Human apo-SRP72 and SRP68/72 complex structures reveal the molecular basis of protein translocation

Abstract

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 essential role in protein translocation. However, structural information on the two largest SRP proteins, SRP68 and SRP72, is limited, especially regarding their interaction. Herein, we report the first crystal structures of human apo-SRP72 and the SRP68/72 complex at 2.91Å and 1.7Å 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 heterodimer. A 23-residue polypeptide of SRP68 is sufficient for tight binding to SRP72 through its unusually hydrophobic and extended surface. 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 interaction in SRP-mediated protein translocation and provide a structural basis for disease diagnosis, pathophysiology, and drug design.

Identification of biallelic germline variants of SRP68 in a sporadic case with severe congenital neutropenia

“… 8 SRP68 functions only as a heterodimeric structure with the SRP72 protein. 9 We could speculate that null SRP68 variants would be lethal on the basis of what was recently shown in Srp72 -/- mice. 10 …”

Crystal structures of REF6 and its complex with DNA reveal diverse recognition mechanisms

“…Constructs of Arabidopsis thaliana REF6 were subcloned into either a modified pET28a vector encoding a His6-MBP fusion tag or the pGEX-4T-2 expression vector 33,46 . Both the MBP and the GST fusion tags are removed from the REF6 constructs by a tobacco etch virus (TEV) protease cleavage site.…”

Proteomic-Based Machine Learning Analysis Reveals PYGB as a Novel Immunohistochemical Biomarker to Distinguish Inverted Urothelial Papilloma From Low-Grade Papillary Urothelial Carcinoma With Inverted Growth

“…Especially, the survival analysis of the BLCA TCGA dataset further validated SRP68 and SETD3 as potential predictive candidates for the ‘NU-like’ (low-risk) or ‘PUC-like’ (high-risk) IUP. SRP68 is a key component of SRP ribonucleoprotein complex regulating endoplasmic reticulum trafficking for protein export and tumor cell mobility ( 63 – 65 ). We showed that low SRP68 mRNA expression was significantly associated with favorable BLCA prognosis in the TCGA dataset.…”