ITSN1 and Ruk/CIN85 colocalized to clathrin-coated pits in MCF-7 cells

Ubiquitin-ligase AIP4 controls differential ubiquitination and stability of isoforms of the scaffold protein ITSN1

At present, the role of ubiquitination of cargoes internalized from the plasma membrane is better understood than the consequences of ubiquitination of proteins comprising the endocytic machinery. Here, we show that the E3 ubiquitin ligase AIP4/ITCH contributes to the differential ubiquitination of isoforms of the endocytic scaffold protein intersectin1 (ITSN1). The major isoform ITSN1-s is monoubiquitinated, whereas the minor one, ITSN1-22a undergoes a combination of mono- and oligoubiquitination. The monoubiquitination is required for ITSN1-s stability, whereas the oligoubiquitination of ITSN1-22a causes its proteasomal degradation. This explains the observed low abundance of the minor isoform in cells. Thus, different modes of ubiquitination regulated by AIP4 have opposite effects on ITSN1 isoform stability.

Intersectin 1 forms complexes with SGIP1 and Reps1 in clathrin-coated pits

Intersectin 1 (ITSN1) is an evolutionarily conserved adaptor protein involved in clathrin-mediated endocytosis, cellular signaling and cytoskeleton rearrangement. ITSN1 gene is located on human chromosome 21 in Down syndrome critical region. Several studies confirmed role of ITSN1 in Down syndrome phenotype. Here we report the identification of novel interconnections in the interaction network of this endocytic adaptor. We show that the membrane-deforming protein SGIP1 (Src homology 3-domain growth factor receptor-bound 2-like (endophilin) interacting protein 1) and the signaling adaptor Reps1 (RalBP associated Eps15-homology domain protein) interact with ITSN1 in vivo. Both interactions are mediated by the SH3 domains of ITSN1 and proline-rich motifs of protein partners. Moreover complexes comprising SGIP1, Reps1 and ITSN1 have been identified. We also identified new interactions between SGIP1, Reps1 and the BAR (Bin/amphiphysin/Rvs) domain-containing protein amphiphysin 1. Immunofluorescent data have demonstrated colocalization of ITSN1 with the newly identified protein partners in clathrin-coated pits. These findings expand the role of ITSN1 as a scaffolding molecule bringing together components of endocytic complexes.