You have recently been hired by a research team that studies cystic fibrosis (CF), a fatal disorder that clogs the lungs and other organs with a viscous, sticky mucous that interferes with breathing and digestion. The leader of your research team has informed you that a major fraction of the mucous in CF patients consists of filamentous actin. His hypothesis is that this actin may play a key role in interfering with breathing and digestion in CF patients. In order to test your laboratory skills and knowledge he decides to give you a quiz.

Your first assignment is to assemble actin monomers into filaments. In a test tube you mix actin monomers with KCl, MgCl2, and ATP and then measure filament assembly

Your boss immediately notices that there is a lag phase before actin filaments can be detected and that the “%actin in filaments” plateaus at less than 100%.

Explain the lag phase and why the percent of actin in filaments peaks at a value less than 100%