed that the intracellular routing of the invariant chain is mediated by its interactions with the clathrin-associated adaptor protein complexes AP1 and AP2. We () have previously demonstrated the in vitro interactions between the cytoplasmic tail of the invariant chain and AP1/AP2 complexes.