  In cell membranes, proteins and lipids diffuse in a highly crowded and
heterogeneous landscape, where aggregates and dense domains of proteins or
lipids obstruct the path of diffusing molecules. In general, hindered motion
gives rise to anomalous transport, though the nature of the onset of this
behavior is still under debate and difficult to investigate experimentally.
Here, we present a systematic study where proteins bound to supported lipid
membranes diffuse freely in two dimensions, but are increasingly hindered by
the presence of other like proteins. In our model system, the surface coverage
of the protein avidin on the lipid bilayer is well controlled by varying the
concentration of biotinylated lipid anchors. Using fluorescence correlation
spectroscopy (FCS), we measure the time correlation function over long times
and convert it to the mean-square displacement of the diffusing proteins. Our
approach allows for high precision data and a clear distinction between
anomalous and normal diffusion. It enables us to investigate the onset of
anomalous diffusion, which takes place when the area coverage of membrane
proteins increases beyond approximately 5%. This transition region exhibits
pronounced spatial heterogeneities. Increasing the packing fraction further,
transport becomes more and more anomalous, manifested in a decrease of the
exponent of subdiffusion.
