Site-specific ubiquitination exposes a linear motif to promote interferon-α receptor endocytosis

Ligand- induced endocytosis and lysosomal degradation of cognate receptors regulate the extent of cell signaling. Along with linear endocytic motifs that recruit the adaptin protein complex 2 ( AP2) - clathrin molecules, monoubiquitination of receptors has emerged as a major endocytic signal. By investigating ubiquitin- dependent lysosomal degradation of the interferon ( IFN)- alpha/beta receptor 1 ( IFNAR1) subunit of the type I IFN receptor, we reveal that IFNAR1 is polyubiquitinated via both Lys48- and Lys63-linked chains. The SCF beta Trcp ( Skp1 - Cullin1 - F- box complex) E3 ubiquitin ligase that mediates IFNAR1 ubiquitination and degradation in cells can conjugate both types of chains in vitro. Although either polyubiquitin linkage suffices for postinternalization sorting, both types of chains are necessary but not sufficient for robust IFNAR1 turnover and internalization. These processes also depend on the proximity of ubiquitin- acceptor lysines to a linear endocytic motif and on its integrity. Furthermore, ubiquitination of IFNAR1 promotes its interaction with the AP2 adaptin complex that is required for the robust internalization of IFNAR1, implicating cooperation between site- specific ubiquitination and the linear endocytic motif in regulating this process.