Kinetic regulation of coated vesicle secretion

Abstract

The secretion of vesicles for intracellular transport often relies 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 energyconsuming 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 behavior, 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.transport vesicle | protein coat | COP vesicles | self-assembly | nonequilibrium phase transition T he plasma membrane and the membrane of cell compartments such as the endoplasmic reticulum (ER) and the Golgi continually produce vesicles for cargo transport. Vesicle formation generally involves specific proteins that aggregate into semirigid coats of dimensions in the 100-nm range, well visible by electronic microscopy (1-3). The process of vesicle formation is now rather well established (4) and is sketched in Fig. 1. First, various cytosolic proteins assemble on the membrane into elementary coat-building units, called monomers. The membrane-bound monomers then polymerize into coat structures that locally bend the membrane and recruit cargo molecules. As the coat expands, the coated membrane invaginates until it forms a nearly spherical vesicle containing cargo (5, 6) that is eventually released from the membrane. The coat components soon disassemble and are ready to participate in the formation of a new vesicle.The coats are classified in three major classes, COPII, COPI, and clathrin. Although they involve distinct proteins, the three types of coat share many common features, from their size and shape to the mechanism by which polymerization, cargo recruitment, and membrane deformation is achieved (4). Our approach is primarily aimed at studying the formation of COPI and COPII vesicles. However, the generality and robustness of its outcome suggest relevance for the more sophisticated clathrin coats as well. The assembly of COPs and clathrin and the fission of COP vesicles can ...

First assembly times and equilibration in stochastic coagulation-fragmentation

“…38,39 Moreover, the mechanisms we describe may contribute to observed effects in self-assembly, such as sample volume-dependent lag times for the formation of critical nuclei.…”

Protein-coat dynamics and cluster phases in intracellular trafficking

“…We explored a range of values for k on , k off , and the interaction strength J . We note that the estimated off rate is 0.1-10 s −1 based on FRAP experiments [16]. For a diffusion constant of 0.1 μ m 2 /s and lattice unit a ∼ 5 nm, this corresponds to an estimated off rate ∼ 10 −5 -10 −3 in our lattice simulations.…”

“…In our model, we do not account for the effects of curvature on line tension and membrane surface tension, which would lead to more complex energy dependence of a cluster on its size. In the work by Foret and Sens [16], this nontrivial energy landscape plays an important role in stabilizing clusters of finite size and governing the switch to a state of vesicle secretion. Here we will demonstrate how even a simple, local energy function can lead to clusters of finite size arising from the kinetics.…”

“…For a diffusion constant of 0.1 μ m 2 /s and lattice unit a ∼ 5 nm, this corresponds to an estimated off rate ∼ 10 −5 -10 −3 in our lattice simulations. Moreover a complete vesicle is expected to have a couple of hundred protein coat units [16], thus in the quiescent membrane we expect clusters typically of size one hundred or smaller. For the results shown in this paper, unless otherwise indicated, the interaction strength, J , was chosen to be 4.0, a value greater than the critical strength J c .…”

Active regeneration unites high- and low-temperature features in cooperative self-assembly

“…Several models have recently been developed that combine high-cooperativity equilibrium dynamics with a nonequilibrium driving force, leading to novel hypotheses about bistability in vesicle coat dynamics [9] and in the behavior of neurotransmitter protein receptors [10]. But these models are formulated in the limit of an infinite thermodynamic force, where at least one of the reaction steps is completely irreversible, and so they are not suitable for investigating the dependence of the system's properties on thermodynamic drive strength.…”