title: Systematic probing of the environment of a translocating secretory protein during translocation through the ER membrane.
abstract: We have extended a previously developed photo-crosslinking approach to systematically probe the protein environment of the secretory protein preprolactin, trapped during its transfer through the endoplasmic reticulum membrane. Single photoreactive groups were placed at various positions of nascent polypeptide chains of various length, corresponding to different stages of the transport process, and photo-crosslinks to membrane proteins were analyzed. In all cases, the polypeptide segment extending from the ribosome was found to be located in a membrane environment that is formed almost exclusively from Sec61 alpha, the multi-spanning subunit of the Sec61p complex that is essential for translocation. At early stages of the translocation process, before cleavage of the signal sequence, almost the entire nascent chain emerged from the ribosome contacts Sec61 alpha. The 'translocating chain-associating membrane' protein interacts mainly with the region of the signal sequence preceding its hydrophobic core. Our results suggest that the nascent chain is transferred directly from the ribosome into a protein-conducting channel, the major constituent of which is Sec61 alpha.
authors.affiliation: Max-Delbrück-Center for Molecular Medicine, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Mothes
authors.forename: S
authors.initials: S
authors.lastname: Prehn
authors.forename: T A
authors.initials: TA
authors.lastname: Rapoport
mesh_terms: D000345:Affinity Labels; D000595:Amino Acid Sequence; D001389:Azirines / Q000737:chemistry; D001565:Benzoates; D001692:Biological Transport; D002451:Cell Compartmentation; D003432:Cross-Linking Reagents; D004252:DNA Mutational Analysis; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D008239:Lysine / Q000235:genetics; D008565:Membrane Proteins / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D011388:Prolactin / Q000378:metabolism*; D014176:Protein Biosynthesis; D011498:Protein Precursors / Q000378:metabolism*; D000069816:SEC Translocation Channels; D013329:Structure-Activity Relationship; D014466:Ultraviolet Rays
chemical_list: D000345:Affinity Labels; D001389:Azirines; D001565:Benzoates; D003432:Cross-Linking Reagents; D008565:Membrane Proteins; D011498:Protein Precursors; D000069816:SEC Translocation Channels; C017671:preprolactin; C051320:4-(3-trifluoromethyldiazirino)benzoic acid; D011388:Prolactin; D008239:Lysine