Human tumor suppressor protein Pdcd4 binds at the mRNA entry channel in the 40S small ribosomal subunit

Area of Science:

• Molecular Biology
• Structural Biology
• Cancer Research

Background:

• Translation initiation is a key regulatory step in gene expression, frequently dysregulated in human diseases.
• Programmed cell death gene 4 (Pdcd4) is a tumor suppressor protein that inhibits cell growth, invasion, and metastasis.
• Pdcd4 is downregulated in tumors, correlating with increased global translation.

Purpose of the Study:

• To elucidate the structural mechanisms by which Pdcd4 regulates translational initiation.
• To understand Pdcd4's interaction with the 40S ribosomal subunit and translation factors.

Main Methods:

• Single-particle cryo-electron microscopy (cryo-EM) was used to determine the structures of Pdcd4 bound to the 40S ribosomal subunit.
• Complexes including Pdcd4-40S and Pdcd4-40S-eIF4A-eIF3-eIF1 were analyzed.
• Quantitative binding and in vitro translation assays were performed.

Main Results:

• The cryo-EM structures revealed Pdcd4's binding site on the 40S subunit at the mRNA entry channel.
• Pdcd4's C-terminal domain (CTD) interacts with eIF4A, while the N-terminal domain (NTD) occupies the mRNA channel and decoding site.
• The NTD is critical for Pdcd4 recruitment to the ribosome.

Conclusions:

• Pdcd4 acts as a translational repressor by directly binding the 40S ribosomal subunit.
• Pdcd4 inhibits the eIF4F-independent function of eIF4A during mRNA recruitment and 5' UTR scanning.
• These findings provide mechanistic insights into Pdcd4's tumor suppressive functions via translational control.