Observing a lipid-dependent alteration in single lactose permeases

Area of Science:

• Membrane biophysics
• Protein structure-function relationships
• Escherichia coli physiology

Background:

• Escherichia coli membranes primarily consist of phosphatidylethanolamine (PE) and phosphatidylglycerol (PG).
• Previous biochemical studies suggest PE depletion alters lactose permease (LacY) structure, specifically causing helix VII inversion.
• Understanding lipid-protein interactions is vital for membrane protein stability and function.

Purpose of the Study:

• To investigate the structural impact of phosphatidylethanolamine (PE) depletion on the lactose permease (LacY) using single-molecule force spectroscopy.
• To elucidate the role of PE in maintaining the native conformation of LacY within the Escherichia coli membrane.

Main Methods:

• Single-molecule force spectroscopy was employed to probe LacY structure in different lipid environments.
• Lipid compositions were varied, focusing on the ratio of PE to PG.
• Conformational states of LacY were analyzed based on force-distance curves.

Main Results:

• In a standard PE:PG (3:1) membrane, approximately 95% of LacY molecules adopted a native structure.
• In a PG-only membrane (lacking PE), LacY populated both native and perturbed conformations almost equally.
• Significant structural changes were observed in helices VI and VII and the connecting loop when PE was absent.

Conclusions:

• Phosphatidylethanolamine (PE) is essential for promoting the native fold of lactose permease (LacY).
• The zwitterionic nature of PE may mitigate electrostatic repulsion between LacY and PG, preventing nonnative conformations.
• PE plays a critical role in stabilizing LacY in a functionally competent state within the E. coli membrane.