  The secretion of vesicles for intracellular transport often rely on the
aggregation of specialized membrane-bound proteins into a coat able to curve
cell membranes. The nucleation and growth of a protein coat is a kinetic
process that competes with the energy-consuming turnover of coat components
between the membrane and the cytosol. We propose a generic kinetic description
of coat assembly and the formation of coated vesicles, and discuss its
implication to the dynamics of COP vesicles that traffic within the Golgi and
with the Endoplasmic Reticulum. We show that stationary coats of fixed area
emerge from the competition between coat growth and the recycling of coat
components, in a fashion resembling the treadmilling of cytoskeletal filaments.
We further show that the turnover of coat components allows for a highly
sensitive switching mechanism between a quiescent and a vesicle producing
membrane, upon a slowing down of the exchange kinetics. We claim that the
existence of this switching behaviour, also triggered by factors such as the
presence of cargo and variation of the membrane mechanical tension, allows for
efficient regulation of vesicle secretion. We propose a model, supported by
different experimental observations, in which vesiculation of secretory
membranes is impaired by the energy consuming desorption of coat proteins,
until the presence of cargo or other factors triggers a dynamical switch into a
vesicle producing state.
