Spontaneous transmembrane helix insertion thermodynamically mimics translocon-guided insertion.

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysics

Background:

• Membrane protein stability is governed by the favorable free energy of transmembrane (TM) α-helix transfer between aqueous and lipid environments.
• The precise relationship between insertion energetics and the in vivo assembly process mediated by the Sec61/SecY translocon complex remains unclear.

Purpose of the Study:

• To directly quantify the partitioning free energies of designed peptides during membrane insertion.
• To elucidate the role of the membrane interface in translocon-mediated protein insertion.

Main Methods:

• Experimental microsomal Sec61 translocon assay.
• Biophysical spectroscopic characterization of peptide insertion into lipid bilayers.
• Unbiased atomic-detail equilibrium folding-partitioning molecular dynamics simulations.

Main Results:

• Quantitative agreement in measured free energies of insertion across all three independent approaches.
• Molecular dynamics simulations indicate TM helix insertion involves equilibrium with the membrane interface.

Conclusions:

• The energetics of transmembrane helix insertion are consistently determined by experimental and computational methods.
• The membrane interface likely plays a significant role in guiding the insertion process by the Sec61/SecY translocon.