Structural analysis of the dynamic ribosome-translocon complex.

Area of Science:

• Molecular Biology
• Structural Biology
• Cell Biology

Background:

• The protein translocon facilitates the biogenesis of secretory and membrane proteins at the endoplasmic reticulum.
• It comprises the Sec61 translocation channel and various accessory factors.

Purpose of the Study:

• To investigate novel configurations of the ribosome-translocon complex using cryo-electron microscopy (cryo-EM) and structure prediction.
• To elucidate the mechanisms of protein insertion and translocon function.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine high-resolution structures.
• Structure prediction algorithms to model complex interactions.
• Analysis of ribosome-translocon complex configurations.

Main Results:

• Observed transmembrane domains (TMDs) in looped configurations passing through the Sec61 lateral gate.
• Identified nascent chain binding and constraint of ribosomal protein uL22.
• Showed dynamic positioning of the translocon-associated protein (TRAP) complex.
• Discovered RAMP4 intercalated into Sec61's lateral gate in many complexes, widening the pore.

Conclusions:

• The translocon is a plastic machinery with dynamically adjusting conformations and composition.
• RAMP4's presence influences Sec61 pore properties, suggesting a role in protein translocation.
• These findings provide mechanistic hypotheses for translocon function in diverse protein biogenesis pathways.