Structural basis of human erythrocyte glucose transporter function in reconstituted vesicles

Area of Science:

• Structural Biology
• Membrane Protein Biophysics
• Spectroscopic Analysis

Background:

• Understanding the transmembrane orientation of transporters is crucial for elucidating their function.
• The human erythrocyte glucose transporter (GLUT1) plays a vital role in cellular glucose uptake.
• Previous studies have proposed models for GLUT1 structure, but direct orientation data is limited.

Purpose of the Study:

• To determine the transmembrane orientation of the human erythrocyte glucose transporter.
• To investigate the secondary structure and helical arrangement within the lipid bilayer.
• To assess the effect of substrate binding on transporter orientation.

Main Methods:

• Utilized polarized Fourier transform infrared (FTIR) spectroscopy on oriented multilamellar films of reconstituted transporter vesicles.
• Employed ultraviolet circular dichroism (UV-CD) spectroscopy to analyze protein secondary structure.
• Analyzed linear dichroism of FTIR spectra to determine the orientation of alpha-helices relative to the lipid bilayer.

Main Results:

• Infrared spectroscopy confirmed the presence of alpha-helical structures and the absence of beta-structures.
• Linear dichroism analysis indicated that the transporter's alpha-helices are predominantly oriented perpendicular to the lipid bilayer plane, with a tilt angle less than 38 degrees from the membrane normal.
• UV-CD spectra supported the helical orientation, and substrate (D-glucose) binding was observed to slightly decrease the tilt angle.

Conclusions:

• The human erythrocyte glucose transporter adopts a transmembrane orientation characterized by alpha-helices positioned nearly perpendicular to the lipid bilayer.
• Spectroscopic data provides direct evidence for the structural arrangement of GLUT1 within the membrane.
• Substrate interaction can subtly modulate the transporter's orientation, potentially influencing its transport mechanism.