Novel regulation of the eEF2K/eEF2 pathway: prospects of ‘PQBP1 promotes translational elongation and regulates hippocampal mGluR-LTD by suppressing eEF2 phosphorylation’

Abstract

Protein synthesis involves initiation, elongation, and termination. A number of studies show that translation elongation is highly regulated and is the most energy-intensive process, which controls the efficiency and accuracy of protein synthesis. In the elongation process, eukaryotic elongation factor 2 (eEF2) binds to the ribosome to induce its conformation change and promotes the translocation of the tRNAs by hydrolyzing guanosine triphosphate, which then allows the next AA-tRNA to enter the A site for translation elongation. The activity of eEF2 is negatively regulated by its kinase eEF2K that phosphorylates eEF2 at threonine-56 (Thr56). Dysregulation of elongation is the core of the pathogenesis of tumorigenesis and neurodegeneration. Thus, eEF2/eEF2K as a potential therapeutic target has attracted more and more attention (Knight et al., 2020). In a recently published work, we identified that the intellectual disability related protein, polyglutamine-binding protein 1 (PQBP1), is a novel regulator that binds directly with eEF2 and affects its phosphorylation (Figure 1; Shen et al., 2021). Renpenning syndrome is a group of Xlinked intellectual disability disease (XLID) caused by mutations in the human PQBP1 gene (Kalscheuer et al., 2003; Stevenson et al., 2005). PQBP protein is mainly located in the nucleus and is generally believed to play an important role in transcription and mRNA splicing (Okazawa, 2018). However, previous studies also show that the cytoplasmic PQBP1 associates with and regulates the translation of specific mRNAs (Kunde et al., 2011; Wan et al., 2015). Our study revealed that the cytoplasmic PQBP1 directly binds to nonphosphorylated eEF2 at a linear peptide around Thr56 through its WW domain, protecting eEF2 from being phosphorylated by eEF2K, thus promoting global protein synthesis (Figure 1; Shen et al., 2021). We elucidate the detailed mechanism of how PQBP1 is involved in protein synthesis and demonstrate that individual domains of PQBP1 have distinct molecular functions in different cellular compartments. This function of PQBP1 in translation may be essential for cells as it is highly conserved. Drosophila homolog dPQBP1 is mainly in the cytoplasm and also associates with the ribosomes through the conserved WW domain (Wan et al., 2015). Although common clinical manifestations are present in all patients with PQBP1 mutations, various PQBP1 mutations cause notable variations in the symptoms exhibited by these patients (Stevenson et al., 2005). PQBP1-Y65C mutation from Golabi-Ito-Hall (GIH) family is the first missense mutation found in patients and the only one mutation in the WW domain so far (Lubs et al., 2006). It is suggested that Cys65 residue may be easy to form an intramolecular disulfide bond with Cys60 residue, which reduces the binding of PQBP1 to WBP11/SIPP1 and affects the cleavage of pre-mRNA (Tapia et al., 2010; Sudol et al., 2012). Our data show that the Y65C mutation enhanced its binding ability to eEF2, suggesting that PQBP1Y65C missense mutation may have some acquired functions. We have generated a Pqbp1 mouse model for further investigation. Our findings and future work will provide insights into the heterogeneity of Renpenning syndrome. The WW domain, similar to the SH3 domain, specifically recognizes prolinerich ligands and has been found in many signaling proteins. Espanel and Sudol (1999) classified WW domains based on their proline-based ligand specificity. In that scheme, PQBP1 belongs to the Group II WW domain that preferentially binds to PPLP motifs. One of the interesting findings in our study is that the linear peptide around eEF2 Thr56 interacting with the PQBP1 WW domain is proline-free. In addition, its interaction with the WW domain is affected by the phosphorylation status at Thr56. It is worth characterizing the exact binding motif in eEF2, which will expand our