  We introduce a simple physical picture to explain the process of molecular
sorting, whereby specific proteins are concentrated and distilled into
submicrometric lipid vesicles in eukaryotic cells. To this purpose, we
formulate a model based on the coupling of spontaneous molecular aggregation
with vesicle nucleation. Its implications are studied by means of a
phenomenological theory describing the diffusion of molecules towards multiple
sorting centers that grow due to molecule absorption and are extracted when
they reach a sufficiently large size. The predictions of the theory are
compared with numerical simulations of a lattice-gas realization of the model
and with experimental observations. The efficiency of the distillation process
is found to be optimal for intermediate aggregation rates, where the density of
sorted molecules is minimal and the process obeys simple scaling laws.
Quantitative measures of endocytic sorting performed in primary endothelial
cells are compatible with the hypothesis that these optimal conditions are
realized in living cells.
