Two translocating hydrophilic segments of a nascent chain span the ER membrane during multispanning protein topogenesis

J Cell Biol. 2007 Dec 31;179(7):1441-52. doi: 10.1083/jcb.200707050.

Abstract

During protein integration into the endoplasmic reticulum, the N-terminal domain preceding the type I signal-anchor sequence is translocated through a translocon. By fusing a streptavidin-binding peptide tag to the N terminus, we created integration intermediates of multispanning membrane proteins. In a cell-free system, N-terminal domain (N-domain) translocation was arrested by streptavidin and resumed by biotin. Even when N-domain translocation was arrested, the second hydrophobic segment mediated translocation of the downstream hydrophilic segment. In one of the defined intermediates, two hydrophilic segments and two hydrophobic segments formed a transmembrane disposition in a productive state. Both of the translocating hydrophilic segments were crosslinked with a translocon subunit, Sec61alpha. We conclude that two translocating hydrophilic segment in a single membrane protein can span the membrane during multispanning topogenesis flanking the translocon. Furthermore, even after six successive hydrophobic segments entered the translocon, N-domain translocation could be induced to restart from an arrested state. These observations indicate the remarkably flexible nature of the translocon.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell-Free System
  • Endoplasmic Reticulum / chemistry
  • Endoplasmic Reticulum / metabolism*
  • Endoplasmic Reticulum / ultrastructure
  • Humans
  • Hydrophobic and Hydrophilic Interactions
  • Intracellular Membranes / chemistry
  • Intracellular Membranes / metabolism*
  • Intracellular Membranes / ultrastructure
  • Membrane Proteins / chemistry
  • Membrane Proteins / metabolism*
  • Molecular Chaperones / metabolism
  • Protein Biosynthesis / physiology
  • Protein Conformation
  • Protein Folding
  • Protein Processing, Post-Translational / physiology
  • Protein Structure, Tertiary / physiology
  • Protein Transport / physiology
  • SEC Translocation Channels

Substances

  • Membrane Proteins
  • Molecular Chaperones
  • SEC Translocation Channels
  • SEC61A1 protein, human