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Cavin‐1/PTRF as a new substrate of the SOCS3 E3 ubiquitin ligase complex

Suppressor of cytokine signalling 3 (SOCS3) is an inducible specificity factor for a multi‐subunit E3 ubiquitin ligase complex. However the full spectrum of ubiquitylated SOCS3 substrates is unknown. By performing an unbiased SILAC‐facilitated comparison of tandem affinity purified ubiquitinomes from WT and SOCS3‐null fibroblasts, we have identified cavin‐1/PTRF as a protein whose ubiquitylation status is significantly enhanced in SOCS3‐expressing versus SOCS3‐null cells. Importantly, studies of endogenous cavin‐1 expressed in adipocytes and fibroblasts have shown it to be ubiquitylated. Co‐immunoprecipitations and overlapping peptide array overlays confirmed that SOCS3 interacted directly with two distinct regions within cavin‐1 in an interaction that did not require tyrosine phosphorylation. Cavin‐1 ubiquitylation was also found to be enhanced in the presence of SOCS3 and its cognate Cul‐Rbx‐elongin B/C E3 ligase scaffold. In summary, our data suggest a potentially novel role for SOCS3 in regulating caveolae assembly via cavin‐1 ubiquitylation. This provides a novel mechanism by which SOCS3‐inducing stimuli may influence caveolar architecture and function.