During endocytosis, the cell membrane deforms to surround extracellular material and
draw it into the cell. Experiments on clathrin-mediated endocytosis in yeast all agree
that (i) actin polymerizes into a network of filaments exerting active forces on the
membrane to deform it and (ii) the large scale membrane deformation is tubular in
shape. Three competing ideas remain as to precisely how the actin filament network
organizes itself to drive the deformation. To begin to address this issue, we use
variational approaches and numerical simulations to analyze a meso-scale model of
clathrin-mediated endocytosis in yeast. The meso-scale model breaks up the invagination
process into three stages: (i) the initiation stage, where clathrin interacts with
the membrane, (ii) the elongation stage, where the membrane is then pulled and/or
squeezed via polymerizing actin filaments, followed by a (iii) final pinch-off stage.
Our results suggest that the pinch-off mechanism is assisted by a pearling-like instability.
In addition, we potentially rule out two of the three competing models for the organization
of the actin filament network during the elongation stage. These two models could
possibly be important in the pinch-off stage, however, where actin polymerization
helps break off the vesicle. Implications and comparisons with earlier modeling of
clathrin-mediated endocytosis in yeast is discussed.