Visualization of a polytopic membrane protein during SecY-mediated membrane insertion.

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Polytopic membrane protein biogenesis is co-translational, involving ribosomes and the Sec-complex.
• The precise pathway of N-terminal transmembrane domains (TMDs) during synthesis remains unclear.

Purpose of the Study:

• To elucidate the fate of N-terminal TMDs during polytopic membrane protein biogenesis.
• To provide structural insights into the early stages of membrane protein insertion.

Main Methods:

• Sub-nanometer cryo-electron microscopy (cryo-EM) of ribosome-SecY complex.
• In vivo generation of a membrane insertion intermediate of proteorhodopsin (PR).

Main Results:

• Structural visualization of a ribosome-SecY complex with a membrane insertion intermediate.
• The Sec-complex is observed in a pre-opened state.
• The first two TMDs of proteorhodopsin are positioned outside the SecY complex, near the lateral gate.

Conclusions:

• The findings support models positioning N-terminal TMDs at the SecY periphery.
• Provides structural evidence for the molecular mechanism of membrane protein topogenesis.
• Offers new understanding of co-translational protein insertion into the membrane.