Stoichiometry and kinetics of transport vesicle fusion with Golgi membranes

The in vitro complementation assay established by Rothman and co-workers continues to be an important tool to study intra-Golgi transport. In this study, kinetic modeling is used to identify four main parameters that, together, explain the basic features of an assay that is a modification of the original assay. First, the assay signal depends on the ratio of Golgi membranes to transport intermediates in the assay. Secondly, an inactivation rate describes how the activity of transport intermediates decreases over time. Thirdly, the rate at which transport intermediates irreversibly bind to Golgi membranes is measured independently of membrane fusion, thus allowing a quantitative distinction between these two steps. Fourthly, a single rate constant describes the remaining reactions, which result in membrane fusion. This approach of kinetic modeling of experiments is generally applicable to other in vitro assays of cell biological phenomena, permitting quantitative interpretations and an increased resolution of the experiments.