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

• Published in: Nature communications Vol. 5; no. 1; p. 4103
• Main Authors: Bischoff, Lukas, Wickles, Stephan, Berninghausen, Otto, van der Sluis, Eli O., Beckmann, Roland
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 10-06-2014; Nature Publishing Group
• Subjects: 101/28; 631/45/535; 631/45/612/1237; Biosynthesis; Cell Membrane - metabolism; Cell Membrane - ultrastructure; Cryoelectron Microscopy; Escherichia coli; Escherichia coli Proteins - biosynthesis; Escherichia coli Proteins - metabolism; Escherichia coli Proteins - ultrastructure; Humanities and Social Sciences; Models, Molecular; multidisciplinary; Protein Conformation; Proteins; Ribonucleic acid; RNA; Science; Science (multidisciplinary); SEC Translocation Channels; Transfer RNA; Visualization
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms5103

The biogenesis of polytopic membrane proteins occurs co-translationally on ribosomes that are tightly bound to a membrane-embedded protein-conducting channel: the Sec-complex. The path that is followed by nascent proteins inside the ribosome and the Sec-complex is relatively well established; however, it is not clear what the fate of the N-terminal transmembrane domains (TMDs) of polytopic membrane proteins is when the C-terminal TMDs domains are not yet synthesized. Here, we present the sub-nanometer cryo-electron microscopy structure of an
in vivo
generated ribosome-SecY complex that carries a membrane insertion intermediate of proteorhodopsin (PR). The structure reveals a pre-opened Sec-complex and the first two TMDs of PR already outside the SecY complex directly in front of its proposed lateral gate. Thus, our structure is in agreement with positioning of N-terminal TMDs at the periphery of SecY, and in addition, it provides clues for the molecular mechanism underlying membrane protein topogenesis.
Membrane protein topogenesis is not fully understood, although the path that proteins take through the ribosome and Sec-complex has been described. Here, Bischoff
et al.
present the structure of a ribosome-SecY complex containing an intermediate of proteorhodopsin, which provides further insight into this topogenesis.