Evolutionary balance between foldability and functionality of a glucose transporter.

Area of Science:

• Structural Biology
• Biophysics
• Membrane Protein Folding

Background:

• Understanding multi-pass membrane protein folding is crucial yet challenging.
• Human glucose transporter 3 (GLUT3) is vital for glucose transport.
• Native folding pathways of membrane proteins remain largely unknown.

Purpose of the Study:

• To determine the folding pathway of purified human glucose transporter 3 (GLUT3).
• To investigate the role of lipid bilayers and cellular machinery in GLUT3 folding.
• To identify factors influencing the assembly of GLUT3's N- and C-terminal domains.

Main Methods:

• Single-molecule magnetic tweezers to study purified GLUT3 reconstituted in synthetic lipid bilayers.
• Analysis of folding challenges posed by polar residues in the glucose conduit.
• Investigating the influence of endoplasmic reticulum membrane protein complex and specific lipids.

Main Results:

• The N-terminal major facilitator superfamily (MFS) fold forms first, acting as a template.
• Polar residues in the glucose conduit present significant folding hurdles.
• Endoplasmic reticulum proteins assist in hydrophilic transmembrane helix insertion, promoting folding.
• Specific lipids facilitate the final domain assembly by reducing desolvation penalties.
• Asymmetric folding propensity is conserved in metazoan sugar porters.

Conclusions:

• GLUT3 folding is an asymmetric process initiated by the N-terminal MFS domain.
• Lipid-protein interactions and cellular factors are critical for proper GLUT3 assembly.
• Evolutionary pressures create conflicts between foldability and functionality in membrane proteins.