The receptor internalization and degradation (RID) complex of adenovirus plays an important role in modulating the immune response by downregulating the surface levels of tumour necrosis factor receptor 1 (TNFR1), thereby inhibiting NF-kB activation. Total cellular content of TNFR1 is also reduced in the presence of RID, which can be inhibited by treatment with lysosomotropic agents. In this report, surface biotinylation experiments revealed that, although RID and TNFR1 were able to form a complex on the cell surface, the rate of TNFR1 endocytosis was not affected by RID. However, the degradation of internalized TNFR1 was enhanced significantly in the presence of RID. Therefore, these data suggest that RID downregulates TNFR1 levels by altering the fate of internalized TNFR1 that becomes associated with RID at the plasma membrane, probably by promoting its sorting into endosomal/lysosomal degradation compartments.The successful replication and survival of viruses in the host requires evasion of the immune system. Many of the immunomodulatory genes of adenovirus (Ad) are encoded in early region 3 (E3) (Fessler et al., 2004b;Horwitz, 2004). The E3 receptor internalization and degradation (RID) complex, composed of two RIDa and one RIDb subunits, downregulates a specific set of plasma-membrane receptors, including FAS (Shisler et al., 1997;Elsing & Burgert, 1998;Tollefson et al., 1998), tumour necrosis factor (TNF)-related apoptosis-inducing ligand receptor 1 (TRAIL-R1) (Benedict et al., 2001;Tollefson et al., 2001) and epidermal growth factor receptor (EGFR) (Carlin et al., 1989;Tollefson et al., 1991). Together with another E3 protein, E3/6.7K, RID also reduces the surface expression of TRAIL-R2 (Lichtenstein et al., 2004). Whilst both of the RID subunits are critical for the downregulation of FAS and TRAIL receptors, it appears that the RIDa subunit is sufficient to downregulate EGFR under certain experimental conditions (Hoffman et al., 1990). Recently, we demonstrated that RID also downregulates TNFR1, thereby inhibiting TNF-induced NF-kB activation, which mediates the transcription of a large number of genes involved in inflammation and immune responses such as chemokine expression (Fessler et al., 2004a;Delgado-Lopez & Horwitz, 2006).In vivo experiments have shown that RID is an essential component of the Ad E3 cassette, which facilitates transplantation of allogeneic pancreatic cells (Efrat et al., 1995) and decreases autoimmune type 1 diabetes incidence (von Herrath et al., 1997;Efrat et al., 2001;Pierce et al., 2003). To potentially use RID as a therapeutic immunomodulator, it is important to elucidate the molecular mechanism of RIDmediated downregulation of receptors. We recently showed that the tyrosine sorting motifs of RIDb and clathrin play important roles in the downregulation of TNFR1, and that RID-mediated TNFR1 degradation occurs via an endosomal/ lysosomal pathway (Chin & Horwitz, 2005). In this report, we further examined the effect of RID on TNFR1 trafficking. Specifically, we tested the abili...