摘要
The co-translational targeting or insertion of secretory and membrane proteins into the endoplasmic reticulum （ER） is a key biological process mediated by the signal recognition particle （SRP）. In eukaryotes, the SRP68-SRP72 （SRP68/72） heterodimer plays an essen- tial role in protein translocation. However, structural information on the two largest SRP proteins, SRP68 and SRP72, is limited, espe- cially regarding their interaction. Herein, we report the first crystal structures of human apo-SRP72 and the SRP68/72 complex at 2.91A. and 1.7A resolution, respectively. The SRP68-binding domain of SRP72 contains four atypical tetratricopeptide repeats （TPR） and a flexible C-terminal cap. Apo-SRP72 exists mainly as dimers in solution. To bind to SRP68, the SRP72 homodimer disassociates, and the indispensable C-terminal cap undergoes a pronounced conformational change to assist formation of the SRP68/72 heterodi- mer. A 23-residue polypeptide of SRP68 is sufficient for tight binding to SRP72 through its unusually hydrophobic and extended sur- face. Structural, biophysical, and mutagenesis analyses revealed that cancer-associated mutations disrupt the SRP68-SRP72 interaction and their co-localization with ER in mammalian cells. The results highlight the essential role of the SRP68-SRP72 inter- action in SRP-mediated protein translocation and provide a structural basis for disease diagnosis, pathophysiology, and drug design.
