Protein Interaction Screening for the Ankyrin Repeats and Suppressor of Cytokine Signaling (SOCS) Box (ASB) Family Identify Asb11 as a Novel Endoplasmic Reticulum Resident Ubiquitin Ligase

The Ankyrin and SOCS (Suppressor of Cytokine Signaling) box (ASB) family of proteins function as the substrate recognition subunit in a subset of Elongin-Cullin-SOCS (ECS) E3 ubiquitin ligases. Despite counting with 18 members in humans, the identity of the physiological targets of the Asb proteins remain largely unexplored. To increase our understanding of the ASB proteins function, we conducted a family-wide SILAC (Stable Isotope Labeling by Amino acids in Cell Culture)-based protein-protein interaction analysis. This investigation led to the identification of novel as well as known ASB associated proteins like Cullin 5, and Elongins B/C. We observed that several proteins can be bound by more than one Asb protein. The additional exploration of this phenomena demonstrated that ASB/Cullin 5 complexes can oligomerize and provide evidence that Cullin 5 forms heterodimeric complexes with the Cullin 4a/DDB1 complex. We also demonstrated that ASB11 is a novel ER associated ubiquitin ligase with the ability to interact and promote the ubiquitination of Ribophorin 1, an integral protein of the OST glycosylation complex. Moreover, expression of ASB11 can increase Ribophorin 1 protein turnover in vivo. In summary, we provide a comprehensive protein-protein interaction data resource that can aid the biological and functional characterization of ASB ubiquitin ligases.