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Membrane Protein Biogenesis
Author(s) -
Johnson Arthur E.
Publication year - 2011
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.25.1_supplement.192.1
Subject(s) - translocon , endoplasmic reticulum , förster resonance energy transfer , ribosome , sec61 , biophysics , microbiology and biotechnology , membrane protein , membrane , chemistry , biology , biochemistry , fluorescence , rna , gene , physics , quantum mechanics
Most membrane proteins in mammalian cells are integrated co‐translationally at translocons in the endoplasmic reticulum (ER) membrane. To examine this complex process, we incorporated fluorescent or photoreactive probes into nascent proteins using modified aminoacyl‐tRNAs in vitro, and then selectively monitored nascent chain conformation and environment at various stages of integration using fluorescence resonance energy transfer (FRET), fluorescence, and photocrosslinking. The combined data revealed several unexpected aspects of the early stages of integration. Each successive nonpolar transmembrane segment (TMS) in a nascent multi‐spanning polytopic membrane protein (PMP) folds into an α‐helix at a specific site inside the ribosomal tunnel, and each ribosome‐induced folding event triggers a structural change in the ribosome‐translocon complex that directs successive nascent PMP loops alternately into the cytosol or the ER lumen. Upon reaching the translocon, every other TMS in a PMP rotates 180° relative to the plane of the membrane while retaining its helical conformation during the inversion. TMSs do not partition immediately into bulk membrane lipid upon entering the translocon pore. Instead, TMSs bind to specific sites in the translocon and their release is regulated by translation termination or the arrival of another TMS at the translocon.