In the Figure above, the polymerization of fluorescently-l…

  In the Figure above, the polymerization of fluorescently-labeled actin was monitored in the absence or presence of TARP, a bacterial virulence factor.  Increased polymerization produces increased fluorescence intensity.  (a) G actin is present at time 0, but the process of actin polymerization does not begin until something else (other than TARP) is added at ~ 500 seconds.  What is added at ~500 seconds to begin polymerization?  Why does this initiate polymerization?  (b)  What effect does TARP have on actin polymerization?  Explain your answer    (c)  For the actin alone control condition, briefly explain what is occurring for each of the three phases of polymerization.