How a circularized tmRNA moves through the ribosome.

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Ribosomes can stall on truncated or damaged messenger RNAs (mRNAs).
• Trans-translation is a conserved cellular mechanism that rescues stalled ribosomes.
• Transfer-messenger RNA (tmRNA) and its binding protein SmpB are essential for trans-translation.

Purpose of the Study:

• To elucidate the structural mechanisms of tmRNA-SmpB-ribosome complex during trans-translation.
• To visualize the dynamic process of ribosome rescue and polypeptide tagging at high resolution.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine complex structures.
• High-resolution structural analysis (3.7–4.4 angstroms) of key intermediate states.

Main Results:

• Three distinct structural states of the tmRNA-SmpB-ribosome complex were resolved.
• The structures reveal the specific interactions of tmRNA and SmpB with stalled ribosomes.
• The mechanism of tmRNA-SmpB complex translocation through the ribosome was visualized.

Conclusions:

• tmRNA and SmpB specifically recognize and bind to stalled ribosomes.
• The dynamic movement of the complex facilitates the switch from a defective mRNA to tmRNA.
• This process enables the tagging of nascent polypeptides for degradation, maintaining cellular proteostasis.