Recycling of furin from the plasma membrane - Functional importance of thecytoplasmic tail sorting signals and interaction with the AP-2 adaptor medium chain subunit

The predominant intracellular localization of the eukaryotic subtilisin-lik e endoprotease furin is the trans-Golgi network (TGN), but a small fraction is also found on the cell surface. Furin on the cell surface is internaliz ed and delivered to the TGN, The identification of three endocytosis motifs , a tyrosine (YKGL(765)) motif, a leucine-isoleucine (LI760) motif, and a p henylalanine (Phe(790)) signal, in the furin cytoplasmic domain suggested t hat endocytosis of furin occurs via an AP-2/clathrin-dependent pathway. Sin ce little is known about proteins containing multiple sorting components in their cytoplasmic domain, the combination of diverse internalization signa ls in the furin tail raised the question of their individual role, Here we present data showing that the furin tail interacts with the medium (mu 2) s ubunit of the AP-2 plasma membrane specific adaptor complex in vitro and th at this interaction primarily depends on recognition of the tyrosine-based sorting signal and to less extent on the leucine-isoleucine motif, We furth er provide evidence that the three endocytosis signals are of different fun ctional importance for furin internalization and retrieval to the TGN in vi vo, with the tyrosine-based motif being the major determinant, followed by the phenylalanine signal, whereas the leucine-isoleucine motif is only a mi nor component, Finally, we report that phosphorylation of the furin tail by casein kinase II is not only important for efficient interaction with mu 2 and internalization from the plasma membrane but also determines fast retr ieval of the protein from the plasma membrane to the TGN.