Type II transmembrane domain hydrophobicity dictates the cotranslational dependence for inversion.
Bottom Line: Membrane insertion by the Sec61 translocon in the endoplasmic reticulum (ER) is highly dependent on hydrophobicity.Overall the cotranslational inversion of marginally hydrophobic NA TMDs initiates once ~70 amino acids past the TMD are synthesized, and the efficiency reaches 50% by ~100 amino acids, consistent with the positioning of this TMD class in type II human membrane proteins.Inversion of the M2 TMD, achieved by elongating its C-terminus, underscores the contribution of cotranslational synthesis to TMD inversion.
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Mentions: In addition to affecting membrane integration and orientation, decreased hydrophobicity in Nin-Cout (type II) TMDs and shorter C-tail length could also potentially affect their SRP recognition and hence their ER targeting capability (Bird et al., 1990; Hatsuzawa et al., 1997). Therefore we created a series of ribosomally arrested chains with elongated C-tails using the hydrophobic and marginally hydrophobic NA TMDs and assayed their ability to direct targeting to rough ER microsomes. As expected, no targeting to the ER microsomes was observed when the marginally hydrophobic TMD was largely sequestered in the ribosome with a 22-aa C-tail (Figure 6A). With a 36-aa C-tail, a higher proportion of the marginally hydrophobic NA TMD ribosomally arrested chains sedimented with the ER microsomes, and the proportion substantially increased with a 46-aa C-tail, which is similar to the 49-aa C-tail on the SRP-dependent M2 TMD (Hull et al., 1988). A comparable but more efficient ER targeting profile was observed with the hydrophobic NA TMD ribosomally arrested chains (Figure 6B).