Endocytosis mediated by clathrin, a cellular process by which cells internalize 
membrane receptors and their extracellular ligands, is an important component of 
cell signaling regulation. Actin polymerization is involved in endocytosis in 
varying degrees depending on the cellular context. In yeast, clathrin-mediated 
endocytosis requires a pulse of polymerized actin and its regulators, which 
recruit and activate the Arp2/3 complex. In this article, we seek to identify 
the main protein-protein interactions that 1) cause actin and its regulators to 
appear in pulses, and 2) determine the effects of key mutations and drug 
treatments on actin and regulator assembly. We perform a joint 
modeling/experimental study of actin and regulator dynamics during endocytosis 
in the budding yeast Saccharomyces cerevisiae. We treat both a stochastic model 
that grows an explicit three-dimensional actin network, and a simpler 
two-variable Fitzhugh-Nagumo type model. The models include a negative-feedback 
interaction of F-actin onto the Arp2/3 regulators. Both models explain the pulse 
time courses and the effects of interventions on actin polymerization: the 
surprising increase in the peak F-actin count caused by reduced regulator 
branching activity, the increase in F-actin resulting from slowing of actin 
disassembly, and the increased Arp2/3 regulator lifetime resulting from 
latrunculin treatment. In addition, they predict that decreases in the regulator 
branching activity lead to increases in accumulation of regulators, and we 
confirmed this prediction with experiments on yeast harboring mutations in the 
Arp2/3 regulators, using quantitative fluorescence microscopy. Our experimental 
measurements suggest that the regulators act quasi-independently, in the sense 
that accumulation of a particular regulator is most strongly affected by 
mutations of that regulator, as opposed to the others.