The poly(A) tail plays an important role in maintaining mRNA stability and influences translation efficiency via binding with PABP. However, the impact of poly(A) tail length on mRNA translation remains incompletely understood. This study explores the effects of poly(A) tail length on human translation. We determined the translation rates in cell lysates using mRNAs with different poly(A) tails. Cap-dependent translation was stimulated by the poly(A) tail, however, it was largely independent of poly(A) tail length, with an exception observed in the case of the 75 nt poly(A) tail. Conversely, cap-independent translation displayed a positive correlation with poly(A) tail length. Examination of translation stages uncovered the dependence of initiation and termination on the presence of the poly(A) tail, but the efficiency of initiation remained unaffected by poly(A) tail extension. Further study unveiled that increased binding of eRFs to the ribosome with the poly(A) tail extension induced more efficient hydrolysis of peptidyl-tRNA. Building upon these findings, we propose a crucial role for the 75 nt poly(A) tail in orchestrating the formation of a double closed-loop mRNA structure within human cells which couples the initiation and termination phases of translation.

Eukaryotic translation initiation factor 4F (eIF4F), comprising subunits eIF4G, eIF4E, and eIF4A, plays a pivotal role in the 48S preinitiation complex assembly and ribosomal scanning. Additionally, eIF4B enhances the helicase activity of eIF4A. eIF4F also interacts with poly (A)-binding protein (PABP) bound to the poly (A) tail of messenger RNA (mRNA), thereby forming a closed-loop structure. PABP, in turn, interacts with eukaryotic release factor 3 (eRF3), stimulating translation termination. Here, we employed a reconstituted mammalian system to directly demonstrate that eIF4F potently enhances translation termination. Specifically, eIF4A and eIF4B promote the loading of eRF1 into the A site of the ribosome, while eIF4G1 stimulates the GTPase activity of eRF3 and facilitates the dissociation of release factors following peptide release. We also identified MIF4G as the minimal domain required for this activity and showed that eIF4G2/DAP5 can also promote termination. Our findings provide compelling evidence that the closed-loop mRNA structure facilitates translation termination, with PABP and eIF4F directly involved in this process.

RACK1 (Receptor for Activated C Kinase 1) is a highly conserved scaffold protein that functions as a central integrator within diverse cellular signaling pathways. Initially identified as a receptor for activated Protein Kinase C, it is now recognized as a dynamic platform coordinating processes such as cell proliferation, migration, apoptosis, and immune responses. A defining feature of RACK1 is its ability to direct cellular fate by determining whether proteins are synthesized or degraded. However, a unified model explaining this functional pleiotropy has been lacking. In this review, we synthesize current knowledge to propose an integrative model centered on a functional dimorphism driven by RACK1’s localization and post-translational modifications. We posit that RACK1 operates in two primary, mutually exclusive states: a ribosome-associated monomer that supports the translation of specific mRNAs and quality control, and a free monomer or dimer that governs signaling cascades and gene expression. Phosphorylation at key sites, such as Thr50 and Ser146, acts as a molecular switch, spatiotemporally redistributing RACK1 between these pools. This mechanism allows the cell to rapidly reprogram its proteomic landscape in response to stimuli, pivoting between protein synthesis and stress adaptation. Our model resolves the apparent dichotomy of RACK1’s roles by framing it as a cellular “resource manager,” whose regulated switching between functional states ensures an optimal response to the extracellular environment, with significant implications for understanding cancer and neurodegenerative diseases.