Targeting of precursor polypeptides to the human ER

Various pathways can target nascent or fully synthesized precursor polypeptides to the human endoplasmic reticulum (ER). Typically, they involve cytosolic proteins or complexes and their respective receptors on the ER surface. The signal recognition particle (SRP) and the heterodimeric SRP-receptor (SR) represent one such targeting system, others are TRC40 and its heterodimeric TRC-receptor (WRB/CAML) and the components of the SND pathway. Apparently, they all can target precursor polypeptides to the Sec61-channel in the ER membrane. To characterize the substrate specificities of these targeting pathways, we combined siRNA-mediated depletion of membrane receptor subunits in HeLa cells with label-free quantitative proteomics and differential protein abundance analyis.