No abstract

Inducible degrader of the low-density lipoprotein receptor (IDOL) is an E3 ubiquitin ligase mediating degradation of low-density lipoprotein (LDL) receptor (LDLR). IDOL also controls its own stability through autoubiquitination, primarily at lysine 293. Whether IDOL may undergo other forms of post-translational modification is unknown. In this study, we show that IDOL can be modified by small ubiquitin-like modifier 1 (SUMO1) at the K293 residue at least. The SUMOylation of IDOL counteracts its ubiquitination and augments IDOL protein levels. SUMOylation and the associated increase of IDOL protein are effectively reversed by SUMO specific peptidase 1 (SENP1) in an activity-dependent manner. We further demonstrate that SENP1 affects LDLR protein levels by modulating IDOL. Overexpression of SENP1 increases LDLR protein levels and enhances LDL uptake in cultured cells. On the contrary, loss of SENP1 lowers LDLR levels in an IDOL-dependent manner and reduces LDL endocytosis. Collectively, our results reveal SUMOylation as a new regulatory post-translational modification of IDOL, and suggest that SENP1 positively regulates the LDLR pathway via deSUMOylation of IDOL and may therefore be exploited for the treatment of cardiovascular disease.

No abstract

Hedgehog (Hh) signaling pathway plays a pivotal role in embryonic development. Hh binding to Patched1 (PTCH1) derepresses Smoothened (SMO), thereby activating the downstream signal transduction. Covalent SMO modification by cholesterol in its cysteine‐rich domain (CRD) is essential for SMO function. SMO cholesterylation is a calcium‐accelerated autoprocessing reaction, and STIM1‐ORAI1‐mediated store‐operated calcium entry promotes cholesterylation and activation of endosome‐localized SMO. However, it is unknown whether the Hh‐PTCH1 interplay regulates the activity of the endoplasmic reticulum (ER)‐localized SMO. Here, we found that PTCH1 inhibited the COPII‐dependent export of SMO from the ER, whereas Hh promoted this process. The RRxWxR amino acid motif in the cytosolic tail of SMO was essential for COPII recognition, ciliary localization, and signal transduction activity. Hh and PTCH1 regulated cholesterol modification of the ER‐localized SMO, and SMO cholesterylation accelerated its exit from ER. The GRAMD1/ASTER sterol transport proteins facilitated cholesterol transfer to ER from PM, resulting in increased SMO cholesterylation and enhanced Hh signaling. Collectively, we reveal a regulatory role of GRAMD‐mediated cholesterol transport in ER‐resident SMO maturation and Hh signaling.