The N terminus of microsomal Delta 9 stearoyl-CoA desaturase contains the sequence determinant for its rapid degradation

Stearoyl-CoA desaturase (SCD) is a key regulator of membrane fluidity, turn s over rapidly, and represents a model for selective degradation of short-l ived proteins of the endoplasmic: reticulum (ER), The mechanism whereby spe cific ER proteins are targeted for degradation in the midst of stable prote ins coexisting in the same membrane is unknown. To investigate the intracel lular fate of SCD and to identify the determinants involved in the rapid tu rnover of SCD, we created chimeras of SCD tagged at the C terminus with the green fluorescent protein (GFP). The fusion proteins were expressed in Chi nese hamster ovary cells and exhibited an ER localization. Unlike native GF P, the SCD-GFP construct was unstable and had a half life of a few hours. T runcated fusion proteins consisting of residues 27-358 and 45-358 of SCD li nked to the N terminus of GFP were stable. To investigate the general appli cability of the N terminus of SCD in the destabilization of proteins, we fu sed residues 1-33 of SCD to the N terminus of GFP, The resulting chimera wa s extremely short lived. To investigate the effect of membrane sidedness on the fusion protein degradation, we attached a lumenal targeting signal to the N terminus of SCD 1-33-GFP, The construct was localized to the lumen of ER and was metabolically stable, indicating that SCD degradation signal fu nctions on the cytosolic rather than the lumenal side of the ER, These resu lts demonstrate that the N-terminal segment of some 30 residues of SCD cons titutes a motif responsible for the rapid degradation of SCD.