  The spatial organization of cell membrane glycoproteins and glycolipids is
critical for mediating the binding of ligands, receptors, and macromolecules on
the plasma membrane. However, we currently do not have the methods to quantify
the spatial heterogeneities of macromolecular crowding on live cell surfaces.
In this work, we combine experiment and simulation to report crowding
heterogeneities on reconstituted membranes and live cell membranes with
nanometer spatial resolution. By quantifying the effective binding affinity of
IgG monoclonal antibodies to engineered antigen sensors, we discovered sharp
gradients in crowding within a few nanometers of the crowded membrane surface.
Our measurements on human cancer cells support the hypothesis that raft-like
membrane domains exclude bulky membrane proteins and glycoproteins. Our facile
and high-throughput method to quantify spatial crowding heterogeneities on live
cell membranes may facilitate monoclonal antibody design and provide a
mechanistic understanding of plasma membrane biophysical organization.
