Probing Orientational Order of MHC Class I Protein and Lipids in Cell Membranes by Fluorescence Polarization-Resolved Microscopy Imaging

Abstract

Biomolecular orientational organization of lipids and proteins in plasma membrane is a crucial factor in biological processes where functions can be closely related to orientation and ordering mechanisms. The concept of transient nanosized phase separations in ordered and disordered domains, called “lipid rafts” is now widely accepted. Furthermore, the ordered domains are enriched in signalling proteins, which highlights the crucial impact of phase separation during the signalling processes. While this field has been so far largely addressed by studying the translational diffusion behaviour of membrane proteins by Single Molecule Tracking or Fluorescence Correlation Spectroscopy, only little is known about the orientational behaviour of signalling proteins in plasma membranes, mainly due to the lack of appropriate rigid fluorescent label which would be able to act as a proper orientation reporter. In this work we develop a fully polarization-resolved fluorescence imaging technique using a tuneable incident polarization state (“fluorescence polarimetry”), in combination with fluorescence anisotropy imaging, in order to provide orientational order information in very general cell membranes shapes.We apply this technique to the measurement of quantitative orientational distribution of MHC Class I proteins in the plasma and nuclear membranes, benefiting from a rigidly attached GFP probe. The surrounding lipid orientational order in the plasma membrane is additionally probed using the fluorescent reporter di-8-ANEPPQ.The MHC Class I protein is found to be more ordered in the plasma membrane as compared to the nuclear membrane. Both MHC I and di-8-ANEPPQ orientational orders in the plasma membrane are furthermore seen to be highly affected by actin depolymerisation upon Latrunculin A treatment, with variations that indicate both a structural change in the membrane morphology and a disruption of MHC I - actin interactions.