Lactose Permease Scrambles Phospholipids

Area of Science:

• Membrane protein function
• Lipid bilayer dynamics
• Biochemistry

Background:

• Lactose permease (LacY) from Escherichia coli is a well-characterized transporter in the major facilitator superfamily.
• LacY facilitates the symport of lactose and other β-galactosides into the cell, driven by a proton gradient.
• The precise mechanisms and potential alternative functions of LacY remain areas of active investigation.

Purpose of the Study:

• To investigate potential non-transport related functions of purified lactose permease (LacY).
• To determine if LacY exhibits any activity in lipid membrane manipulation.
• To elucidate the relationship between LacY's transport function and any observed lipid scrambling activity.

Main Methods:

• Purified LacY was reconstituted into liposomes.
• The ability of reconstituted LacY to translocate fluorescently and radiolabeled glycerophospholipids across the liposome membrane was assessed.
• Mutant LacY proteins with impaired lactose transport or locked conformations were utilized to differentiate functions.

Main Results:

• Purified LacY demonstrated significant glycerophospholipid scrambling activity when reconstituted into liposomes.
• This lipid scrambling was independent of the proton/lactose symport activity of LacY.
• A double mutant of LacY, locked in an occluded conformation, showed significantly reduced lipid scrambling, highlighting the importance of specific conformational states.

Conclusions:

• Lactose permease (LacY) possesses an intrinsic ability to scramble glycerophospholipids across lipid bilayers.
• The lipid scrambling function of LacY is mechanistically distinct from its canonical proton/lactose transport role.
• Specific amino acid residues and conformational states, such as those involving G46 and G262, are crucial for LacY-mediated glycerophospholipid scrambling.